btrfs: check page->mapping when loading free space cache

commit 3797136b626ad4b6582223660c041efdea8f26b2 upstream.

While testing 5.2 we ran into the following panic

[52238.017028] BUG: kernel NULL pointer dereference, address: 0000000000000001
[52238.105608] RIP: 0010:drop_buffers+0x3d/0x150
[52238.304051] Call Trace:
[52238.308958]  try_to_free_buffers+0x15b/0x1b0
[52238.317503]  shrink_page_list+0x1164/0x1780
[52238.325877]  shrink_inactive_list+0x18f/0x3b0
[52238.334596]  shrink_node_memcg+0x23e/0x7d0
[52238.342790]  ? do_shrink_slab+0x4f/0x290
[52238.350648]  shrink_node+0xce/0x4a0
[52238.357628]  balance_pgdat+0x2c7/0x510
[52238.365135]  kswapd+0x216/0x3e0
[52238.371425]  ? wait_woken+0x80/0x80
[52238.378412]  ? balance_pgdat+0x510/0x510
[52238.386265]  kthread+0x111/0x130
[52238.392727]  ? kthread_create_on_node+0x60/0x60
[52238.401782]  ret_from_fork+0x1f/0x30

The page we were trying to drop had a page->private, but had no
page->mapping and so called drop_buffers, assuming that we had a
buffer_head on the page, and then panic'ed trying to deref 1, which is
our page->private for data pages.

This is happening because we're truncating the free space cache while
we're trying to load the free space cache.  This isn't supposed to
happen, and I'll fix that in a followup patch.  However we still
shouldn't allow those sort of mistakes to result in messing with pages
that do not belong to us.  So add the page->mapping check to verify that
we still own this page after dropping and re-acquiring the page lock.

This page being unlocked as:
btrfs_readpage
  extent_read_full_page
    __extent_read_full_page
      __do_readpage
        if (!nr)
	   unlock_page  <-- nr can be 0 only if submit_extent_page
			    returns an error

CC: stable@vger.kernel.org # 4.4+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[ add callchain ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 69a3c11af9d45d8a89bf25785d51a62dafb7b301..a84a1ceb260af102475c1f0934d3600246ba8e6e 100644 (file)
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -391,6 +391,12 @@ static int io_ctl_prepare_pages(struct btrfs_io_ctl *io_ctl, struct inode *inode
                if (uptodate && !PageUptodate(page)) {
                        btrfs_readpage(NULL, page);
                        lock_page(page);
+                       if (page->mapping != inode->i_mapping) {
+                               btrfs_err(BTRFS_I(inode)->root->fs_info,
+                                         "free space cache page truncated");
+                               io_ctl_drop_pages(io_ctl);
+                               return -EIO;
+                       }
                        if (!PageUptodate(page)) {
                                btrfs_err(BTRFS_I(inode)->root->fs_info,
                                           "error reading free space cache");