From 99579ccec4e271c3d4d4e7c946058766812afdab Mon Sep 17 00:00:00 2001 From: Brian Foster Date: Fri, 22 Jul 2016 09:50:38 +1000 Subject: xfs: skip dirty pages in ->releasepage() XFS has had scattered reports of delalloc blocks present at ->releasepage() time. This results in a warning with a stack trace similar to the following: ... Call Trace: [] dump_stack+0x63/0x84 [] warn_slowpath_common+0x97/0xe0 [] warn_slowpath_null+0x1a/0x20 [] xfs_vm_releasepage+0x10f/0x140 [] ? page_mkclean_one+0xd0/0xd0 [] ? anon_vma_prepare+0x150/0x150 [] try_to_release_page+0x32/0x50 [] shrink_active_list+0x3ce/0x3e0 [] shrink_lruvec+0x687/0x7d0 [] shrink_zone+0xdc/0x2c0 [] kswapd+0x4f9/0x970 [] ? mem_cgroup_shrink_node_zone+0x1a0/0x1a0 [] kthread+0xc9/0xe0 [] ? kthread_stop+0x100/0x100 [] ret_from_fork+0x3f/0x70 [] ? kthread_stop+0x100/0x100 This occurs because it is possible for shrink_active_list() to send pages marked dirty to ->releasepage() when certain buffer_head threshold conditions are met. shrink_active_list() doesn't check the page dirty state apparently to handle an old ext3 corner case where in some cases clean pages would not have the dirty bit cleared, thus it is up to the filesystem to determine how to handle the page. XFS currently handles the delalloc case properly, but this behavior makes the warning spurious. Update the XFS ->releasepage() handler to explicitly skip dirty pages. Retain the existing delalloc/unwritten checks so we continue to warn if such buffers exist on clean pages when they shouldn't. Diagnosed-by: Dave Chinner Signed-off-by: Brian Foster Reviewed-by: Dave Chinner Signed-off-by: Dave Chinner --- fs/xfs/xfs_aops.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'fs/xfs/xfs_aops.c') diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c index 3ba0809e0be8..6135787500fc 100644 --- a/fs/xfs/xfs_aops.c +++ b/fs/xfs/xfs_aops.c @@ -1040,6 +1040,20 @@ xfs_vm_releasepage( trace_xfs_releasepage(page->mapping->host, page, 0, 0); + /* + * mm accommodates an old ext3 case where clean pages might not have had + * the dirty bit cleared. Thus, it can send actual dirty pages to + * ->releasepage() via shrink_active_list(). Conversely, + * block_invalidatepage() can send pages that are still marked dirty + * but otherwise have invalidated buffers. + * + * We've historically freed buffers on the latter. Instead, quietly + * filter out all dirty pages to avoid spurious buffer state warnings. + * This can likely be removed once shrink_active_list() is fixed. + */ + if (PageDirty(page)) + return 0; + xfs_count_page_state(page, &delalloc, &unwritten); if (WARN_ON_ONCE(delalloc)) -- cgit v1.2.3 From 28b783e47ad702b8e0f4861ef94cdfce6abd7c80 Mon Sep 17 00:00:00 2001 From: Dave Chinner Date: Fri, 22 Jul 2016 09:56:38 +1000 Subject: 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: [] dump_stack+0x68/0x94 [] print_trailer+0x115/0x1a0 [] object_err+0x34/0x40 [] kasan_report_error+0x217/0x530 [] __asan_report_load8_noabort+0x43/0x50 [] xfs_destroy_ioend+0x3bf/0x4c0 [] xfs_end_bio+0x154/0x220 [] bio_endio+0x158/0x1b0 [] blk_update_request+0x18b/0xb80 [] scsi_end_request+0x97/0x5a0 [] scsi_io_completion+0x438/0x1690 [] scsi_finish_command+0x375/0x4e0 [] 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: # 4.7 Signed-off-by: Dave Chinner Reported-and-Tested-by: Calvin Owens Reviewed-by: Christoph Hellwig Reviewed-by: Brian Foster Signed-off-by: Dave Chinner --- fs/xfs/xfs_aops.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) (limited to 'fs/xfs/xfs_aops.c') diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c index 6135787500fc..f1c7f8cec22a 100644 --- a/fs/xfs/xfs_aops.c +++ b/fs/xfs/xfs_aops.c @@ -87,6 +87,12 @@ xfs_find_bdev_for_inode( * We're now finished for good with this page. Update the page state via the * associated buffer_heads, paying attention to the start and end offsets that * we need to process on the page. + * + * Landmine Warning: bh->b_end_io() will call end_page_writeback() on the last + * buffer in the IO. Once it does this, it is unsafe to access the bufferhead or + * the page at all, as we may be racing with memory reclaim and it can free both + * the bufferhead chain and the page as it will see the page as clean and + * unused. */ static void xfs_finish_page_writeback( @@ -95,8 +101,9 @@ xfs_finish_page_writeback( int error) { unsigned int end = bvec->bv_offset + bvec->bv_len - 1; - struct buffer_head *head, *bh; + struct buffer_head *head, *bh, *next; unsigned int off = 0; + unsigned int bsize; ASSERT(bvec->bv_offset < PAGE_SIZE); ASSERT((bvec->bv_offset & ((1 << inode->i_blkbits) - 1)) == 0); @@ -105,15 +112,17 @@ xfs_finish_page_writeback( bh = head = page_buffers(bvec->bv_page); + bsize = bh->b_size; do { + next = bh->b_this_page; 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); + off += bsize; + } while ((bh = next) != head); } /* -- cgit v1.2.3