Re: CVE-2025-22029: exec: fix the racy usage of fs_struct->in_exec

Re: CVE-2025-22029: exec: fix the racy usage of fs_struct->in_exec

[Michal Hocko]

Based on a follow up update from Oleg[1] I would like to dispute this
CVE.

TL;DR version
: with or without this patch the "if (fs->in_exec)" check in copy_fs()
: can obviously hit the 1 -> 0 transition.
: 
: This is harmless, but should be probably fixed just to avoid another report
: from KCSAN.

On Wed 16-04-25 16:12:00, Greg KH wrote:
> Description
> ===========
> 
> In the Linux kernel, the following vulnerability has been resolved:
> 
> exec: fix the racy usage of fs_struct->in_exec
> 
> check_unsafe_exec() sets fs->in_exec under cred_guard_mutex, then execve()
> paths clear fs->in_exec lockless. This is fine if exec succeeds, but if it
> fails we have the following race:
> 
> 	T1 sets fs->in_exec = 1, fails, drops cred_guard_mutex
> 
> 	T2 sets fs->in_exec = 1
> 
> 	T1 clears fs->in_exec
> 
> 	T2 continues with fs->in_exec == 0
> 
> Change fs/exec.c to clear fs->in_exec with cred_guard_mutex held.
> 
> The Linux kernel CVE team has assigned CVE-2025-22029 to this issue.
> 
> 
> Affected and fixed versions
> ===========================
> 
> 	Fixed in 6.6.87 with commit 753a620a7f8e134b444f89fe90873234e894e21a
> 	Fixed in 6.12.23 with commit e2d8e7bd3314485e0b3b08380c659b3d1d67ed6a
> 	Fixed in 6.13.11 with commit a6b5070721503fb6021ebed51c925ffc66b1c5ab
> 	Fixed in 6.14.2 with commit b519f2e5800fe2391b7545ba6889df795828e885
> 	Fixed in 6.15-rc1 with commit af7bb0d2ca459f15cb5ca604dab5d9af103643f0
> 
> Please see https://www.kernel.org for a full list of currently supported
> kernel versions by the kernel community.
> 
> Unaffected versions might change over time as fixes are backported to
> older supported kernel versions.  The official CVE entry at
> 	https://cve.org/CVERecord/?id=CVE-2025-22029
> will be updated if fixes are backported, please check that for the most
> up to date information about this issue.
> 
> 
> Affected files
> ==============
> 
> The file(s) affected by this issue are:
> 	fs/exec.c
> 
> 
> Mitigation
> ==========
> 
> The Linux kernel CVE team recommends that you update to the latest
> stable kernel version for this, and many other bugfixes.  Individual
> changes are never tested alone, but rather are part of a larger kernel
> release.  Cherry-picking individual commits is not recommended or
> supported by the Linux kernel community at all.  If however, updating to
> the latest release is impossible, the individual changes to resolve this
> issue can be found at these commits:
> 	https://git.kernel.org/stable/c/753a620a7f8e134b444f89fe90873234e894e21a
> 	https://git.kernel.org/stable/c/e2d8e7bd3314485e0b3b08380c659b3d1d67ed6a
> 	https://git.kernel.org/stable/c/a6b5070721503fb6021ebed51c925ffc66b1c5ab
> 	https://git.kernel.org/stable/c/b519f2e5800fe2391b7545ba6889df795828e885
> 	https://git.kernel.org/stable/c/af7bb0d2ca459f15cb5ca604dab5d9af103643f0

-- 
Michal Hocko
SUSE Labs

Re: CVE-2025-22029: exec: fix the racy usage of fs_struct->in_exec

[Greg Kroah-Hartman]

On Wed, Apr 30, 2025 at 12:39:26PM +0200, Michal Hocko wrote:
> Based on a follow up update from Oleg[1] I would like to dispute this
> CVE.
> 
> TL;DR version
> : with or without this patch the "if (fs->in_exec)" check in copy_fs()
> : can obviously hit the 1 -> 0 transition.
> : 
> : This is harmless, but should be probably fixed just to avoid another report
> : from KCSAN.

What is the link to [1]?

thanks,

greg k-h

Re: CVE-2025-22029: exec: fix the racy usage of fs_struct->in_exec

[Oleg Nesterov]

On 04/30, Michal Hocko wrote:
>
> Based on a follow up update from Oleg[1] I would like to dispute this
> CVE.

Agreed. Let me quote my reply to my "fix", see
https://lore.kernel.org/all/20250429154944.GA18907@redhat.com/

	Damn, I am stupid.

	On 03/24, Oleg Nesterov wrote:
	>
	> check_unsafe_exec() sets fs->in_exec under cred_guard_mutex, then execve()
	> paths clear fs->in_exec lockless. This is fine if exec succeeds, but if it
	> fails we have the following race:
	>
	> 	T1 sets fs->in_exec = 1, fails, drops cred_guard_mutex
	>
	> 	T2 sets fs->in_exec = 1
	>
	> 	T1 clears fs->in_exec

	When I look at this code again, I think this race was not possible and thus
	this patch (applied as af7bb0d2ca45) was not needed.

	Yes, begin_new_exec() can drop cred_guard_mutex on failure, but only after
	de_thread() succeeds, when we can't race with another sub-thread.

	I hope this patch didn't make the things worse so we don't need to revert it.
	Plus I think it makes this (confusing) logic a bit more clear. Just, unless
	I am confused again, it wasn't really needed.

Sorry for the confusion caused by my patch :/

Oleg.

Re: CVE-2025-22029: exec: fix the racy usage of fs_struct->in_exec

[Greg Kroah-Hartman]

On Wed, Apr 30, 2025 at 01:20:33PM +0200, Oleg Nesterov wrote:
> On 04/30, Michal Hocko wrote:
> >
> > Based on a follow up update from Oleg[1] I would like to dispute this
> > CVE.
> 
> Agreed. Let me quote my reply to my "fix", see
> https://lore.kernel.org/all/20250429154944.GA18907@redhat.com/
> 
> 	Damn, I am stupid.
> 
> 	On 03/24, Oleg Nesterov wrote:
> 	>
> 	> check_unsafe_exec() sets fs->in_exec under cred_guard_mutex, then execve()
> 	> paths clear fs->in_exec lockless. This is fine if exec succeeds, but if it
> 	> fails we have the following race:
> 	>
> 	> 	T1 sets fs->in_exec = 1, fails, drops cred_guard_mutex
> 	>
> 	> 	T2 sets fs->in_exec = 1
> 	>
> 	> 	T1 clears fs->in_exec
> 
> 	When I look at this code again, I think this race was not possible and thus
> 	this patch (applied as af7bb0d2ca45) was not needed.
> 
> 	Yes, begin_new_exec() can drop cred_guard_mutex on failure, but only after
> 	de_thread() succeeds, when we can't race with another sub-thread.
> 
> 	I hope this patch didn't make the things worse so we don't need to revert it.
> 	Plus I think it makes this (confusing) logic a bit more clear. Just, unless
> 	I am confused again, it wasn't really needed.
> 
> Sorry for the confusion caused by my patch :/

Sorry for the delay, the CVE is now rejected, thanks.

greg k-h