Add capabilities file to securityfs

RFC PATCH In review Francis Laniel 4 participants 12 messages Jul 25, 2022 – Aug 17, 2022

Subsystems (3)

BPF Security (LSM) Capabilities

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Francis Laniel

author

2022-07-25 12:42 UTC #0 lore raw

v4 cover Add capabilities file to securityfs

Hi.

First, I hope you are fine and the same for your relatives.

Capabilities are used to check if a thread can perform a given action [1]. For example, a thread with CAP_BPF set can use the bpf() syscall.

Capabilities are used in the container world.
In terms of code, several projects related to container maintain code where the capabilities are written alike include/uapi/linux/capability.h [2][3][4][5]. For these projects, their codebase should be updated when a new capability is added to the kernel.
Some other projects rely on <sys/capability.h> [6].
In this case, this header file should reflect the capabilities offered by the kernel.

The delay between adding a new capability to the kernel and this capability being used by "container stack" software users can be long. Indeed, CAP_BPF was added in a17b53c4a4b5 which was part of v5.8 released in August 2020.
Almost 2 years later, none of the "container stack" software authorize using this capability in their last stable release.
The only way to use CAP_BPF with moby is to use v22.06.0-beta.0 release which contains a commit enabling CAP_BPF, CAP_PERFMON and CAP_CHECKPOINT_RESTORE [7]. This situation can be easily explained by the following:
1. moby depends on containerd which in turns depends on runc.
2. runc depends on github.com/syndtr/gocapability which is golang package to deal with capabilities.
This high number of dependencies explain the delay and the big amount of human work to add support in the "container stack" software for a new capability.

A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.

The "file" is read-only and its content is the capability number associated with the capability name:

root@vm-amd64:~# cat /sys/kernel/security/capabilities
0       CAP_CHOWN
1       CAP_DAC_OVERRIDE
...
40      CAP_CHECKPOINT_RESTORE
root@vm-amd64:~# wc -c /sys/kernel/security/capabilities
698 /sys/kernel/security/capabilities
So, the "container stack" software just have to read this file to know if they
can use the capabilities the user asked for.
For example, if user asks for CAP_BPF on kernel 5.8, then this capability will
be present in the file and so it can be used.
Nonetheless, if the underlying kernel is 5.4, this capability will not be
present and so it cannot be used.

The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap
So, I think there should not be any issue exposing all the capabilities it offers.
If there is any, please share it as I do not want to introduce issue with this series.
Also, the data exchanged with userspace are less than 700 bytes long which represent 17% of PAGE_SIZE.

Note that I am open to any better way for the userspace to ask the kernel for known capabilities.
And if you see any way to improve this series please share it as it would increase this contribution quality.

Change since:
v3:
* Use securityfs_create_file() to create securityfs file.
v2:
* Use a char * for cap_string instead of an array, each line of this char * contains the capability number and its name.
* Move the file under /sys/kernel/security instead of /sys/kernel.

Francis Laniel (2):
capability: Add cap_string.
security/inode.c: Add capabilities file.

 include/uapi/linux/capability.h |  1 +
 kernel/capability.c             | 45 +++++++++++++++++++++++++++++++++
 security/inode.c                | 16 ++++++++++++
 3 files changed, 62 insertions(+)

Best regards and thank you in advance for your reviews.
---
[1] man capabilities
[2] https://github.com/containerd/containerd/blob/1a078e6893d07fec10a4940a5664fab21d6f7d1e/pkg/cap/cap_linux.go#L135
[3] https://github.com/moby/moby/commit/485cf38d48e7111b3d1f584d5e9eab46a902aabc#diff-2e04625b209932e74c617de96682ed72fbd1bb0d0cb9fb7c709cf47a86b6f9c1
moby relies on containerd code.
[4] https://github.com/syndtr/gocapability/blob/42c35b4376354fd554efc7ad35e0b7f94e3a0ffb/capability/enum.go#L47
[5] https://github.com/opencontainers/runc/blob/00f56786bb220b55b41748231880ba0e6380519a/libcontainer/capabilities/capabilities.go#L12
runc relies on syndtr package.
[6] https://github.com/containers/crun/blob/fafb556f09e6ffd4690c452ff51856b880c089f1/src/libcrun/linux.c#L35
[7] https://github.com/moby/moby/commit/c1c973e81b0ff36c697fbeabeb5ea7d09566ddc0
--
2.25.1

Francis Laniel

author

2022-07-25 12:42 UTC #1 lore raw

RFC PATCH v4 1/2 capability: Add cap_string.

This string contains on each line the number of the capability associated to its name.
For example, first line is:
__stringify(CAP_CHOWN) "\tCAP_CHOWN\n"
which the preprocessor will replace by:
"0\tCAP_CHOWN\n"

Acked-by Casey Schaufler <[email protected]>

Signed-off-by Francis Laniel <[email protected]>

---
 include/uapi/linux/capability.h |  1 +
 kernel/capability.c             | 45 +++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

include/uapi/linux/capability.h +1 -0

index 463d1ba2232a..115f4fef00da 100644
@@ -428,5 +428,6 @@ struct vfs_ns_cap_data {
 #define CAP_TO_INDEX(x)     ((x) >> 5)        /* 1 << 5 == bits in __u32 */
 #define CAP_TO_MASK(x)      (1 << ((x) & 31)) /* mask for indexed __u32 */
 
+extern const char *cap_string;
 
 #endif /* _UAPI_LINUX_CAPABILITY_H */

kernel/capability.c +45 -0

57 lines not on this page · raw message

Signature · 1 line

2.25.1

Francis Laniel

author

2022-07-25 12:42 UTC #2 lore raw

RFC PATCH v4 2/2 security/inode.c: Add capabilities file.

In reply to Francis Laniel

This new read-only file prints the capabilities values with their names:
cat /sys/kernel/security/capabilities
0 CAP_CHOWN
1 CAP_DAC_OVERRIDE
...
40 CAP_CHECKPOINT_RESTORE

Acked-by Casey Schaufler <[email protected]>

Signed-off-by Francis Laniel <[email protected]>

---
 security/inode.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

security/inode.c +16 -0

index 6c326939750d..6da87ae5a8d6 100644
@@ -21,6 +21,7 @@
 #include <linux/security.h>
 #include <linux/lsm_hooks.h>
 #include <linux/magic.h>
+#include <linux/capability.h>
 
 static struct vfsmount *mount;
 static int mount_count;

@@ -328,6 +329,19 @@ static const struct file_operations lsm_ops = {
 };
 #endif
 
+static struct dentry *capabilities_dentry;
+static ssize_t capabilities_read(struct file *unused, char __user *buf,
+				 size_t count, loff_t *ppos)
+{
+	return simple_read_from_buffer(buf, count, ppos, cap_string,
+				       strlen(cap_string));
+}
+
+static const struct file_operations capabilities_ops = {
+	.read = capabilities_read,
+	.llseek = generic_file_llseek,
+};
+
 static int __init securityfs_init(void)
 {
 	int retval;

@@ -345,6 +359,8 @@ static int __init securityfs_init(void)
 	lsm_dentry = securityfs_create_file("lsm", 0444, NULL, NULL,
 						&lsm_ops);
 #endif
+	capabilities_dentry = securityfs_create_file("capabilities", 0444, NULL,
+						     NULL, &capabilities_ops);
 	return 0;
 }
 core_initcall(securityfs_init);

Signature · 1 line

2.25.1

Paul Moore

maintainer

2022-08-16 21:59 UTC #3 lore raw

In reply to Francis Laniel

Quoted 3 lines · Francis Laniel

Hi.
First, I hope you are fine and the same for your relatives.

Hi Francis :)

Quoted 6 lines

A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.

...

Quoted 2 lines

The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap

I'm not clear on why this patchset is needed, why can't the application simply read from "cap_last_cap" to determine what capabilities the kernel supports?

Signature · 1 line

paul-moore.com

Francis Laniel

author

2022-08-17 11:53 UTC #4 lore raw

Hi.

Quoted 16 lines · Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit :

On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel
Quoted 3 lines · <[email protected]> wrote:
Hi.
First, I hope you are fine and the same for your relatives.
Hi Francis :)
Quoted 6 lines
A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.

Quoted 8 lines

...
Quoted 2 lines
The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap
I'm not clear on why this patchset is needed, why can't the application simply read from "cap_last_cap" to determine what capabilities the kernel supports?

When you capabilities with, for example, docker, you will fill capabilities like this:
docker run --rm --cap-add SYS_ADMIN debian:latest echo foo
As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set.

Sadly, each time a new capability is added to the kernel, it means "container stack" software should add a new string corresponding to the number of the capabilities [1].

The solution I propose would lead to "container stack" software to get rid of such an array and to test at runtime, if the name provided by user on the command line matches the name of a capability known by the kernel. If it is the case, the number associated to the capability will be get by "container stack" code to be used as argument of capset() system call.

The advantage of this solution is that it would reduce the time taken between a new capability added to the kernel (e.g. CAP_BPF) and the time users can use it.
More generally, a solution to this problem would be a way for the kernel to expose the capabilities it knows.

Do not hesitate to ask for clarification if I was not clear.

Best regards.
---
[1] https://github.com/containerd/containerd/blob/
1a078e6893d07fec10a4940a5664fab21d6f7d1e/pkg/cap/cap_linux.go#L135

Paul Moore

maintainer

2022-08-17 14:52 UTC #5 lore raw

Quoted 26 lines · Francis Laniel

Quoted 16 lines · Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit :
On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel
Quoted 3 lines · <[email protected]> wrote:
Hi.
First, I hope you are fine and the same for your relatives.
Hi Francis :)
Quoted 6 lines
A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.
Quoted 8 lines
...
Quoted 2 lines
The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap
I'm not clear on why this patchset is needed, why can't the application simply read from "cap_last_cap" to determine what capabilities the kernel supports?

Quoted 8 lines

When you capabilities with, for example, docker, you will fill capabilities like this:
docker run --rm --cap-add SYS_ADMIN debian:latest echo foo
As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set.
Sadly, each time a new capability is added to the kernel, it means "container stack" software should add a new string corresponding to the number of the capabilities [1].

Thanks for clarifying things, I thought you were more concerned about detecting what capabilities the running kernel supported, I didn't realize it was getting a string literal for each supported capability. Unless there is a significant show of support for this - and I'm guessing there isn't due to the lack of comments - I don't think this is something we want to add to the kernel, especially since the kernel doesn't really care about the capabilities' names, it's the number that matters.

Signature · 1 line

paul-moore.com

Casey Schaufler

2022-08-17 15:50 UTC #6 lore raw

Quoted 29 lines · Paul Moore

Quoted 27 lines · Francis Laniel
Quoted 14 lines · Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit :
On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel
Quoted 3 lines · <[email protected]> wrote:
Hi.
First, I hope you are fine and the same for your relatives.
Hi Francis :)
Quoted 6 lines
A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.
Quoted 7 lines
...
Quoted 2 lines
The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap
I'm not clear on why this patchset is needed, why can't the application simply read from "cap_last_cap" to determine what capabilities the kernel supports?
When you capabilities with, for example, docker, you will fill capabilities like this:
docker run --rm --cap-add SYS_ADMIN debian:latest echo foo
As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set.

Quoted 7 lines

Quoted 3 lines
Sadly, each time a new capability is added to the kernel, it means "container stack" software should add a new string corresponding to the number of the capabilities [1].
Thanks for clarifying things, I thought you were more concerned about detecting what capabilities the running kernel supported, I didn't realize it was getting a string literal for each supported capability. Unless there is a significant show of support for this

I believe this could be a significant help in encouraging the use of capabilities. An application that has to know the list of capabilities at compile time but is expected to run unmodified for decades isn't going to be satisfied with cap_last_cap. The best it can do with that is abort, not being able to ask an admin what to do in the presence of a capability that wasn't around before because the name isn't known.

On the other hand, it's possible that capabilities are a failure, and that any effort to make them easier to use is pointless.

Quoted 6 lines

- and I'm
guessing there isn't due to the lack of comments - I don't think this is something we want to add to the kernel, especially since the kernel doesn't really care about the capabilities' names, it's the number that matters.

Paul Moore

maintainer

2022-08-17 16:10 UTC #7 lore raw

Quoted 38 lines · Casey Schaufler

Quoted 29 lines · Paul Moore
Quoted 27 lines · Francis Laniel
Quoted 14 lines · Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit :
On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel
Quoted 3 lines · <[email protected]> wrote:
Hi.
First, I hope you are fine and the same for your relatives.
Hi Francis :)
Quoted 6 lines
A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.
Quoted 7 lines
...
Quoted 2 lines
The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap
I'm not clear on why this patchset is needed, why can't the application simply read from "cap_last_cap" to determine what capabilities the kernel supports?
When you capabilities with, for example, docker, you will fill capabilities like this:
docker run --rm --cap-add SYS_ADMIN debian:latest echo foo
As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set.
Quoted 7 lines
Quoted 3 lines
Sadly, each time a new capability is added to the kernel, it means "container stack" software should add a new string corresponding to the number of the capabilities [1].
Thanks for clarifying things, I thought you were more concerned about detecting what capabilities the running kernel supported, I didn't realize it was getting a string literal for each supported capability. Unless there is a significant show of support for this

Quoted 6 lines

I believe this could be a significant help in encouraging the use of capabilities. An application that has to know the list of capabilities at compile time but is expected to run unmodified for decades isn't going to be satisfied with cap_last_cap. The best it can do with that is abort, not being able to ask an admin what to do in the presence of a capability that wasn't around before because the name isn't known.

An application isn't going to be able to deduce the semantic value of a capability based solely on a string value, an integer is just as meaningful in that regard. What might be useful is if the application simply accepts a set of capabilities from the user and then checks those against the maximum supported by the kernel, but once again that doesn't require a string value, it just requires the application taking a set of integers and passing those into the kernel when a capability set is required. I still don't see how adding the capability string names to the kernel is useful here.

Signature · 1 line

paul-moore.com

Serge E. Hallyn

maintainer

2022-08-17 16:19 UTC #8 lore raw

Quoted 46 lines · Paul Moore

Quoted 38 lines · Casey Schaufler
Quoted 29 lines · Paul Moore
Quoted 27 lines · Francis Laniel
Quoted 14 lines · Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit :
On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel
Quoted 3 lines · <[email protected]> wrote:
Hi.
First, I hope you are fine and the same for your relatives.
Hi Francis :)
Quoted 6 lines
A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.
Quoted 7 lines
...
Quoted 2 lines
The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap
I'm not clear on why this patchset is needed, why can't the application simply read from "cap_last_cap" to determine what capabilities the kernel supports?
When you capabilities with, for example, docker, you will fill capabilities like this:
docker run --rm --cap-add SYS_ADMIN debian:latest echo foo
As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set.
Quoted 7 lines
Quoted 3 lines
Sadly, each time a new capability is added to the kernel, it means "container stack" software should add a new string corresponding to the number of the capabilities [1].
Thanks for clarifying things, I thought you were more concerned about detecting what capabilities the running kernel supported, I didn't realize it was getting a string literal for each supported capability. Unless there is a significant show of support for this
Quoted 6 lines
I believe this could be a significant help in encouraging the use of capabilities. An application that has to know the list of capabilities at compile time but is expected to run unmodified for decades isn't going to be satisfied with cap_last_cap. The best it can do with that is abort, not being able to ask an admin what to do in the presence of a capability that wasn't around before because the name isn't known.

Quoted 3 lines

An application isn't going to be able to deduce the semantic value of a capability based solely on a string value, an integer is just as meaningful in that regard. What might be useful is if the application

Maybe it's important to point out that an integer value capability in kernel will NEVER change its string value (or semantic meaning).

The libcap tools like capsh accept integer capabilities, other tools probably should as well. (see man 3 cap_from_text)

Quoted 9 lines

simply accepts a set of capabilities from the user and then checks those against the maximum supported by the kernel, but once again that doesn't require a string value, it just requires the application taking a set of integers and passing those into the kernel when a capability set is required. I still don't see how adding the capability string names to the kernel is useful here.
Signature · 1 line
paul-moore.com

Paul Moore

maintainer

2022-08-17 16:23 UTC #9 lore raw

Quoted 51 lines · Serge E. Hallyn

Quoted 46 lines · Paul Moore
Quoted 38 lines · Casey Schaufler
Quoted 29 lines · Paul Moore
Quoted 27 lines · Francis Laniel
Quoted 14 lines · Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit :
On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel
Quoted 3 lines · <[email protected]> wrote:
Hi.
First, I hope you are fine and the same for your relatives.
Hi Francis :)
Quoted 6 lines
A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.
Quoted 7 lines
...
Quoted 2 lines
The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap
I'm not clear on why this patchset is needed, why can't the application simply read from "cap_last_cap" to determine what capabilities the kernel supports?
When you capabilities with, for example, docker, you will fill capabilities like this:
docker run --rm --cap-add SYS_ADMIN debian:latest echo foo
As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set.
Quoted 7 lines
Quoted 3 lines
Sadly, each time a new capability is added to the kernel, it means "container stack" software should add a new string corresponding to the number of the capabilities [1].
Thanks for clarifying things, I thought you were more concerned about detecting what capabilities the running kernel supported, I didn't realize it was getting a string literal for each supported capability. Unless there is a significant show of support for this
Quoted 6 lines
I believe this could be a significant help in encouraging the use of capabilities. An application that has to know the list of capabilities at compile time but is expected to run unmodified for decades isn't going to be satisfied with cap_last_cap. The best it can do with that is abort, not being able to ask an admin what to do in the presence of a capability that wasn't around before because the name isn't known.
Quoted 3 lines
An application isn't going to be able to deduce the semantic value of a capability based solely on a string value, an integer is just as meaningful in that regard. What might be useful is if the application

Quoted 5 lines

Maybe it's important to point out that an integer value capability in kernel will NEVER change its string value (or semantic meaning).
The libcap tools like capsh accept integer capabilities, other tools probably should as well. (see man 3 cap_from_text)

Seems like a reasonable thing to me, I would much prefer that than the approach in this patchset.

Signature · 1 line

paul-moore.com

Casey Schaufler

2022-08-17 16:50 UTC #10 lore raw

In reply to Paul Moore

Quoted 38 lines · Paul Moore

Quoted 37 lines · Casey Schaufler
Quoted 36 lines · Paul Moore
Quoted 22 lines · Francis Laniel
Quoted 14 lines · Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit :
On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel
Quoted 3 lines · <[email protected]> wrote:
Hi.
First, I hope you are fine and the same for your relatives.
Hi Francis :)
Quoted 6 lines
A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.
Quoted 7 lines
...
Quoted 2 lines
The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap
I'm not clear on why this patchset is needed, why can't the application simply read from "cap_last_cap" to determine what capabilities the kernel supports?
Quoted 8 lines
When you capabilities with, for example, docker, you will fill capabilities like this:
docker run --rm --cap-add SYS_ADMIN debian:latest echo foo
As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set.
Sadly, each time a new capability is added to the kernel, it means "container stack" software should add a new string corresponding to the number of the capabilities [1].
Thanks for clarifying things, I thought you were more concerned about detecting what capabilities the running kernel supported, I didn't realize it was getting a string literal for each supported capability. Unless there is a significant show of support for this

Quoted 8 lines

Quoted 6 lines
I believe this could be a significant help in encouraging the use of capabilities. An application that has to know the list of capabilities at compile time but is expected to run unmodified for decades isn't going to be satisfied with cap_last_cap. The best it can do with that is abort, not being able to ask an admin what to do in the presence of a capability that wasn't around before because the name isn't known.
An application isn't going to be able to deduce the semantic value of a capability based solely on a string value,

True, but it can ask someone what to do, and in that case a string is much better than a number:

thwonkd: Unknown capability 42 - update thwonkd.conf policy section
thwonkd: Unknown capability butter_toast - update thwonkd.conf policy section

The thwonkd configuration could be updated to use that capability correctly. Yes, you could look capability 42 up in the system header files, but only if they're installed and there's no guarantee that the header files match the running kernel. That said, I can't think of a case where this would be useful in real life except for systemd and chcap. I can't speak to the container manager proposed, as I don't see containers being deployed with finer granularity than "privileged" or "unprivileged".

Quoted 9 lines

an integer is just as
meaningful in that regard. What might be useful is if the application simply accepts a set of capabilities from the user and then checks those against the maximum supported by the kernel, but once again that doesn't require a string value, it just requires the application taking a set of integers and passing those into the kernel when a capability set is required. I still don't see how adding the capability string names to the kernel is useful here.

Paul Moore

maintainer

2022-08-17 17:20 UTC #11 lore raw

Quoted 45 lines · Casey Schaufler

Quoted 44 lines · Paul Moore
Quoted 43 lines · Casey Schaufler
Quoted 29 lines · Paul Moore
Quoted 27 lines · Francis Laniel
Quoted 14 lines · Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit :
On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel
Quoted 3 lines · <[email protected]> wrote:
Hi.
First, I hope you are fine and the same for your relatives.
Hi Francis :)
Quoted 6 lines
A solution to this problem could be to add a way for the userspace to ask the kernel about the capabilities it offers.
So, in this series, I added a new file to securityfs:
/sys/kernel/security/capabilities.
The goal of this file is to be used by "container world" software to know kernel capabilities at run time instead of compile time.
Quoted 7 lines
...
Quoted 2 lines
The kernel already exposes the last capability number under:
/proc/sys/kernel/cap_last_cap
I'm not clear on why this patchset is needed, why can't the application simply read from "cap_last_cap" to determine what capabilities the kernel supports?
When you capabilities with, for example, docker, you will fill capabilities like this:
docker run --rm --cap-add SYS_ADMIN debian:latest echo foo
As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set.
Quoted 7 lines
Quoted 3 lines
Sadly, each time a new capability is added to the kernel, it means "container stack" software should add a new string corresponding to the number of the capabilities [1].
Thanks for clarifying things, I thought you were more concerned about detecting what capabilities the running kernel supported, I didn't realize it was getting a string literal for each supported capability. Unless there is a significant show of support for this
I believe this could be a significant help in encouraging the use of capabilities. An application that has to know the list of capabilities at compile time but is expected to run unmodified for decades isn't going to be satisfied with cap_last_cap. The best it can do with that is abort, not being able to ask an admin what to do in the presence of a capability that wasn't around before because the name isn't known.

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An application isn't going to be able to deduce the semantic value of a capability based solely on a string value,
True, but it can ask someone what to do, and in that case a string is much better than a number ...

If you are asking a user what to do, that user can just as easily look up the capability list to translate numbers to intent. If your security approach requires a user knowing all of the subtle details around a capability based on 10~15 character string, I wish you the best of luck :)

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