linux-ext4 - [PATCH] ext4: set h_journal if there is a failure starting a reserved handle

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Date:   Wed, 18 Apr 2018 11:49:46 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
        stable@...nel.org
Subject: [PATCH] ext4: set h_journal if there is a failure starting a reserved handle

If ext4 tries to start a reserved handle via
jbd2_journal_start_reserved(), and the journal has been aborted, this
can result in a NULL pointer dereference.  This is because the fields
h_journal and h_transaction in the handle structure share the same
meory, via a union, so jbd2_journal_start_reserved() will clear
h_journal before calling start_this_handle().  If this function fails
due to an aborted handle, h_journal will still be NULL, and the call
to jbd2_journal_free_reserved() will pass a NULL journal to
sub_reserve_credits().

This can be reproduced by running "kvm-xfstests -c dioread_nolock
generic/475".

Fixes: 8f7d89f36829b ("jbd2: transaction reservation support")
Signed-off-by: Theodore Ts'o <tytso@....edu>
Cc: Jan Kara <jack@...e.cz>
Cc: stable@...nel.org # 3.11
---
 fs/jbd2/transaction.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index ac311037d7a5..8aa453784402 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -532,6 +532,7 @@ int jbd2_journal_start_reserved(handle_t *handle, unsigned int type,
 	 */
 	ret = start_this_handle(journal, handle, GFP_NOFS);
 	if (ret < 0) {
+		handle->h_journal = journal;
 		jbd2_journal_free_reserved(handle);
 		return ret;
 	}
-- 
2.16.1.72.g5be1f00a9a