From 83f59076a1ae6f5c6845d6f7ed3a1a373d883684 Mon Sep 17 00:00:00 2001 From: Leo Martins Date: Fri, 12 Dec 2025 17:26:26 -0800 Subject: [PATCH] btrfs: fix use-after-free warning in btrfs_get_or_create_delayed_node() Previously, btrfs_get_or_create_delayed_node() set the delayed_node's refcount before acquiring the root->delayed_nodes lock. Commit e8513c012de7 ("btrfs: implement ref_tracker for delayed_nodes") moved refcount_set inside the critical section, which means there is no longer a memory barrier between setting the refcount and setting btrfs_inode->delayed_node. Without that barrier, the stores to node->refs and btrfs_inode->delayed_node may become visible out of order. Another thread can then read btrfs_inode->delayed_node and attempt to increment a refcount that hasn't been set yet, leading to a refcounting bug and a use-after-free warning. The fix is to move refcount_set back to where it was to take advantage of the implicit memory barrier provided by lock acquisition. Because the allocations now happen outside of the lock's critical section, they can use GFP_NOFS instead of GFP_ATOMIC. Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-lkp/202511262228.6dda231e-lkp@intel.com Fixes: e8513c012de7 ("btrfs: implement ref_tracker for delayed_nodes") Tested-by: kernel test robot Reviewed-by: Filipe Manana Signed-off-by: Leo Martins Signed-off-by: David Sterba --- fs/btrfs/delayed-inode.c | 32 +++++++++++++++++--------------- 1 file changed, 17 insertions(+), 15 deletions(-) diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c index ce6e9f8812e0..4b7d9015e0da 100644 --- a/fs/btrfs/delayed-inode.c +++ b/fs/btrfs/delayed-inode.c @@ -152,37 +152,39 @@ again: return ERR_PTR(-ENOMEM); btrfs_init_delayed_node(node, root, ino); + /* Cached in the inode and can be accessed. */ + refcount_set(&node->refs, 2); + btrfs_delayed_node_ref_tracker_alloc(node, tracker, GFP_NOFS); + btrfs_delayed_node_ref_tracker_alloc(node, &node->inode_cache_tracker, GFP_NOFS); + /* Allocate and reserve the slot, from now it can return a NULL from xa_load(). */ ret = xa_reserve(&root->delayed_nodes, ino, GFP_NOFS); - if (ret == -ENOMEM) { - btrfs_delayed_node_ref_tracker_dir_exit(node); - kmem_cache_free(delayed_node_cache, node); - return ERR_PTR(-ENOMEM); - } + if (ret == -ENOMEM) + goto cleanup; + xa_lock(&root->delayed_nodes); ptr = xa_load(&root->delayed_nodes, ino); if (ptr) { /* Somebody inserted it, go back and read it. */ xa_unlock(&root->delayed_nodes); - btrfs_delayed_node_ref_tracker_dir_exit(node); - kmem_cache_free(delayed_node_cache, node); - node = NULL; - goto again; + goto cleanup; } ptr = __xa_store(&root->delayed_nodes, ino, node, GFP_ATOMIC); ASSERT(xa_err(ptr) != -EINVAL); ASSERT(xa_err(ptr) != -ENOMEM); ASSERT(ptr == NULL); - - /* Cached in the inode and can be accessed. */ - refcount_set(&node->refs, 2); - btrfs_delayed_node_ref_tracker_alloc(node, tracker, GFP_ATOMIC); - btrfs_delayed_node_ref_tracker_alloc(node, &node->inode_cache_tracker, GFP_ATOMIC); - btrfs_inode->delayed_node = node; xa_unlock(&root->delayed_nodes); return node; +cleanup: + btrfs_delayed_node_ref_tracker_free(node, tracker); + btrfs_delayed_node_ref_tracker_free(node, &node->inode_cache_tracker); + btrfs_delayed_node_ref_tracker_dir_exit(node); + kmem_cache_free(delayed_node_cache, node); + if (ret) + return ERR_PTR(ret); + goto again; } /*