[PATCH 4.9 92/94] xfs: avoid mount-time deadlock in CoW extent recovery

Greg Kroah-Hartman
2017-06-05T16:57:54Z

From: Darrick J. Wong <darrick.wong@oracle.com>

commit 3ecb3ac7b950ff8f6c6a61e8b7b0d6e3546429a0 upstream.

If a malicious user corrupts the refcount btree to cause a cycle between
different levels of the tree, the next mount attempt will deadlock in
the CoW recovery routine while grabbing buffer locks. We can use the
ability to re-grab a buffer that was previous locked to a transaction to
avoid deadlocks, so do that here.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/xfs/libxfs/xfs_refcount.c | 43 +++++++++++++++++++++++++++++++------------
1 file changed, 31 insertions(+), 12 deletions(-)

--- a/fs/xfs/libxfs/xfs_refcount.c
+++ b/fs/xfs/libxfs/xfs_refcount.c
@@ -1629,13 +1629,28 @@ xfs_refcount_recover_cow_leftovers(
if (mp->m_sb.sb_agblocks >= XFS_REFC_COW_START)
return -EOPNOTSUPP;

- error = xfs_alloc_read_agf(mp, NULL, agno, 0, &agbp);
+ INIT_LIST_HEAD(&debris);
+
+ /*
+ * In this first part, we use an empty transaction to gather up
+ * all the leftover CoW extents so that we can subsequently
+ * delete them. The empty transaction is used to avoid
+ * a buffer lock deadlock if there happens to be a loop in the
+ * refcountbt because we're allowed to re-grab a buffer that is
+ * already attached to our transaction. When we're done
+ * recording the CoW debris we cancel the (empty) transaction
+ * and everything goes away cleanly.
+ */
+ error = xfs_trans_alloc_empty(mp, &tp);
if (error)
return error;
- cur = xfs_refcountbt_init_cursor(mp, NULL, agbp, agno, NULL);
+
+ error = xfs_alloc_read_agf(mp, tp, agno, 0, &agbp);
+ if (error)
+ goto out_trans;
+ cur = xfs_refcountbt_init_cursor(mp, tp, agbp, agno, NULL);

/* Find all the leftover CoW staging extents. */
- INIT_LIST_HEAD(&debris);
memset(&low, 0, sizeof(low));
memset(&high, 0, sizeof(high));
low.rc.rc_startblock = XFS_REFC_COW_START;
@@ -1645,10 +1660,11 @@ xfs_refcount_recover_cow_leftovers(
if (error)
goto out_cursor;
xfs_btree_del_cursor(cur, XFS_BTREE_NOERROR);
- xfs_buf_relse(agbp);
+ xfs_trans_brelse(tp, agbp);
+ xfs_trans_cancel(tp);

/* Now iterate the list to free the leftovers */
- list_for_each_entry(rr, &debris, rr_list) {
+ list_for_each_entry_safe(rr, n, &debris, rr_list) {
/* Set up transaction. */
error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, 0, 0, 0, &tp);
if (error)
@@ -1676,8 +1692,16 @@ xfs_refcount_recover_cow_leftovers(
error = xfs_trans_commit(tp);
if (error)
goto out_free;
+
+ list_del(&rr->rr_list);
+ kmem_free(rr);
}

+ return error;
+out_defer:
+ xfs_defer_cancel(&dfops);
+out_trans:
+ xfs_trans_cancel(tp);
out_free:
/* Free the leftover list */
list_for_each_entry_safe(rr, n, &debris, rr_list) {
@@ -1688,11 +1712,6 @@ out_free:

out_cursor:
xfs_btree_del_cursor(cur, XFS_BTREE_ERROR);
- xfs_buf_relse(agbp);
- goto out_free;
-
-out_defer:
- xfs_defer_cancel(&dfops);
- xfs_trans_cancel(tp);
- goto out_free;
+ xfs_trans_brelse(tp, agbp);
+ goto out_trans;
}