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

[Andrii Nakryiko]

On Tue, Feb 9, 2021 at 10:00 PM Martin KaFai Lau <kafai@xxxxxx> wrote:
>
> 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>

Added Fixes: 3a08c2fd7634 ("bpf: LRU List") and applied to bpf-next, thanks.