Re: BUG? "Call fasync() functions without the BKL" is racy

[Andi Kleen <ak@linux.intel.com>]

Oleg Nesterov wrote:
> Hi.
> 
> Let's suppose we have the tasks T1, T2, T3 which share the same file,
> all do sys_fcntl(file, F_SETFL) in parallel. file->f_flags == 0.
> 
> setfl(arg) does:
> 
> 	if ((arg ^ filp->f_flags) & FASYNC)
> // --- WINDOW_1 ---
> 		filp->f_op->fasync(fd, filp, (arg & FASYNC) != 0)
> // --- WINDOW_2 ---
> 	filp->f_flags = arg;
> 
> T1 calls setfl(FASYNC), preempted in WINDOW_1.
> 
> T2 calls setfl(FASYNC), does all job and returns.
> 
> T3 calls setfl(0), sees ->f_flags & FASYNC, does ->fasync(on => 0),
> preempted in WINDOW_2.
> 
> T1 resumes, does ->fasync(on => 1) again, update ->f_flags (it
> already has FASYNC) and returns.
> 
> T3 resumes, and clears FASYNC from ->f_flags.
> 
> 
> 
> Now, this file was added into some "struct fasync_struct", but
> ->f_flags doesn't have FASYNC. This means __fput() will skip
> ->fasync(on => 0) and the next kill_fasync() can crash because
> fa_file points to the freed/reused memory.

Nasty. Thanks for catching.
> 
> I think a238b790d5f99c7832f9b73ac8847025815b85f7 should be reverted.
> Or do you see the better fix?

Hmm, about checking for this case and retrying?

Or put a fasync mutex into files_struct.

-Andi