linux-ext4 - [PATCH] ext4: Fix buffer double free in ext4_alloc

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Date:	Fri, 10 Oct 2014 16:23:16 +0200
From:	Jan Kara <jack@...e.cz>
To:	linux-fsdevel@...r.kernel.org
Cc:	linux-ext4@...r.kernel.org, Dave Chinner <david@...morbit.com>,
	xfs@....sgi.com, cluster-devel@...hat.com,
	Steven Whitehouse <swhiteho@...hat.com>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>, ocfs2-devel@....oracle.com,
	reiserfs-devel@...r.kernel.org, Jeff Mahoney <jeffm@...e.de>,
	Dave Kleikamp <shaggy@...nel.org>,
	jfs-discussion@...ts.sourceforge.net, tytso@....edu,
	viro@...iv.linux.org.uk, Jan Kara <jack@...e.cz>
Subject: [PATCH] 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@...e.cz>
---
 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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