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

[Mateusz Guzik <mjguzik@gmail.com>]

On Fri, Mar 21, 2025 at 11:26:03PM -0700, Kees Cook wrote:
> 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.
> 
> Yeah, you found it. Thank you!
> 
> > 1) A enters check_unsafe_execve(), sets ->in_exec to 1
> > 2) B enters check_unsafe_execve(), sets ->in_exec to 1
> 
> With 3 threads A, B, and C already running, fs->users == 3, so steps (1)
> and (2) happily pass.
> 
> > 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
> 
> D's creation bumps fs->users == 4.
> 
> > 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.
> 
> Yeah, I would agree. Totally untested patch:
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 506cd411f4ac..988b8621c079 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1632,7 +1632,7 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
>  	if (p->fs->users > n_fs)
>  		bprm->unsafe |= LSM_UNSAFE_SHARE;
>  	else
> -		p->fs->in_exec = 1;
> +		refcount_inc(&p->fs->in_exec);
>  	spin_unlock(&p->fs->lock);
>  }
>  
> @@ -1862,7 +1862,7 @@ static int bprm_execve(struct linux_binprm *bprm)
>  
>  	sched_mm_cid_after_execve(current);
>  	/* execve succeeded */
> -	current->fs->in_exec = 0;
> +	refcount_dec(&current->fs->in_exec);
>  	current->in_execve = 0;
>  	rseq_execve(current);
>  	user_events_execve(current);
> @@ -1881,7 +1881,7 @@ static int bprm_execve(struct linux_binprm *bprm)
>  		force_fatal_sig(SIGSEGV);
>  
>  	sched_mm_cid_after_execve(current);
> -	current->fs->in_exec = 0;
> +	refcount_dec(&current->fs->in_exec);
>  	current->in_execve = 0;
>  
>  	return retval;

The bump is conditional and with this patch you may be issuing
refcount_dec when you declined to refcount_inc.

A special case where there are others to worry about and which proceeds
with an exec without leaving in any indicators is imo sketchy.

I would argue it would make the most sense to serialize these execs.

Vast majority of programs are single-threaded when they exec with an
unshared ->fs, so they don't need to bear any overhead nor complexity
modulo a branch.

For any fucky case you can park yourself waiting for any pending exec to
finish.