[PATCH] lockd: fix use-after-free in nlmsvc_retry_blocked

nlmsvc_retry_blocked() examines the block at the head of nlm_blocked
under nlm_blocked_lock, then releases the lock before calling
nlmsvc_grant_blocked() or retry_deferred_block(). It holds no
reference of its own across that window, so the nlm_blocked list
reference is all that keeps the block alive. Unmounting the nfsd
filesystem while a lock request is still blocked reaches
nlmsvc_traverse_blocks(), which drops that reference.
nlmsvc_grant_blocked() does take a reference, but only after
nlm_blocked_lock has been dropped, so the block and the nlm_rqst
hanging off it can be freed before it runs.

  BUG: KASAN: slab-use-after-free in nlm_async_call+0xd6/0x230
  Read of size 8 at addr ffff88811b04c808 by task lockd/8377
    nlm_async_call+0xd6/0x230
    nlmsvc_retry_blocked+0x61c/0x800
    lockd+0x144/0x1c0
  Freed by task 8392:
    nlmsvc_release_block+0x231/0x290
    nlmsvc_traverse_blocks+0x139/0x1b0
    nlm_traverse_files+0x1aa/0xa00
    nlmsvc_free_host_resources+0x12/0x60
    nlm_shutdown_hosts_net+0x127/0x280
    lockd_down+0xd5/0x1c0

Take a reference before releasing nlm_blocked_lock and drop it
once the retry has run. retry_deferred_block() borrows its
caller's reference as well, so this covers both arms of the loop.

Fixes: 0e4ac9d93515 ("lockd: handle fl_grant callbacks")
Reported-by: Shuangpeng Bai <[email protected]>
Closes: https://lore.kernel.org/linux-nfs/[email protected]/
Signed-off-by: Chuck Lever <[email protected]>
---
 fs/lockd/svclock.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/lockd/svclock.c b/fs/lockd/svclock.c
index e628b5d35507..8d83283d3e21 100644
--- a/fs/lockd/svclock.c
+++ b/fs/lockd/svclock.c
@@ -1023,6 +1023,7 @@ nlmsvc_retry_blocked(struct svc_rqst *rqstp)
 			timeout = block->b_when - jiffies;
 			break;
 		}
+		kref_get(&block->b_count);
 		spin_unlock(&nlm_blocked_lock);
 
 		dprintk("nlmsvc_retry_blocked(%p, when=%ld)\n",
@@ -1033,6 +1034,7 @@ nlmsvc_retry_blocked(struct svc_rqst *rqstp)
 			retry_deferred_block(block);
 		} else
 			nlmsvc_grant_blocked(block);
+		nlmsvc_release_block(block);
 		spin_lock(&nlm_blocked_lock);
 	}
 	spin_unlock(&nlm_blocked_lock);
-- 
2.54.0