Re: [PATCH bpf-next] bpf, seccomp: fix false positive preemption splat for cbpf->ebpf progs

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Feb 20, 2019 at 12:06:29PM +0100, Daniel Borkmann wrote:
> In 568f196756ad ("bpf: check that BPF programs run with preemption disabled")
> a check was added for BPF_PROG_RUN() that for every invocation preemption has
> to be disabled to not break eBPF assumptions (e.g. per-cpu map). Of course this
> does not count for seccomp because only cBPF -> eBPF is loaded here and it does
> not make use of any functionality that would require this assertion. Fix this
> false positive by adding and using __BPF_PROG_RUN() variant that does not have
> the cant_sleep(); check.
> 
> Fixes: 568f196756ad ("bpf: check that BPF programs run with preemption disabled")
> Reported-by: syzbot+8bf19ee2aa580de7a2a7@syzkaller.appspotmail.com
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> ---
>  include/linux/filter.h | 9 ++++++++-
>  kernel/seccomp.c       | 2 +-
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index f32b3ec..2648fd7 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -533,7 +533,14 @@ struct sk_filter {
>  	struct bpf_prog	*prog;
>  };
>  
> -#define BPF_PROG_RUN(filter, ctx)  ({ cant_sleep(); (*(filter)->bpf_func)(ctx, (filter)->insnsi); })
> +#define bpf_prog_run__non_preempt(prog, ctx)	\
> +	({ cant_sleep(); __BPF_PROG_RUN(prog, ctx); })
> +/* Native eBPF or cBPF -> eBPF transitions. Preemption must be disabled. */
> +#define BPF_PROG_RUN(prog, ctx)			\
> +	bpf_prog_run__non_preempt(prog, ctx)
> +/* Direct use for cBPF -> eBPF only, but not for native eBPF. */

I think the comment is too abstract.
May be it should say that this is seccomp cBPF only ?
And macro name should be explicit as well ?

> +#define __BPF_PROG_RUN(prog, ctx)		\
> +	(*(prog)->bpf_func)(ctx, (prog)->insnsi)
>  
>  #define BPF_SKB_CB_LEN QDISC_CB_PRIV_LEN
>  
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index e815781..826d4e4 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -268,7 +268,7 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
>  	 * value always takes priority (ignoring the DATA).
>  	 */
>  	for (; f; f = f->prev) {
> -		u32 cur_ret = BPF_PROG_RUN(f->prog, sd);
> +		u32 cur_ret = __BPF_PROG_RUN(f->prog, sd);
>  
>  		if (ACTION_ONLY(cur_ret) < ACTION_ONLY(ret)) {
>  			ret = cur_ret;
> -- 
> 2.9.5
>