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

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Date:   Wed, 20 Feb 2019 19:27:54 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     ast@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf, seccomp: fix false positive preemption
 splat for cbpf->ebpf progs

On 02/20/2019 06:07 PM, Alexei Starovoitov wrote:
> 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@...kaller.appspotmail.com
>> Signed-off-by: Daniel Borkmann <daniel@...earbox.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 ?

I think macro naming is probably okay imho as used internally as
well from BPF_PROG_RUN(), but I'll improve the comment to state
seccomp specifically as an example there and providing some more
background.

>> +#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
>>