linux-kernel - Re: [syzbot] [fs?] [mm?] KCSAN: data-race in bprm_execve / copy

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Message-ID: <20250321-languste-farbig-e68aef9f4ac8@brauner>
Date: Fri, 21 Mar 2025 09:49:22 +0100
From: Christian Brauner <brauner@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Al Viro <viro@...iv.linux.org.uk>, Oleg Nesterov <oleg@...hat.com>, 
	jack@...e.cz, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, syzkaller-bugs@...glegroups.com, 
	syzbot <syzbot+1c486d0b62032c82a968@...kaller.appspotmail.com>
Subject: Re: [syzbot] [fs?] [mm?] KCSAN: data-race in bprm_execve / copy_fs
 (4)

On Fri, Mar 21, 2025 at 01:10:26AM -0700, Kees Cook wrote:
> 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.

This is a harmless data race afaict. See my other mail.