fix-buffer-double-free-in-ext4_alloc_branch

   1 ext4: Fix buffer double free in ext4_alloc_branch()
   2
   3 From: Jan Kara <jack@suse.cz>
   4
   5 Error recovery in ext4_alloc_branch() calls ext4_forget() even for
   6 buffer corresponding to indirect block it did not allocate. This leads
   7 to brelse() being called twice for that buffer (once from ext4_forget()
   8 and once from cleanup in ext4_ind_map_blocks()) leading to buffer use
   9 count misaccounting. Eventually (but often much later because there
  10 are other users of the buffer) we will see messages like:
  11 VFS: brelse: Trying to free free buffer
  12
  13 Another manifestation of this problem is an error:
  14 JBD2 unexpected failure: jbd2_journal_revoke: !buffer_revoked(bh);
  15 inconsistent data on disk
  16
  17 The fix is easy - don't forget buffer we did not allocate. Also add an
  18 explanatory comment because the indexing at ext4_alloc_branch() is
  19 somewhat subtle.
  20
  21 Signed-off-by: Jan Kara <jack@suse.cz>
  22 Signed-off-by: Theodore Ts'o <tytso@mit.edu>
  23 Cc: stable@vger.kernel.org
  24 ---
  25  fs/ext4/indirect.c | 8 +++++++-
  26  1 file changed, 7 insertions(+), 1 deletion(-)
  27
  28 diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
  29 index 594009f5f523..3b91d240da4d 100644
  30 --- a/fs/ext4/indirect.c
  31 +++ b/fs/ext4/indirect.c
  32 @@ -389,7 +389,13 @@ static int ext4_alloc_branch(handle_t *handle, struct inode *inode,
  33         return 0;
  34  failed:
  35         for (; i >= 0; i--) {
  36 -               if (i != indirect_blks && branch[i].bh)
  37 +               /*
  38 +                * We want to ext4_forget() only freshly allocated indirect
  39 +                * blocks.  Buffer for new_blocks[i-1] is at branch[i].bh and
  40 +                * buffer at branch[0].bh is indirect block / inode already
  41 +                * existing before ext4_alloc_branch() was called.
  42 +                */
  43 +               if (i > 0 && i != indirect_blks && branch[i].bh)
  44                         ext4_forget(handle, 1, inode, branch[i].bh,
  45                                     branch[i].bh->b_blocknr);
  46                 ext4_free_blocks(handle, inode, NULL, new_blocks[i],
  47 --
  48 1.8.1.4
  49
  50