lkml.org 
[lkml]   [2021]   [Feb]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] bpf_lru_list: Read double-checked variable once without lock
On Tue, Feb 09, 2021 at 12:27:01PM +0100, Marco Elver wrote:
> 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.
>
> Reported-by: syzbot+3536db46dfa58c573458@syzkaller.appspotmail.com
> Reported-by: syzbot+516acdb03d3e27d91bcd@syzkaller.appspotmail.com
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> Detailed reports:
> https://groups.google.com/g/syzkaller-upstream-moderation/c/PwsoQ7bfi8k/m/NH9Ni2WxAQAJ
> https://groups.google.com/g/syzkaller-upstream-moderation/c/-fXQO9ehxSM/m/RmQEcI2oAQAJ
> ---
> kernel/bpf/bpf_lru_list.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/bpf_lru_list.c b/kernel/bpf/bpf_lru_list.c
> index 1b6b9349cb85..d99e89f113c4 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) {
I think this can be bpf-next.

Acked-by: Martin KaFai Lau <kafai@fb.com>

\
 
 \ /
  Last update: 2021-02-10 07:02    [W:0.059 / U:1.480 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site