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

Oleg Nesterov
2008-12-01T19:17:35Z

On 12/01, Andi Kleen wrote:
>
> Oleg Nesterov wrote:
>>
>> 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.

Actually, this is more simple. Somehow I missed that setf() _always_
updates ->f_flags, so we have a trivial race with 2 threads.

>>
>> 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.

Perhaps, we can add O_LOCK_FLAGS, then something like

--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -175,6 +175,15 @@ static int setfl(int fd, struct file * f
if (error)
return error;

+ spin_lock(&current->files->file_lock);
+ if (!(filp->f_flags & O_LOCK_FLAGS))
+ filp->f_flags |= O_LOCK_FLAGS;
+ else
+ error = -EAGAIN;
+ spin_unlock(&current->files->file_lock);
+ if (error) /* pretend ->f_flags was changed after us */
+ return 0;
+
if ((arg ^ filp->f_flags) & FASYNC) {
if (filp->f_op && filp->f_op->fasync) {
error = filp->f_op->fasync(fd, filp, (arg & FASYNC) != 0);
@@ -183,7 +192,8 @@ static int setfl(int fd, struct file * f
}
}

- filp->f_flags = (arg & SETFL_MASK) | (filp->f_flags & ~SETFL_MASK);
+ filp->f_flags = (arg & SETFL_MASK) |
+ (filp->f_flags & ~(SETFL_MASK | O_LOCK_FLAGS));
out:
return error;
}

What do you think?

(btw, ioctl_fioasync() calls lock_kernel() but it doesn't protect ->f_flags)

Oleg.