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

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Message-ID: <20250322010008.GG2023217@ZenIV>
Date: Sat, 22 Mar 2025 01:00:08 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Christian Brauner <brauner@...nel.org>
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 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.

IOW, it really is racy.  It's a counter, not a flag.