landlock.7: Document Landlock ABI v2 (file reparenting; kernel 5.19)

PATCH Superseded Günther Noack 2 participants 7 messages Feb 28, 2023 – Mar 6, 2023 Man-pages: manual pages for Linux -- Sections 2, 3, 4, 5, and 7

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Günther Noack

author

2023-02-28 20:52 UTC #0 lore raw

PATCH v2 1/2 landlock.7: Document Landlock ABI v2 (file reparenting; kernel 5.19)

* Add the description for LANDLOCK_ACCESS_FS_REFER,
in line with recent update to the uapi headers:
https://lore.kernel.org/linux-security-module/[email protected]/T/
* VERSIONS: Add a table of Landlock versions and their changes. Briefly talk about how to probe ABI levels and warn users about the special semantics of the LANDLOCK_ACCESS_FS_REFER right.
* Add LANDLOCK_ACCESS_FS_REFER to the code example.

Code review threads for the "refer" feature:
* https://lore.kernel.org/all/[email protected]/ (initial commit)
* https://lore.kernel.org/all/[email protected]/ (bugfix)
* https://lore.kernel.org/all/[email protected]/ (documentation update)

---
 man7/landlock.7 | 102 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 100 insertions(+), 2 deletions(-)

man7/landlock.7 +100 -2

index b2bc9e10b..f70a01484 100644
@@ -105,6 +105,56 @@ Create (or rename or link) a block device.
 .TP
 .B LANDLOCK_ACCESS_FS_MAKE_SYM
 Create (or rename or link) a symbolic link.
+.TP
+.B LANDLOCK_ACCESS_FS_REFER
+Link or rename a file from or to a different directory
+(i.e. reparent a file hierarchy).
+.IP
+This access right is available since the second version of the Landlock ABI.
+.IP
+This is the only access right which is denied by default by any ruleset,
+even if the right is not specified as handled at ruleset creation time.
+The only way to make a ruleset grant this right
+is to explicitly allow it for a specific directory
+by adding a matching rule to the ruleset.
+.IP
+In particular, when using the first Landlock ABI version,
+Landlock will always deny attempts to reparent files
+between different directories.
+.IP
+In addition to the source and destination directories having the
+.B LANDLOCK_ACCESS_FS_REFER
+access right,
+the attempted link or rename operation must meet the following constraints:
+.RS
+.IP \[bu] 3
+The reparented file may not gain more access rights in the destination directory
+than it previously had in the source directory.
+If this is attempted, the operation results in an
+.B EXDEV
+error.
+.IP \[bu] 3
+When linking or renaming, the
+.B LANDLOCK_ACCESS_FS_MAKE_*
+right for the respective file type must be granted
+for the destination directory.
+Otherwise, the operation results in an
+.BR EACCES
+error.
+.IP \[bu] 3
+When renaming, the
+.B LANDLOCK_ACCESS_FS_REMOVE_*
+right for the respective file type must be granted
+for the source directory.
+Otherwise, the operation results in an
+.B EACCES
+error.
+.RE
+.IP
+If multiple requirements are not met, the
+.B EACCES
+error code takes precedence over
+.BR EXDEV .
 .\"
 .SS Layers of file path access rights
 Each time a thread enforces a ruleset on itself,

@@ -182,7 +232,54 @@ and related syscalls on a target process,
 a sandboxed process should have a subset of the target process rules,
 which means the tracee must be in a sub-domain of the tracer.
 .SH VERSIONS
-Landlock was added in Linux 5.13.
+Landlock was introduced in Linux 5.13.
+.PP
+To determine which Landlock features are available,
+users should query the Landlock ABI version:
+.TS
+box;
+ntb| ntb| lbx
+nt| nt| lbx.
+ABI	Kernel	Newly introduced access rights
+_	_	_
+1	5.13	LANDLOCK_ACCESS_FS_EXECUTE
+\^	\^	LANDLOCK_ACCESS_FS_WRITE_FILE
+\^	\^	LANDLOCK_ACCESS_FS_READ_FILE
+\^	\^	LANDLOCK_ACCESS_FS_READ_DIR
+\^	\^	LANDLOCK_ACCESS_FS_REMOVE_DIR
+\^	\^	LANDLOCK_ACCESS_FS_REMOVE_FILE
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_CHAR
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_DIR
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_REG
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_SOCK
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_FIFO
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_BLOCK
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_SYM
+_	_	_
+2	5.19	LANDLOCK_ACCESS_FS_REFER
+.TE
+.sp 1
+.PP
+Users should use the Landlock ABI version rather than the kernel version
+to determine which features are available.
+The mainline kernel versions listed here are only included for orientation.
+Kernels from other sources may contain backported features,
+and their version numbers may not match.
+.PP
+To query the running kernel's Landlock ABI version,
+programs may pass the
+.B LANDLOCK_CREATE_RULESET_VERSION
+flag to
+.BR landlock_create_ruleset (2).
+.PP
+When building fallback mechanisms for compatibility with older kernels,
+users are advised to consider the special semantics of the
+.B LANDLOCK_ACCESS_FS_REFER
+access right:
+In ABI v1,
+linking and moving of files between different directories is always forbidden,
+so programs relying on such operations are only compatible
+with Landlock ABI v2 and higher.
 .SH NOTES
 Landlock is enabled by
 .BR CONFIG_SECURITY_LANDLOCK .

@@ -242,7 +339,8 @@ attr.handled_access_fs =
         LANDLOCK_ACCESS_FS_MAKE_SOCK |
         LANDLOCK_ACCESS_FS_MAKE_FIFO |
         LANDLOCK_ACCESS_FS_MAKE_BLOCK |
-        LANDLOCK_ACCESS_FS_MAKE_SYM;
+        LANDLOCK_ACCESS_FS_MAKE_SYM |
+        LANDLOCK_ACCESS_FS_REFER;
 
 ruleset_fd = landlock_create_ruleset(&attr, sizeof(attr), 0);
 if (ruleset_fd == -1) {

base-commit: 53a7e5dfc3554a2e8dbdfdc4504e99652e1d6382

Signature · 1 line

2.39.2

Günther Noack

author

2023-02-28 20:52 UTC #1 lore raw

PATCH v2 2/2 landlock.7: Document Landlock ABI v3 (file truncation; kernel 6.2)

https://git.kernel.org/torvalds/c/299e2b1967578b1442128ba8b3e86ed3427d3651

---
 man7/landlock.7 | 82 +++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 80 insertions(+), 2 deletions(-)

man7/landlock.7 +80 -2

index f70a01484..9ddb17ae8 100644
@@ -64,9 +64,38 @@ Execute a file.
 .TP
 .B LANDLOCK_ACCESS_FS_WRITE_FILE
 Open a file with write access.
+Note that you might additionally need the
+.B LANDLOCK_ACCESS_FS_TRUNCATE
+right in order to overwrite files with
+.BR open (2)
+using
+.B O_TRUNC
+or
+.BR creat (2).
 .TP
 .B LANDLOCK_ACCESS_FS_READ_FILE
 Open a file with read access.
+.TP
+.B LANDLOCK_ACCESS_FS_TRUNCATE
+Truncate a file with
+.BR truncate (2),
+.BR ftruncate (2),
+.BR creat (2),
+or
+.BR open (2)
+with
+.BR O_TRUNC .
+Whether an opened file can be truncated with
+.BR ftruncate (2)
+is determined during
+.BR open (2),
+in the same way as read and write permissions are checked during
+.BR open (2)
+using
+.B LANDLOCK_ACCESS_FS_READ_FILE
+and
+.BR LANDLOCK_ACCESS_FS_WRITE_FILE .
+This access right is available since the third version of the Landlock ABI.
 .PP
 A directory can receive access rights related to files or directories.
 The following access right is applied to the directory itself,

@@ -231,6 +260,53 @@ To be allowed to use
 and related syscalls on a target process,
 a sandboxed process should have a subset of the target process rules,
 which means the tracee must be in a sub-domain of the tracer.
+.\"
+.SS Truncating files
+The operations covered by
+.B LANDLOCK_ACCESS_FS_WRITE_FILE
+and
+.B LANDLOCK_ACCESS_FS_TRUNCATE
+both change the contents of a file and sometimes overlap in
+non-intuitive ways.
+It is recommended to always specify both of these together.
+.PP
+A particularly surprising example is
+.BR creat (2).
+The name suggests that this system call requires
+the rights to create and write files.
+However, it also requires the truncate right
+if an existing file under the same name is already present.
+.PP
+It should also be noted that truncating files does not require the
+.B LANDLOCK_ACCESS_FS_WRITE_FILE
+right.
+Apart from the
+.BR truncate (2)
+system call, this can also be done through
+.BR open (2)
+with the flags
+.BR "O_RDONLY | O_TRUNC" .
+.PP
+When opening a file, the availability of the
+.B LANDLOCK_ACCESS_FS_TRUNCATE
+right is associated with the newly created file descriptor
+and will be used for subsequent truncation attempts using
+.BR ftruncate (2).
+The behavior is similar to opening a file for reading or writing,
+where permissions are checked during
+.BR open (2),
+but not during the subsequent
+.BR read (2)
+and
+.BR write (2)
+calls.
+.PP
+As a consequence,
+it is possible to have multiple open file descriptors for the same file,
+where one grants the right to truncate the file and the other does not.
+It is also possible to pass such file descriptors between processes,
+keeping their Landlock properties,
+even when these processes do not have an enforced Landlock ruleset.
 .SH VERSIONS
 Landlock was introduced in Linux 5.13.
 .PP

@@ -257,6 +333,8 @@ _	_	_
 \^	\^	LANDLOCK_ACCESS_FS_MAKE_SYM
 _	_	_
 2	5.19	LANDLOCK_ACCESS_FS_REFER
+_	_	_
+3	6.2	LANDLOCK_ACCESS_FS_TRUNCATE
 .TE
 .sp 1
 .PP

@@ -302,7 +380,6 @@ in kernel logs.
 It is currently not possible to restrict some file-related actions
 accessible through these system call families:
 .BR chdir (2),
-.BR truncate (2),
 .BR stat (2),
 .BR flock (2),
 .BR chmod (2),

@@ -340,7 +417,8 @@ attr.handled_access_fs =
         LANDLOCK_ACCESS_FS_MAKE_FIFO |
         LANDLOCK_ACCESS_FS_MAKE_BLOCK |
         LANDLOCK_ACCESS_FS_MAKE_SYM |
-        LANDLOCK_ACCESS_FS_REFER;
+        LANDLOCK_ACCESS_FS_REFER |
+        LANDLOCK_ACCESS_FS_TRUNCATE;
 
 ruleset_fd = landlock_create_ruleset(&attr, sizeof(attr), 0);
 if (ruleset_fd == -1) {

Signature · 1 line

2.39.2

Alejandro Colomar

2023-03-01 21:21 UTC #2 lore raw

Re: [PATCH v2 2/2] landlock.7: Document Landlock ABI v3 (file truncation; kernel 6.2)

In the subject it's not so important, but for consistency with the language used within the pages, I'd ask you to rewrite it as Linux 6.2 (and similarly for patch 1/2).

Quoted 10 lines · Günther Noack

https://git.kernel.org/torvalds/c/299e2b1967578b1442128ba8b3e86ed3427d3651
---
 man7/landlock.7 | 82 +++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 80 insertions(+), 2 deletions(-)
man7/landlock.7 +0 -0
index f70a01484..9ddb17ae8 100644

man7/landlock.7 Execute a file. · line 67

Quoted 4 lines

man7/landlock.7 +1 -0

@@ -64,9 +64,38 @@ Execute a file.
 .TP
 .B LANDLOCK_ACCESS_FS_WRITE_FILE
 Open a file with write access.
+Note that you might additionally need the

"Note that" is usually redundant.
See:
<https://lore.kernel.org/linux-man/[email protected]/>
<https://lore.kernel.org/linux-man/20230105225235.6cjtz6orjzxzvo6v@illithid/>

Quoted 54 lines

man7/landlock.7 +44 -0

+.B LANDLOCK_ACCESS_FS_TRUNCATE
+right in order to overwrite files with
+.BR open (2)
+using
+.B O_TRUNC
+or
+.BR creat (2).
 .TP
 .B LANDLOCK_ACCESS_FS_READ_FILE
 Open a file with read access.
+.TP
+.B LANDLOCK_ACCESS_FS_TRUNCATE
+Truncate a file with
+.BR truncate (2),
+.BR ftruncate (2),
+.BR creat (2),
+or
+.BR open (2)
+with
+.BR O_TRUNC .
+Whether an opened file can be truncated with
+.BR ftruncate (2)
+is determined during
+.BR open (2),
+in the same way as read and write permissions are checked during
+.BR open (2)
+using
+.B LANDLOCK_ACCESS_FS_READ_FILE
+and
+.BR LANDLOCK_ACCESS_FS_WRITE_FILE .
+This access right is available since the third version of the Landlock ABI.
 .PP
 A directory can receive access rights related to files or directories.
 The following access right is applied to the directory itself,

@@ -231,6 +260,53 @@ To be allowed to use
 and related syscalls on a target process,
 a sandboxed process should have a subset of the target process rules,
 which means the tracee must be in a sub-domain of the tracer.
+.\"
+.SS Truncating files
+The operations covered by
+.B LANDLOCK_ACCESS_FS_WRITE_FILE
+and
+.B LANDLOCK_ACCESS_FS_TRUNCATE
+both change the contents of a file and sometimes overlap in
+non-intuitive ways.
+It is recommended to always specify both of these together.
+.PP
+A particularly surprising example is
+.BR creat (2).
+The name suggests that this system call requires
+the rights to create and write files.
+However, it also requires the truncate right
+if an existing file under the same name is already present.

man7/landlock.7 To be allowed to use · line 288

Quoted 10 lines

man7/landlock.7 +10 -0

+.PP
+It should also be noted that truncating files does not require the
+.B LANDLOCK_ACCESS_FS_WRITE_FILE
+right.
+Apart from the
+.BR truncate (2)
+system call, this can also be done through
+.BR open (2)
+with the flags
+.BR "O_RDONLY | O_TRUNC" .

Expressions should go in italics. See man-pages(7):

Expressions, if not written on a separate indented line, should be specified in italics. Again, the use of nonbreaking spaces may be appropriate if the expression is inlined with normal text.

Cheers,

Alex

Quoted 51 lines

man7/landlock.7 +25 -2

+.PP
+When opening a file, the availability of the
+.B LANDLOCK_ACCESS_FS_TRUNCATE
+right is associated with the newly created file descriptor
+and will be used for subsequent truncation attempts using
+.BR ftruncate (2).
+The behavior is similar to opening a file for reading or writing,
+where permissions are checked during
+.BR open (2),
+but not during the subsequent
+.BR read (2)
+and
+.BR write (2)
+calls.
+.PP
+As a consequence,
+it is possible to have multiple open file descriptors for the same file,
+where one grants the right to truncate the file and the other does not.
+It is also possible to pass such file descriptors between processes,
+keeping their Landlock properties,
+even when these processes do not have an enforced Landlock ruleset.
 .SH VERSIONS
 Landlock was introduced in Linux 5.13.
 .PP

@@ -257,6 +333,8 @@ _	_	_
 \^	\^	LANDLOCK_ACCESS_FS_MAKE_SYM
 _	_	_
 2	5.19	LANDLOCK_ACCESS_FS_REFER
+_	_	_
+3	6.2	LANDLOCK_ACCESS_FS_TRUNCATE
 .TE
 .sp 1
 .PP

@@ -302,7 +380,6 @@ in kernel logs.
 It is currently not possible to restrict some file-related actions
 accessible through these system call families:
 .BR chdir (2),
-.BR truncate (2),
 .BR stat (2),
 .BR flock (2),
 .BR chmod (2),

@@ -340,7 +417,8 @@ attr.handled_access_fs =
         LANDLOCK_ACCESS_FS_MAKE_FIFO |
         LANDLOCK_ACCESS_FS_MAKE_BLOCK |
         LANDLOCK_ACCESS_FS_MAKE_SYM |
-        LANDLOCK_ACCESS_FS_REFER;
+        LANDLOCK_ACCESS_FS_REFER |
+        LANDLOCK_ACCESS_FS_TRUNCATE;
 
 ruleset_fd = landlock_create_ruleset(&attr, sizeof(attr), 0);
 if (ruleset_fd == -1) {

Signature · 2 lines

<http://www.alejandro-colomar.es/>
GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5

Günther Noack

author

2023-03-02 13:42 UTC #3 lore raw

Re: [PATCH v2 2/2] landlock.7: Document Landlock ABI v3 (file truncation; kernel 6.2)

Hello Alex!

Quoted 3 lines · Alejandro Colomar

In the subject it's not so important, but for consistency with the language used within the pages, I'd ask you to rewrite it as Linux 6.2 (and similarly for patch 1/2).

Done, sure thing.

Quoted 11 lines · Günther Noack

man7/landlock.7 Execute a file. · line 67
Quoted 5 lines
man7/landlock.7 +1 -0
index f70a01484..9ddb17ae8 100644
+Note that you might additionally need the
"Note that" is usually redundant.
See:
<https://lore.kernel.org/linux-man/[email protected]/>
<https://lore.kernel.org/linux-man/20230105225235.6cjtz6orjzxzvo6v@illithid/>

Thank you, that is a good observation, the "Kemper notectomy". This is very helpful, exactly the kind of review I was hoping for. :)

How about this rewording:

LANDLOCK_ACCESS_FS_WRITE

Open a file with write access.

When opening files for writing, you will often additionally need the LANDLOCK_ACCESS_FS_TRUNCATE right. In many cases, these system calls truncate existing files when overwriting them (e.g. creat(2)).

This paragraph started with "note that" because it talks about an unintuitive side-aspect of the LANDLOCK_ACCESS_FS_WRITE_FILE right.

I reworded it to make it more clear why that should be relevant to the reader (because creat(2) and its open(2) equivalent are used very commonly).

Does that sound better?

The man page also explains it in more detail in the "Truncating files" section further below.

Quoted 11 lines

man7/landlock.7 To be allowed to use · line 288
Quoted 4 lines
man7/landlock.7 +4 -0
+system call, this can also be done through
+.BR open (2)
+with the flags
+.BR "O_RDONLY | O_TRUNC" .
Expressions should go in italics. See man-pages(7):
Expressions, if not written on a separate indented line, should be specified in italics. Again, the use of nonbreaking spaces may be appropriate if the expression is inlined with normal text.

Done.

Thank you for the review!
–Günther

Alejandro Colomar

2023-03-06 12:33 UTC #4 lore raw

Re: [PATCH v2 2/2] landlock.7: Document Landlock ABI v3 (file truncation; kernel 6.2)

Quoted 1 line · Günther Noack

Hello Alex!

Hello Günther!

Quoted 21 lines · Alejandro Colomar

Quoted 3 lines
In the subject it's not so important, but for consistency with the language used within the pages, I'd ask you to rewrite it as Linux 6.2 (and similarly for patch 1/2).
Done, sure thing.
Quoted 11 lines · Günther Noack
man7/landlock.7 Execute a file. · line 67
Quoted 5 lines
man7/landlock.7 +1 -0
index f70a01484..9ddb17ae8 100644
+Note that you might additionally need the
"Note that" is usually redundant.
See:
<https://lore.kernel.org/linux-man/[email protected]/>
<https://lore.kernel.org/linux-man/20230105225235.6cjtz6orjzxzvo6v@illithid/>

Quoted 2 lines

Thank you, that is a good observation, the "Kemper notectomy". This is very helpful, exactly the kind of review I was hoping for. :)

Quoted 11 lines

How about this rewording:
LANDLOCK_ACCESS_FS_WRITE
Open a file with write access.
When opening files for writing, you will often additionally need the LANDLOCK_ACCESS_FS_TRUNCATE right. In many cases, these system calls truncate existing files when overwriting them (e.g. creat(2)).

Sounds good.

Quoted 26 lines

This paragraph started with "note that" because it talks about an unintuitive side-aspect of the LANDLOCK_ACCESS_FS_WRITE_FILE right.
I reworded it to make it more clear why that should be relevant to the reader (because creat(2) and its open(2) equivalent are used very commonly).
Does that sound better?
The man page also explains it in more detail in the "Truncating files" section further below.
Quoted 11 lines
man7/landlock.7 To be allowed to use · line 288
Quoted 4 lines
man7/landlock.7 +4 -0
+system call, this can also be done through
+.BR open (2)
+with the flags
+.BR "O_RDONLY | O_TRUNC" .
Expressions should go in italics. See man-pages(7):
Expressions, if not written on a separate indented line, should be specified in italics. Again, the use of nonbreaking spaces may be appropriate if the expression is inlined with normal text.

Quoted 4 lines

Done.
Thank you for the review!
–Günther

Cheers,

Alex

Signature · 2 lines

<http://www.alejandro-colomar.es/>
GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5

Alejandro Colomar

2023-03-01 21:25 UTC #5 lore raw

In reply to Günther Noack

Hi Günther,

Quoted 43 lines · Günther Noack

* Add the description for LANDLOCK_ACCESS_FS_REFER,
in line with recent update to the uapi headers:
https://lore.kernel.org/linux-security-module/[email protected]/T/
* VERSIONS: Add a table of Landlock versions and their changes. Briefly talk about how to probe ABI levels and warn users about the special semantics of the LANDLOCK_ACCESS_FS_REFER right.
* Add LANDLOCK_ACCESS_FS_REFER to the code example.
Code review threads for the "refer" feature:
* https://lore.kernel.org/all/[email protected]/ (initial commit)
* https://lore.kernel.org/all/[email protected]/ (bugfix)
* https://lore.kernel.org/all/[email protected]/ (documentation update)
---
 man7/landlock.7 | 102 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 100 insertions(+), 2 deletions(-)
man7/landlock.7 +19 -0
index b2bc9e10b..f70a01484 100644
@@ -105,6 +105,56 @@ Create (or rename or link) a block device.
 .TP
 .B LANDLOCK_ACCESS_FS_MAKE_SYM
 Create (or rename or link) a symbolic link.
+.TP
+.B LANDLOCK_ACCESS_FS_REFER
+Link or rename a file from or to a different directory
+(i.e. reparent a file hierarchy).
+.IP
+This access right is available since the second version of the Landlock ABI.
+.IP
+This is the only access right which is denied by default by any ruleset,
+even if the right is not specified as handled at ruleset creation time.
+The only way to make a ruleset grant this right
+is to explicitly allow it for a specific directory
+by adding a matching rule to the ruleset.
+.IP
+In particular, when using the first Landlock ABI version,
+Landlock will always deny attempts to reparent files
+between different directories.
+.IP
+In addition to the source and destination directories having the
+.B LANDLOCK_ACCESS_FS_REFER

man7/landlock.7 Create (or rename or link) a block device. · line 136

Quoted 10 lines

man7/landlock.7 +10 -0

+access right,
+the attempted link or rename operation must meet the following constraints:
+.RS
+.IP \[bu] 3
+The reparented file may not gain more access rights in the destination directory
+than it previously had in the source directory.
+If this is attempted, the operation results in an
+.B EXDEV
+error.
+.IP \[bu] 3

You only need to specify the indentation (the "3") in the first consecutive .IP. All others reuse the indentation level until a .PP appears (or a few other situations that I won't enumerate for brevity).

man7/landlock.7 Create (or rename or link) a block device. · line 146

Quoted 10 lines

man7/landlock.7 +10 -0

+When linking or renaming, the
+.B LANDLOCK_ACCESS_FS_MAKE_*
+right for the respective file type must be granted
+for the destination directory.
+Otherwise, the operation results in an
+.BR EACCES
+error.
+.IP \[bu] 3
+When renaming, the
+.B LANDLOCK_ACCESS_FS_REMOVE_*

The * should be in italics, since it's not part of the literal, but rather a variable part. I know the pages are not very consistent in this, but I'd like to make them consistent in the future.

Cheers,

Alex

Quoted 82 lines

man7/landlock.7 +61 -2

+right for the respective file type must be granted
+for the source directory.
+Otherwise, the operation results in an
+.B EACCES
+error.
+.RE
+.IP
+If multiple requirements are not met, the
+.B EACCES
+error code takes precedence over
+.BR EXDEV .
 .\"
 .SS Layers of file path access rights
 Each time a thread enforces a ruleset on itself,

@@ -182,7 +232,54 @@ and related syscalls on a target process,
 a sandboxed process should have a subset of the target process rules,
 which means the tracee must be in a sub-domain of the tracer.
 .SH VERSIONS
-Landlock was added in Linux 5.13.
+Landlock was introduced in Linux 5.13.
+.PP
+To determine which Landlock features are available,
+users should query the Landlock ABI version:
+.TS
+box;
+ntb| ntb| lbx
+nt| nt| lbx.
+ABI	Kernel	Newly introduced access rights
+_	_	_
+1	5.13	LANDLOCK_ACCESS_FS_EXECUTE
+\^	\^	LANDLOCK_ACCESS_FS_WRITE_FILE
+\^	\^	LANDLOCK_ACCESS_FS_READ_FILE
+\^	\^	LANDLOCK_ACCESS_FS_READ_DIR
+\^	\^	LANDLOCK_ACCESS_FS_REMOVE_DIR
+\^	\^	LANDLOCK_ACCESS_FS_REMOVE_FILE
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_CHAR
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_DIR
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_REG
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_SOCK
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_FIFO
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_BLOCK
+\^	\^	LANDLOCK_ACCESS_FS_MAKE_SYM
+_	_	_
+2	5.19	LANDLOCK_ACCESS_FS_REFER
+.TE
+.sp 1
+.PP
+Users should use the Landlock ABI version rather than the kernel version
+to determine which features are available.
+The mainline kernel versions listed here are only included for orientation.
+Kernels from other sources may contain backported features,
+and their version numbers may not match.
+.PP
+To query the running kernel's Landlock ABI version,
+programs may pass the
+.B LANDLOCK_CREATE_RULESET_VERSION
+flag to
+.BR landlock_create_ruleset (2).
+.PP
+When building fallback mechanisms for compatibility with older kernels,
+users are advised to consider the special semantics of the
+.B LANDLOCK_ACCESS_FS_REFER
+access right:
+In ABI v1,
+linking and moving of files between different directories is always forbidden,
+so programs relying on such operations are only compatible
+with Landlock ABI v2 and higher.
 .SH NOTES
 Landlock is enabled by
 .BR CONFIG_SECURITY_LANDLOCK .

@@ -242,7 +339,8 @@ attr.handled_access_fs =
         LANDLOCK_ACCESS_FS_MAKE_SOCK |
         LANDLOCK_ACCESS_FS_MAKE_FIFO |
         LANDLOCK_ACCESS_FS_MAKE_BLOCK |
-        LANDLOCK_ACCESS_FS_MAKE_SYM;
+        LANDLOCK_ACCESS_FS_MAKE_SYM |
+        LANDLOCK_ACCESS_FS_REFER;
 
 ruleset_fd = landlock_create_ruleset(&attr, sizeof(attr), 0);
 if (ruleset_fd == -1) {

base-commit: 53a7e5dfc3554a2e8dbdfdc4504e99652e1d6382

Signature · 2 lines

<http://www.alejandro-colomar.es/>
GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5

Günther Noack

author

2023-03-02 13:52 UTC #6 lore raw

Hello Alex!

Quoted 7 lines · Alejandro Colomar

Quoted 1 line · Günther Noack
+.IP \[bu] 3
You only need to specify the indentation (the "3") in the first consecutive .IP. All others reuse the indentation level until a .PP appears (or a few other situations that I won't enumerate for brevity).

Thanks, I missed that.

Quoted 7 lines

Quoted 2 lines
+When renaming, the
+.B LANDLOCK_ACCESS_FS_REMOVE_*
The * should be in italics, since it's not part of the literal, but rather a variable part. I know the pages are not very consistent in this, but I'd like to make them consistent in the future.

Done.

Thank you for the review!
–Günther