[PATCH 3.16 169/366] ext4: set h_journal if there is a failure starting a reserved handle

[Ben Hutchings]

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Theodore Ts'o <ty...@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".

Fixes: 8f7d89f36829b ("jbd2: transaction reservation support")
Signed-off-by: Theodore Ts'o <ty...@mit.edu>
Reviewed-by: Andreas Dilger <adil...@dilger.ca>
Reviewed-by: Jan Kara <j...@suse.cz>
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
 fs/jbd2/transaction.c | 1 +
 1 file changed, 1 insertion(+)

--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -515,6 +515,7 @@ 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;
        }