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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Theodore Tso <tytso@mit.edu>,
        Andreas Dilger <adilger@dilger.ca>, Jan Kara <jack@suse.cz>
Subject: [PATCH 4.16 002/113] ext4: set h_journal if there is a failure starting a reserved handle
Date:   Mon, 30 Apr 2018 12:23:33 -0700
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4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Theodore Ts'o <tytso@mit.edu>

commit b2569260d55228b617bd82aba6d0db2faeeb4116 upstream.

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
memory, 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".

Cc: stable@kernel.org # 3.11
Fixes: 8f7d89f36829b ("jbd2: transaction reservation support")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/jbd2/transaction.c |    1 +
 1 file changed, 1 insertion(+)

--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -532,6 +532,7 @@ int jbd2_journal_start_reserved(handle_t
 	 */
 	ret = start_this_handle(journal, handle, GFP_NOFS);
 	if (ret < 0) {
+		handle->h_journal = journal;
 		jbd2_journal_free_reserved(handle);
 		return ret;
 	}