On 11/03/2017 03:31 PM, Josef Bacik wrote:
> On Fri, Nov 03, 2017 at 12:12:13AM +0100, Daniel Borkmann wrote:
>> Hi Josef,
>>
>> one more issue I just noticed, see comment below:
>>
>> On 11/02/2017 03:37 PM, Josef Bacik wrote:
>> [...]
>>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>>> index cdd78a7beaae..dfa44fd74bae 100644
>>> --- a/include/linux/filter.h
>>> +++ b/include/linux/filter.h
>>> @@ -458,7 +458,8 @@ struct bpf_prog {
>>> locked:1, /* Program image locked? */
>>> gpl_compatible:1, /* Is filter GPL compatible? */
>>> cb_access:1, /* Is control block accessed? */
>>> - dst_needed:1; /* Do we need dst entry? */
>>> + dst_needed:1, /* Do we need dst entry? */
>>> + kprobe_override:1; /* Do we override a kprobe? */
>>> kmemcheck_bitfield_end(meta);
>>> enum bpf_prog_type type; /* Type of BPF program */
>>> u32 len; /* Number of filter blocks */
>> [...]
>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>> index d906775e12c1..f8f7927a9152 100644
>>> --- a/kernel/bpf/verifier.c
>>> +++ b/kernel/bpf/verifier.c
>>> @@ -4189,6 +4189,8 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
>>> prog->dst_needed = 1;
>>> if (insn->imm == BPF_FUNC_get_prandom_u32)
>>> bpf_user_rnd_init_once();
>>> + if (insn->imm == BPF_FUNC_override_return)
>>> + prog->kprobe_override = 1;
>>> if (insn->imm == BPF_FUNC_tail_call) {
>>> /* If we tail call into other programs, we
>>> * cannot make any assumptions since they can
>>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>>> index 9660ee65fbef..0d7fce52391d 100644
>>> --- a/kernel/events/core.c
>>> +++ b/kernel/events/core.c
>>> @@ -8169,6 +8169,13 @@ static int perf_event_set_bpf_prog(struct perf_event *event, u32 prog_fd)
>>> return -EINVAL;
>>> }
>>>
>>> + /* Kprobe override only works for kprobes, not uprobes. */
>>> + if (prog->kprobe_override &&
>>> + !(event->tp_event->flags & TRACE_EVENT_FL_KPROBE)) {
>>> + bpf_prog_put(prog);
>>> + return -EINVAL;
>>> + }
>>
>> Can we somehow avoid the prog->kprobe_override flag here completely
>> and also same in the perf_event_attach_bpf_prog() handler?
>>
>> Reason is that it's not reliable for bailing out this way: Think of
>> the main program you're attaching doesn't use bpf_override_return()
>> helper, but it tail-calls into other BPF progs that make use of it
>> instead. So above check would be useless and will fail and we continue
>> to attach the prog for probes where it's not intended to be used.
>>
>> We've had similar issues in the past e.g. c2002f983767 ("bpf: fix
>> checking xdp_adjust_head on tail calls") is just one of those. Thus,
>> can we avoid the flag altogether and handle such error case differently?
>
> So if I'm reading this right there's no way to know what we'll tail call at any
> given point, so I need to go back to my previous iteration of this patch and
> always save the state of the kprobe in the per-cpu variable to make sure we
> don't use bpf_override_return in the wrong case?
Yeah.
> The tail call functions won't be in the BPF_PROG_ARRAY right? It'll be just
> some other arbitrary function? If that's the case then we really need something
> like this
With BPF_PROG_ARRAY you mean BPF_MAP_TYPE_PROG_ARRAY or the prog array
for the tracing/multiprog attach point? The program you're calling into
is inside the BPF_MAP_TYPE_PROG_ARRAY map, but can change at any time
and can have nesting as well.
> https://patchwork.kernel.org/patch/10034815/
>
> and I need to bring that back right? Thanks,
I'm afraid so. The thing with skb cb_access which was brought up there is
that once you have a tail call in the prog you cannot make any assumptions
anymore, therefore the cb_access flag is set to 1 so we save/restore for
those cases precautionary since it could be accessed or not later on. In
your case I think this wouldn't work since legitimate bpf kprobes progs could
use tail calls today, so setting prog->kprobe_override there would prevent
attaching for non-kprobes due to subsequent flags & TRACE_EVENT_FL_KPROBE
check.
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