xfs: avoid mount-time deadlock in CoW extent recovery

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>

diff --git a/fs/xfs/libxfs/xfs_refcount.c b/fs/xfs/libxfs/xfs_refcount.c
index b177ef3..82a38d8 100644 (file)
--- 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;
 }