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@xxxxxxxxxx/
> 
> 	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