Re: [PATCH] bpf_lru_list: Read double-checked variable once without lock

[Martin KaFai Lau]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: syzbot+516acdb03d3e27d91bcd@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> ---
> 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@xxxxxx>