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[PATCH] ext4: Fix buffer double free in ext4_alloc_branch()

To: linux-fsdevel@xxxxxxxxxxxxxxx
Subject: [PATCH] ext4: Fix buffer double free in ext4_alloc_branch()
From: Jan Kara <jack@xxxxxxx>
Date: Fri, 10 Oct 2014 16:23:16 +0200
Cc: linux-ext4@xxxxxxxxxxxxxxx, Dave Chinner <david@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, cluster-devel@xxxxxxxxxx, Steven Whitehouse <swhiteho@xxxxxxxxxx>, Mark Fasheh <mfasheh@xxxxxxxx>, Joel Becker <jlbec@xxxxxxxxxxxx>, ocfs2-devel@xxxxxxxxxxxxxx, reiserfs-devel@xxxxxxxxxxxxxxx, Jeff Mahoney <jeffm@xxxxxxx>, Dave Kleikamp <shaggy@xxxxxxxxxx>, jfs-discussion@xxxxxxxxxxxxxxxxxxxxx, tytso@xxxxxxx, viro@xxxxxxxxxxxxxxxxxx, Jan Kara <jack@xxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1412951028-4085-1-git-send-email-jack@xxxxxxx>
References: <1412951028-4085-1-git-send-email-jack@xxxxxxx>
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@xxxxxxx>
---
 fs/ext4/indirect.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index 594009f5f523..3b91d240da4d 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],
-- 
1.8.1.4

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