Re: [syzbot] [fs?] [mm?] KCSAN: data-race in bprm_execve / copy_fs (4)

[Kees Cook <kees@kernel.org>]

On Fri, Mar 21, 2025 at 01:44:23AM +0000, Al Viro wrote:
> On Thu, Mar 20, 2025 at 01:09:38PM -0700, Kees Cook wrote:
> 
> > What I can imagine here is two failing execs racing a fork:
> > 
> > 	A start execve
> > 	B fork with CLONE_FS
> > 	C start execve, reach check_unsafe_exec(), set fs->in_exec
> > 	A bprm_execve() failure, clear fs->in_exec
> > 	B copy_fs() increment fs->users.
> > 	C bprm_execve() failure, clear fs->in_exec
> > 
> > But I don't think this is a "real" flaw, though, since the locking is to
> > protect a _successful_ execve from a fork (i.e. getting the user count
> > right). A successful execve will de_thread, and I don't see any wrong
> > counting of fs->users with regard to thread lifetime.
> > 
> > Did I miss something in the analysis? Should we perform locking anyway,
> > or add data race annotations, or something else?
> 
> Umm...  What if C succeeds, ending up with suid sharing ->fs?

I still can't quite construct it -- fs->users is always correct, I
think?

Below would be the bad set of events, but it's wrong that "fs->users==1".
If A and C are both running with CLONE_FS then fs->users==2. A would need to
exit first, but it can't do that and also set fs->in_exec=0

A execve, reaches bprm_execve() failure path
B fork with CLONE_FS, reaches copy_fs()
C execve, reaches check_unsafe_exec()
C takes fs->lock, counts, finds safe fs->users==1, sets in_exec=1, unlocks
A sets fs->in_exec=0
B takes fs->lock, sees in_exec==0, does fs->users++, unlocks
C goes setuid, sharing fs with unpriv B

Something still feels very weird, though. Does fs->in_exec not matter at
all? Hmm, no, it stops fs->users++ happening after it was validated to be 1.

-- 
Kees Cook