sysctl: Make sure proc handlers can't expose heap memory

PATCH In review Kees Cook 4 participants 9 messages May 4, 2020 – May 5, 2020

Subsystems (4)

Memory management VFS Proc filesystem Proc sysctl

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Kees Cook

author

2020-05-04 19:08 UTC #0 lore raw

PATCH 1/1 sysctl: Make sure proc handlers can't expose heap memory

Just as a precaution, make sure that proc handlers don't accidentally grow "count" beyond the allocated kbuf size.

Signed-off-by Kees Cook <[email protected]>

---
This applies to hch's sysctl cleanup tree...

---
 fs/proc/proc_sysctl.c | 3 +++
 1 file changed, 3 insertions(+)

fs/proc/proc_sysctl.c +3 -0

index 15030784566c..535ab26473af 100644
@@ -546,6 +546,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 	struct inode *inode = file_inode(filp);
 	struct ctl_table_header *head = grab_header(inode);
 	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+	size_t count_max = count;
 	void *kbuf;
 	ssize_t error;

@@ -590,6 +591,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 
 	if (!write) {
 		error = -EFAULT;
+		if (WARN_ON(count > count_max))
+			count = count_max;
 		if (copy_to_user(ubuf, kbuf, count))
 			goto out_free_buf;
 	}

Signature · 5 lines

2.20.1


-- 
Kees Cook

Luis Chamberlain

2020-05-04 19:59 UTC #1 lore raw

Quoted 23 lines · Kees Cook

Just as a precaution, make sure that proc handlers don't accidentally grow "count" beyond the allocated kbuf size.
Signed-off-by Kees Cook <[email protected]>
---
This applies to hch's sysctl cleanup tree...
---
 fs/proc/proc_sysctl.c | 3 +++
 1 file changed, 3 insertions(+)
fs/proc/proc_sysctl.c +1 -0
index 15030784566c..535ab26473af 100644
@@ -546,6 +546,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 	struct inode *inode = file_inode(filp);
 	struct ctl_table_header *head = grab_header(inode);
 	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+	size_t count_max = count;
 	void *kbuf;
 	ssize_t error;
 

fs/proc/proc_sysctl.c static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf, · line 595

Quoted 5 lines

fs/proc/proc_sysctl.c +2 -0

@@ -590,6 +591,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 
 	if (!write) {
 		error = -EFAULT;
+		if (WARN_ON(count > count_max))
+			count = count_max;

That crash a system with panic-on-warn. I don't think we want that?

Luis

Kees Cook

author

2020-05-04 20:32 UTC #2 lore raw

Quoted 23 lines · Luis Chamberlain

Quoted 22 lines · Kees Cook
Just as a precaution, make sure that proc handlers don't accidentally grow "count" beyond the allocated kbuf size.
Signed-off-by Kees Cook <[email protected]>
---
This applies to hch's sysctl cleanup tree...
---
 fs/proc/proc_sysctl.c | 3 +++
 1 file changed, 3 insertions(+)
fs/proc/proc_sysctl.c +1 -0
index 15030784566c..535ab26473af 100644
@@ -546,6 +546,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 	struct inode *inode = file_inode(filp);
 	struct ctl_table_header *head = grab_header(inode);
 	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+	size_t count_max = count;
 	void *kbuf;
 	ssize_t error;
 

Quoted 8 lines

fs/proc/proc_sysctl.c static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf, · line 595
Quoted 6 lines
fs/proc/proc_sysctl.c +2 -0
@@ -590,6 +591,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 
 	if (!write) {
 		error = -EFAULT;
+		if (WARN_ON(count > count_max))
+			count = count_max;
That crash a system with panic-on-warn. I don't think we want that?

Eh? None of the handlers should be making this mistake currently and it's not a mistake that can be controlled from userspace. WARN() is absolutely what's wanted here: report an impossible situation (and handle it gracefully for the bulk of users that don't have panic_on_warn set).

Signature · 1 line

Kees Cook

Luis Chamberlain

2020-05-04 21:59 UTC #3 lore raw

Quoted 33 lines · Kees Cook

Quoted 23 lines · Luis Chamberlain
Quoted 22 lines · Kees Cook
Just as a precaution, make sure that proc handlers don't accidentally grow "count" beyond the allocated kbuf size.
Signed-off-by Kees Cook <[email protected]>
---
This applies to hch's sysctl cleanup tree...
---
 fs/proc/proc_sysctl.c | 3 +++
 1 file changed, 3 insertions(+)
fs/proc/proc_sysctl.c +1 -0
index 15030784566c..535ab26473af 100644
@@ -546,6 +546,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 	struct inode *inode = file_inode(filp);
 	struct ctl_table_header *head = grab_header(inode);
 	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+	size_t count_max = count;
 	void *kbuf;
 	ssize_t error;
 
Quoted 8 lines
fs/proc/proc_sysctl.c static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf, · line 595
Quoted 6 lines
fs/proc/proc_sysctl.c +2 -0
@@ -590,6 +591,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 
 	if (!write) {
 		error = -EFAULT;
+		if (WARN_ON(count > count_max))
+			count = count_max;
That would crash a system with panic-on-warn. I don't think we want that?

Quoted 5 lines

Eh? None of the handlers should be making this mistake currently and it's not a mistake that can be controlled from userspace. WARN() is absolutely what's wanted here: report an impossible situation (and handle it gracefully for the bulk of users that don't have panic_on_warn set).

Alrighty, Greg are you OK with this type of WARN_ON()? You recently expressed concerns over its use due to panic-on-warn on another patch.

LUis

Greg KH

2020-05-05 06:34 UTC #4 lore raw

Quoted 34 lines · Luis Chamberlain

Quoted 33 lines · Kees Cook
Quoted 31 lines · Luis Chamberlain
Quoted 23 lines · Kees Cook
Just as a precaution, make sure that proc handlers don't accidentally grow "count" beyond the allocated kbuf size.
Signed-off-by Kees Cook <[email protected]>
---
This applies to hch's sysctl cleanup tree...
---
 fs/proc/proc_sysctl.c | 3 +++
 1 file changed, 3 insertions(+)
fs/proc/proc_sysctl.c +1 -0
index 15030784566c..535ab26473af 100644
@@ -546,6 +546,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 	struct inode *inode = file_inode(filp);
 	struct ctl_table_header *head = grab_header(inode);
 	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+	size_t count_max = count;
 	void *kbuf;
 	ssize_t error;
 
fs/proc/proc_sysctl.c static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf, · line 595
Quoted 5 lines
fs/proc/proc_sysctl.c +2 -0
@@ -590,6 +591,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 
 	if (!write) {
 		error = -EFAULT;
+		if (WARN_ON(count > count_max))
+			count = count_max;
That would crash a system with panic-on-warn. I don't think we want that?

Quoted 8 lines

Quoted 5 lines
Eh? None of the handlers should be making this mistake currently and it's not a mistake that can be controlled from userspace. WARN() is absolutely what's wanted here: report an impossible situation (and handle it gracefully for the bulk of users that don't have panic_on_warn set).
Alrighty, Greg are you OK with this type of WARN_ON()? You recently expressed concerns over its use due to panic-on-warn on another patch.

We should never call WARN() on any path that a user can trigger.

If it is just a "the developer called this api in a foolish way" then we could use a WARN_ON() to have them realize their mistake, but in my personal experience, foolish developers don't even notice that kind of mistake :(

thanks,

greg k-h

Kees Cook

author

2020-05-05 20:41 UTC #5 lore raw

Quoted 44 lines · Greg KH

Quoted 34 lines · Luis Chamberlain
Quoted 33 lines · Kees Cook
Quoted 31 lines · Luis Chamberlain
Quoted 23 lines · Kees Cook
Just as a precaution, make sure that proc handlers don't accidentally grow "count" beyond the allocated kbuf size.
Signed-off-by Kees Cook <[email protected]>
---
This applies to hch's sysctl cleanup tree...
---
 fs/proc/proc_sysctl.c | 3 +++
 1 file changed, 3 insertions(+)
fs/proc/proc_sysctl.c +1 -0
index 15030784566c..535ab26473af 100644
@@ -546,6 +546,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 	struct inode *inode = file_inode(filp);
 	struct ctl_table_header *head = grab_header(inode);
 	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+	size_t count_max = count;
 	void *kbuf;
 	ssize_t error;
 
fs/proc/proc_sysctl.c static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf, · line 595
Quoted 5 lines
fs/proc/proc_sysctl.c +2 -0
@@ -590,6 +591,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 
 	if (!write) {
 		error = -EFAULT;
+		if (WARN_ON(count > count_max))
+			count = count_max;
That would crash a system with panic-on-warn. I don't think we want that?
Quoted 8 lines
Quoted 5 lines
Eh? None of the handlers should be making this mistake currently and it's not a mistake that can be controlled from userspace. WARN() is absolutely what's wanted here: report an impossible situation (and handle it gracefully for the bulk of users that don't have panic_on_warn set).
Alrighty, Greg are you OK with this type of WARN_ON()? You recently expressed concerns over its use due to panic-on-warn on another patch.

Quoted 6 lines

We should never call WARN() on any path that a user can trigger.
If it is just a "the developer called this api in a foolish way" then we could use a WARN_ON() to have them realize their mistake, but in my personal experience, foolish developers don't even notice that kind of mistake :(

Right -- while it'd be nice if the developer noticed it, it is _usually_ an unsuspecting end user (or fuzzer), in which case we absolutely want a WARN (and not a BUG![1]) and have the situations handled gracefully, so it can be reported and fixed.

-Kees

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on

Signature · 1 line

Kees Cook

Luis Chamberlain

2020-05-05 22:03 UTC #6 lore raw

Quoted 52 lines · Kees Cook

Quoted 44 lines · Greg KH
Quoted 34 lines · Luis Chamberlain
Quoted 33 lines · Kees Cook
Quoted 31 lines · Luis Chamberlain
Quoted 23 lines · Kees Cook
Just as a precaution, make sure that proc handlers don't accidentally grow "count" beyond the allocated kbuf size.
Signed-off-by Kees Cook <[email protected]>
---
This applies to hch's sysctl cleanup tree...
---
 fs/proc/proc_sysctl.c | 3 +++
 1 file changed, 3 insertions(+)
fs/proc/proc_sysctl.c +1 -0
index 15030784566c..535ab26473af 100644
@@ -546,6 +546,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 	struct inode *inode = file_inode(filp);
 	struct ctl_table_header *head = grab_header(inode);
 	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+	size_t count_max = count;
 	void *kbuf;
 	ssize_t error;
 
fs/proc/proc_sysctl.c static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf, · line 595
Quoted 5 lines
fs/proc/proc_sysctl.c +2 -0
@@ -590,6 +591,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
 
 	if (!write) {
 		error = -EFAULT;
+		if (WARN_ON(count > count_max))
+			count = count_max;
That would crash a system with panic-on-warn. I don't think we want that?
Quoted 8 lines
Quoted 5 lines
Eh? None of the handlers should be making this mistake currently and it's not a mistake that can be controlled from userspace. WARN() is absolutely what's wanted here: report an impossible situation (and handle it gracefully for the bulk of users that don't have panic_on_warn set).
Alrighty, Greg are you OK with this type of WARN_ON()? You recently expressed concerns over its use due to panic-on-warn on another patch.
Quoted 6 lines
We should never call WARN() on any path that a user can trigger.
If it is just a "the developer called this api in a foolish way" then we could use a WARN_ON() to have them realize their mistake, but in my personal experience, foolish developers don't even notice that kind of mistake :(

Quoted 4 lines

Right -- while it'd be nice if the developer noticed it, it is _usually_ an unsuspecting end user (or fuzzer), in which case we absolutely want a WARN (and not a BUG![1]) and have the situations handled gracefully, so it can be reported and fixed.

I've been using WARN*() for this exact purpose before, so I am as surprised as you are bout these concerns. However if we have folks shipping with panic-on-warn this would be rather detrimental to our goals.

Greg, are you aware of folks shipping with panic-on-warn on some products?

Luis

Kees Cook

author

2020-05-05 23:23 UTC #7 lore raw

Quoted 8 lines · Luis Chamberlain

Quoted 4 lines · Kees Cook
Right -- while it'd be nice if the developer noticed it, it is _usually_ an unsuspecting end user (or fuzzer), in which case we absolutely want a WARN (and not a BUG![1]) and have the situations handled gracefully, so it can be reported and fixed.
I've been using WARN*() for this exact purpose before, so I am as surprised as you are bout these concerns. However if we have folks

I don't see any mismatch here: it's not user-reachable, which is what Greg said. WARN is for non-user-reachable "impossible situations". We want to know if those can be hit (via bad API usage, races, etc). If it's reachable from userspace, then it can't be a WARN() any more and needs to be pr_warn().

Quoted 4 lines

shipping with panic-on-warn this would be rather detrimental to our goals.
Greg, are you aware of folks shipping with panic-on-warn on some products?

People shipping with panic_on_warn are expecting to panic for WARNs like this. :P

Signature · 1 line

Kees Cook

Christoph Hellwig

Acked-by on v1 1/1

2020-05-05 05:58 UTC #8 lore raw

In reply to Kees Cook

Quoted 6 lines · Kees Cook

Just as a precaution, make sure that proc handlers don't accidentally grow "count" beyond the allocated kbuf size.
Signed-off-by Kees Cook <[email protected]>
---
This applies to hch's sysctl cleanup tree...

This looks ok o me. You should probably add Al to the Cc list as he has picked up my series into a branch of vfs.git.

Acked-by Christoph Hellwig <[email protected]>