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

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Message-ID: <20250322-nettigkeiten-weitreichend-fa9e8ee6875b@brauner>
Date: Sat, 22 Mar 2025 11:23:21 +0100
From: Christian Brauner <brauner@...nel.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Kees Cook <kees@...nel.org>, 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 Sat, Mar 22, 2025 at 01:00:08AM +0000, Al Viro wrote:
> On Fri, Mar 21, 2025 at 09:45:39AM +0100, Christian Brauner wrote:
> 
> > Afaict, the only way this data race can happen is if we jump to the
> > cleanup label and then reset current->fs->in_exec. If the execve was
> > successful there's no one to race us with CLONE_FS obviously because we
> > took down all other threads.
> 
> Not really.
> 
> 1) A enters check_unsafe_execve(), sets ->in_exec to 1
> 2) B enters check_unsafe_execve(), sets ->in_exec to 1
> 3) A calls exec_binprm(), fails (bad binary)
> 4) A clears ->in_exec
> 5) C calls clone(2) with CLONE_FS and spawns D - ->in_exec is 0
> 6) B gets through exec_binprm(), kills A and C, but not D.
> 7) B clears ->in_exec, returns
> 
> Result: B and D share ->fs, B runs suid binary.
> 
> Had (5) happened prior to (2), (2) wouldn't have set ->in_exec;
> had (5) happened prior to (4), clone() would've failed; had
> (5) been delayed past (6), there wouldn't have been a thread
> to call clone().
> 
> But in the window between (4) and (6), clone() doesn't see
> execve() in progress and check_unsafe_execve() has already
> been done, so it hadn't seen the extra thread.

Eewww, you're right. That's ugly as hell.