linux-ext4 - [patch 3/3] jbd: fix race in buffer processing in commit code

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Date:	Tue, 09 Jun 2009 16:26:26 -0700
From:	akpm@...ux-foundation.org
To:	torvalds@...ux-foundation.org
Cc:	akpm@...ux-foundation.org, jack@...e.cz,
	linux-ext4@...r.kernel.org, stable@...nel.org
Subject: [patch 3/3] jbd: fix race in buffer processing in commit code

From: Jan Kara <jack@...e.cz>

In commit code, we scan buffers attached to a transaction.  During this
scan, we sometimes have to drop j_list_lock and then we recheck whether
the journal buffer head didn't get freed by journal_try_to_free_buffers().
 But checking for buffer_jbd(bh) isn't enough because a new journal head
could get attached to our buffer head.  So add a check whether the journal
head remained the same and whether it's still at the same transaction and
list.

This is a nasty bug and can cause problems like memory corruption (use after
free) or trigger various assertions in JBD code (observed).

Signed-off-by: Jan Kara <jack@...e.cz>
Cc: <stable@...nel.org>
Cc: <linux-ext4@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 fs/jbd/commit.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff -puN fs/jbd/commit.c~jbd-fix-race-in-buffer-processing-in-commit-code fs/jbd/commit.c
--- a/fs/jbd/commit.c~jbd-fix-race-in-buffer-processing-in-commit-code
+++ a/fs/jbd/commit.c
@@ -241,7 +241,7 @@ write_out_data:
 			spin_lock(&journal->j_list_lock);
 		}
 		/* Someone already cleaned up the buffer? */
-		if (!buffer_jbd(bh)
+		if (!buffer_jbd(bh) || bh2jh(bh) != jh
 			|| jh->b_transaction != commit_transaction
 			|| jh->b_jlist != BJ_SyncData) {
 			jbd_unlock_bh_state(bh);
@@ -478,7 +478,9 @@ void journal_commit_transaction(journal_
 			spin_lock(&journal->j_list_lock);
 			continue;
 		}
-		if (buffer_jbd(bh) && jh->b_jlist == BJ_Locked) {
+		if (buffer_jbd(bh) && bh2jh(bh) == jh &&
+		    jh->b_transaction == commit_transaction &&
+		    jh->b_jlist == BJ_Locked) {
 			__journal_unfile_buffer(jh);
 			jbd_unlock_bh_state(bh);
 			journal_remove_journal_head(bh);
_
--
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