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

[Christian Brauner <brauner@kernel.org>]

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.