Re: [PATCH] aio: fix a use after free (and fix freeze protection of aio writes)

[Peter Zijlstra]

On Mon, Oct 17, 2016 at 03:40:24PM -0400, Jeff Moyer wrote:
> Christoph Hellwig <hch@xxxxxx> writes:
> 
> > On Mon, Oct 17, 2016 at 02:19:47PM -0400, Jeff Moyer wrote:
> >> This ends up being a call to __sb_end_write:
> >> 
> >> void __sb_end_write(struct super_block *sb, int level)
> >> {
> >>         percpu_up_read(sb->s_writers.rw_sem + level-1);
> >> }
> >> 
> >> Nothing guarantees that submission and completion happen on the same
> >> CPU.  Is this safe?
> >
> > Good point.  From my reading of the percpu_rwsem implementation it
> > is not safe to release it from a different CPU.  Which makes me
> > wonder how we can protect aio writes properly here..
> 
> Could we just change percpu_rw_semaphore->read_count to be a signed
> integer?  The down_write path sums up the counters from all cpus...

To what point?