ext4: Fix buffer double free in ext4_alloc_branch()

Error recovery in ext4_alloc_branch() calls ext4_forget() even for
buffer corresponding to indirect block it did not allocate. This leads
to brelse() being called twice for that buffer (once from ext4_forget()
and once from cleanup in ext4_ind_map_blocks()) leading to buffer use
count misaccounting. Eventually (but often much later because there
are other users of the buffer) we will see messages like:
VFS: brelse: Trying to free free buffer

Another manifestation of this problem is an error:
JBD2 unexpected failure: jbd2_journal_revoke: !buffer_revoked(bh);
inconsistent data on disk

The fix is easy - don't forget buffer we did not allocate. Also add an
explanatory comment because the indexing at ext4_alloc_branch() is
somewhat subtle.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

1 files changed, 7 insertions, 1 deletions

diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index 8a57e9fcd1b9..f85bafd474dc 100644
--- a/fs/ext4/indirect.c
+++ b/fs/ext4/indirect.c
@@ -389,7 +389,13 @@ static int ext4_alloc_branch(handle_t *handle, struct inode *inode,
         return 0;
 failed:
         for (; i >= 0; i--) {
-                if (i != indirect_blks && branch[i].bh)
+                /*
+                 * We want to ext4_forget() only freshly allocated indirect
+                 * blocks.  Buffer for new_blocks[i-1] is at branch[i].bh and
+                 * buffer at branch[0].bh is indirect block / inode already
+                 * existing before ext4_alloc_branch() was called.
+                 */
+                if (i > 0 && i != indirect_blks && branch[i].bh)
                         ext4_forget(handle, 1, inode, branch[i].bh,
                                     branch[i].bh->b_blocknr);
                 ext4_free_blocks(handle, inode, NULL, new_blocks[i],