[PATCH] xfs: fix mount failure crash on invalid iclog memory access

[Brian Foster <bfoster@redhat.com>]

syzbot (via KASAN) reports a use-after-free in the error path of
xlog_alloc_log(). Specifically, the iclog freeing loop doesn't
handle the case of a fully initialized ->l_iclog linked list.
Instead, it assumes that the list is partially constructed and NULL
terminated.

This bug manifested because there was no possible error scenario
after iclog list setup when the original code was added.  Subsequent
code and associated error conditions were added some time later,
while the original error handling code was never updated. Fix up the
error loop to terminate either on a NULL iclog or reaching the end
of the list.

Reported-by: syzbot+c732f8644185de340492@syzkaller.appspotmail.com
Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/xfs/xfs_log.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 6a147c63a8a6..f6006d94a581 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -1542,6 +1542,8 @@ xlog_alloc_log(
 		prev_iclog = iclog->ic_next;
 		kmem_free(iclog->ic_data);
 		kmem_free(iclog);
+		if (prev_iclog == log->l_iclog)
+			break;
 	}
 out_free_log:
 	kmem_free(log);
-- 
2.20.1