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

[Al Viro <viro@zeniv.linux.org.uk>]

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.