This is a note to let you know that I've just added the patch titled bpf_lru_list: Read double-checked variable once without lock to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: bpf_lru_list-read-double-checked-variable-once-witho.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 4458d878659efb4360f88e7bcd8ca5bd7bc1a0b9 Author: Marco Elver <elver@xxxxxxxxxx> Date: Tue Feb 9 12:27:01 2021 +0100 bpf_lru_list: Read double-checked variable once without lock [ Upstream commit 6df8fb83301d68ea0a0c0e1cbcc790fcc333ed12 ] For double-checked locking in bpf_common_lru_push_free(), node->type is read outside the critical section and then re-checked under the lock. However, concurrent writes to node->type result in data races. For example, the following concurrent access was observed by KCSAN: write to 0xffff88801521bc22 of 1 bytes by task 10038 on cpu 1: __bpf_lru_node_move_in kernel/bpf/bpf_lru_list.c:91 __local_list_flush kernel/bpf/bpf_lru_list.c:298 ... read to 0xffff88801521bc22 of 1 bytes by task 10043 on cpu 0: bpf_common_lru_push_free kernel/bpf/bpf_lru_list.c:507 bpf_lru_push_free kernel/bpf/bpf_lru_list.c:555 ... Fix the data races where node->type is read outside the critical section (for double-checked locking) by marking the access with READ_ONCE() as well as ensuring the variable is only accessed once. Fixes: 3a08c2fd7634 ("bpf: LRU List") Reported-by: syzbot+3536db46dfa58c573458@xxxxxxxxxxxxxxxxxxxxxxxxx Reported-by: syzbot+516acdb03d3e27d91bcd@xxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Marco Elver <elver@xxxxxxxxxx> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> Acked-by: Martin KaFai Lau <kafai@xxxxxx> Link: https://lore.kernel.org/bpf/20210209112701.3341724-1-elver@xxxxxxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/kernel/bpf/bpf_lru_list.c b/kernel/bpf/bpf_lru_list.c index 1b6b9349cb857..d99e89f113c43 100644 --- a/kernel/bpf/bpf_lru_list.c +++ b/kernel/bpf/bpf_lru_list.c @@ -502,13 +502,14 @@ struct bpf_lru_node *bpf_lru_pop_free(struct bpf_lru *lru, u32 hash) static void bpf_common_lru_push_free(struct bpf_lru *lru, struct bpf_lru_node *node) { + u8 node_type = READ_ONCE(node->type); unsigned long flags; - if (WARN_ON_ONCE(node->type == BPF_LRU_LIST_T_FREE) || - WARN_ON_ONCE(node->type == BPF_LRU_LOCAL_LIST_T_FREE)) + if (WARN_ON_ONCE(node_type == BPF_LRU_LIST_T_FREE) || + WARN_ON_ONCE(node_type == BPF_LRU_LOCAL_LIST_T_FREE)) return; - if (node->type == BPF_LRU_LOCAL_LIST_T_PENDING) { + if (node_type == BPF_LRU_LOCAL_LIST_T_PENDING) { struct bpf_lru_locallist *loc_l; loc_l = per_cpu_ptr(lru->common_lru.local_list, node->cpu);