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

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Message-ID: <20250322185007.GI2023217@ZenIV>
Date: Sat, 22 Mar 2025 18:50:07 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Christian Brauner <brauner@...nel.org>, Kees Cook <kees@...nel.org>,
	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 04:55:39PM +0100, Oleg Nesterov wrote:

> No, check_unsafe_execve() is called with cred_guard_mutex held,
> see prepare_bprm_creds()

Point...

> > 3) A calls exec_binprm(), fails (bad binary)
> > 4) A clears ->in_exec
> 
> So (2) can only happen after A fails and drops cred_guard_mutex.
> 
> And this means that we just need to ensure that ->in_exec is cleared
> before this mutex is dropped, no? Something like below?

Probably should work, but I wonder if it would be cleaner to have
->in_exec replaced with pointer to task_struct responsible.  Not
"somebody with that fs_struct for ->fs is trying to do execve(),
has verified that nothing outside of their threads is using this
and had been holding ->signal->cred_guard_mutex ever since then",
but "this is the thread that..."