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

[Oleg Nesterov <oleg@redhat.com>]

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.