xfs: bufferhead chains are invalid after end_page_writeback

In xfs_finish_page_writeback(), we have a loop that looks like this:

        do {
                if (off < bvec->bv_offset)
                        goto next_bh;
                if (off > end)
                        break;
                bh->b_end_io(bh, !error);
next_bh:
                off += bh->b_size;
        } while ((bh = bh->b_this_page) != head);

The b_end_io function is end_buffer_async_write(), which will call
end_page_writeback() once all the buffers have marked as no longer
under IO.  This issue here is that the only thing currently
protecting both the bufferhead chain and the page from being
reclaimed is the PageWriteback state held on the page.

While we attempt to limit the loop to just the buffers covered by
the IO, we still read from the buffer size and follow the next
pointer in the bufferhead chain. There is no guarantee that either
of these are valid after the PageWriteback flag has been cleared.
Hence, loops like this are completely unsafe, and result in
use-after-free issues. One such problem was caught by Calvin Owens
with KASAN:

.....
 INFO: Freed in 0x103fc80ec age=18446651500051355200 cpu=2165122683 pid=-1
  free_buffer_head+0x41/0x90
  __slab_free+0x1ed/0x340
  kmem_cache_free+0x270/0x300
  free_buffer_head+0x41/0x90
  try_to_free_buffers+0x171/0x240
  xfs_vm_releasepage+0xcb/0x3b0
  try_to_release_page+0x106/0x190
  shrink_page_list+0x118e/0x1a10
  shrink_inactive_list+0x42c/0xdf0
  shrink_zone_memcg+0xa09/0xfa0
  shrink_zone+0x2c3/0xbc0
.....
 Call Trace:
  <IRQ>  [<ffffffff81e8b8e4>] dump_stack+0x68/0x94
  [<ffffffff8153a995>] print_trailer+0x115/0x1a0
  [<ffffffff81541174>] object_err+0x34/0x40
  [<ffffffff815436e7>] kasan_report_error+0x217/0x530
  [<ffffffff81543b33>] __asan_report_load8_noabort+0x43/0x50
  [<ffffffff819d651f>] xfs_destroy_ioend+0x3bf/0x4c0
  [<ffffffff819d69d4>] xfs_end_bio+0x154/0x220
  [<ffffffff81de0c58>] bio_endio+0x158/0x1b0
  [<ffffffff81dff61b>] blk_update_request+0x18b/0xb80
  [<ffffffff821baf57>] scsi_end_request+0x97/0x5a0
  [<ffffffff821c5558>] scsi_io_completion+0x438/0x1690
  [<ffffffff821a8d95>] scsi_finish_command+0x375/0x4e0
  [<ffffffff821c3940>] scsi_softirq_done+0x280/0x340


Where the access is occuring during IO completion after the buffer
had been freed from direct memory reclaim.

Prevent use-after-free accidents in this end_io processing loop by
pre-calculating the loop conditionals before calling bh->b_end_io().
The loop is already limited to just the bufferheads covered by the
IO in progress, so the offset checks are sufficient to prevent
accessing buffers in the chain after end_page_writeback() has been
called by the the bh->b_end_io() callout.

Yet another example of why Bufferheads Must Die.

cc: <stable@vger.kernel.org> # 4.7
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reported-and-Tested-by: Calvin Owens <calvinowens@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>