Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples

Jin, Yao
2020-08-06T02:26:36Z

Hi Peter,

On 8/5/2020 8:44 PM, peterz@infradead.org wrote:
> On Wed, Aug 05, 2020 at 10:15:26AM +0800, Jin, Yao wrote:
>> Hi Peter,
>>
>> On 8/4/2020 7:49 PM, peterz@infradead.org wrote:
>>> On Fri, Jul 31, 2020 at 10:56:17AM +0800, Jin Yao wrote:
>>>> @@ -6973,7 +6973,8 @@ static struct perf_callchain_entry __empty_callchain = { .nr = 0, };
>>>> struct perf_callchain_entry *
>>>> perf_callchain(struct perf_event *event, struct pt_regs *regs)
>>>> {
>>>> - bool kernel = !event->attr.exclude_callchain_kernel;
>>>> + bool kernel = !event->attr.exclude_callchain_kernel &&
>>>> + !event->attr.exclude_kernel;
>>>
>>> This seems weird; how can we get there. Also it seems to me that if you
>>> have !exclude_callchain_kernel you already have permission for kernel
>>> bits, so who cares.
>>>
>>
>> In perf tool, exclude_callchain_kernel is set to 1 when perf-record only
>> collects the user callchains and exclude_kernel is set to 1 when events are
>> configured to run in user space.
>>
>> So if an event is configured to run in user space, that should make sense we
>> don't need it's kernel callchains.
>>
>> But it seems to me there is no code logic in perf tool which can make sure
>> !exclude_callchain_kernel -> !exclude_kernel.
>>
>> Jiri, Arnaldo, is my understanding correct?
>
> What the perf tool does or does not do is irrelevant. It is a valid,
> (albeit slightly silly) configuration to have:
>
> exclude_kernel && !exclude_callchain_kernel
>
> You're now saying that when you configure things like this you're not
> allowed kernel IPs, that's wrong I think.
>
> Also, !exclude_callchain_kernel should require privilidge, whcih needs
> fixing, see below.
>

I see you add '!exclude_callchain_kernel' check before perf_allow_kernel() at syscall entry in below
code.

So if we allow callchain_kernel collection that means we allow kernel by default. That makes sense,
thanks!

>> So the new code looks like:
>>
>> if (event->attr.exclude_kernel && !user_mode(regs)) {
>> if (!(current->flags & PF_KTHREAD)) {
>> regs_fake = task_pt_regs(current);
>> if (!regs_fake)
>> instruction_pointer_set(regs, -1L);
>> } else {
>> instruction_pointer_set(regs, -1L);
>> }
>
> Again:
>
> if (!(current->flags & PF_KTHREAD))
> regs_fake = task_pt_regs(current);
>
> if (!regs_fake)
> instruction_pointer_set(regs, -1L);
>
> Is much simpler and more readable.
>

Yes, agree. Your code is much simpler and better.

>>>> + if ((header->misc & PERF_RECORD_MISC_CPUMODE_MASK) ==
>>>> + PERF_RECORD_MISC_KERNEL) {
>>>> + header->misc &= ~PERF_RECORD_MISC_CPUMODE_MASK;
>>>> + header->misc |= PERF_RECORD_MISC_USER;
>>>> + }
>>>
>>> Why the conditional? At this point it had better be unconditionally
>>> user, no?
>>>
>>> headers->misc &= ~PERF_RECORD_MISC_CPUMODE_MASK;
>>> headers->misc |= PERF_RECORD_MISC_USER;
>>>
>>
>> #define PERF_RECORD_MISC_CPUMODE_MASK (7 << 0)
>> #define PERF_RECORD_MISC_CPUMODE_UNKNOWN (0 << 0)
>> #define PERF_RECORD_MISC_KERNEL (1 << 0)
>> #define PERF_RECORD_MISC_USER (2 << 0)
>> #define PERF_RECORD_MISC_HYPERVISOR (3 << 0)
>> #define PERF_RECORD_MISC_GUEST_KERNEL (4 << 0)
>> #define PERF_RECORD_MISC_GUEST_USER (5 << 0)
>>
>> If we unconditionally set user, it will reset for hypervisor, guest
>> kernel and guest_user.
>
> At the same time :u had better not get any of those either. Which seems
> to suggest we're going about this wrong.
>
> Also, if we call this before perf_misc_flags() we don't need to fix it
> up.
>
> How's this?
>
> ---
> kernel/events/core.c | 38 +++++++++++++++++++++++++++++++++-----
> 1 file changed, 33 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 7c436d705fbd..3e4e328b521a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6988,23 +6988,49 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
> return callchain ?: &__empty_callchain;
> }
>
> +/*
> + * Due to interrupt latency (skid), we may enter the kernel before taking the
> + * PMI, even if the PMU is configured to only count user events. To avoid
> + * leaking kernel addresses, use task_pt_regs(), when available.
> + */
> +static struct pt_regs *sanitize_sample_regs(struct perf_event *event, struct pt_regs *regs)
> +{
> + struct pt_regs *sample_regs = regs;
> +
> + /* user only */
> + if (!event->attr.exclude_kernel || !event->attr.exclude_hv ||
> + !event->attr.exclude_host || !event->attr.exclude_guest)
> + return sample_regs;
> +

Is this condition correct?

Say counting user event on host, exclude_kernel = 1 and exclude_host = 0. It will go "return
sample_regs" path.

> + if (sample_regs(regs))
> + return sample_regs;
> +

In your another mail, you said it should be:

if (user_regs(regs))
return sample_regs;

> + if (!(current->flags & PF_KTHREAD)) {

No '{', you mentioned in another mail.

> + sample_regs = task_pt_regs(current);
> + else
> + instruction_pointer_set(regs, -1L);
> +
> + return sample_regs;
> +}
> +
> void perf_prepare_sample(struct perf_event_header *header,
> struct perf_sample_data *data,
> struct perf_event *event,
> struct pt_regs *regs)
> {
> + struct pt_regs *sample_regs = sanitize_sample_regs(event, regs);
> u64 sample_type = event->attr.sample_type;
>
> header->type = PERF_RECORD_SAMPLE;
> header->size = sizeof(*header) + event->header_size;
>
> header->misc = 0;
> - header->misc |= perf_misc_flags(regs);
> + header->misc |= perf_misc_flags(sample_regs);
>
> __perf_event_header__init_id(header, data, event);
>
> if (sample_type & PERF_SAMPLE_IP)
> - data->ip = perf_instruction_pointer(regs);
> + data->ip = perf_instruction_pointer(sample_regs);
>
> if (sample_type & PERF_SAMPLE_CALLCHAIN) {
> int size = 1;
> @@ -7054,9 +7080,10 @@ void perf_prepare_sample(struct perf_event_header *header,
> header->size += size;
> }
>
> - if (sample_type & (PERF_SAMPLE_REGS_USER | PERF_SAMPLE_STACK_USER))
> + if (sample_type & (PERF_SAMPLE_REGS_USER | PERF_SAMPLE_STACK_USER)) {
> perf_sample_regs_user(&data->regs_user, regs,
> &data->regs_user_copy);
> + }
>
> if (sample_type & PERF_SAMPLE_REGS_USER) {
> /* regs dump ABI info */
> @@ -7099,7 +7126,7 @@ void perf_prepare_sample(struct perf_event_header *header,
> /* regs dump ABI info */
> int size = sizeof(u64);
>
> - perf_sample_regs_intr(&data->regs_intr, regs);
> + perf_sample_regs_intr(&data->regs_intr, sample_regs);
>
> if (data->regs_intr.regs) {
> u64 mask = event->attr.sample_regs_intr;
> @@ -11609,7 +11636,8 @@ SYSCALL_DEFINE5(perf_event_open,
> if (err)
> return err;
>
> - if (!attr.exclude_kernel) {
> + if (!attr.exclude_kernel || !attr.exclude_callchain_kernel ||
> + !attr.exclude_hv || !attr.exclude_host || !attr.exclude_guest) {
> err = perf_allow_kernel(&attr);
> if (err)
> return err;
>

I can understand the conditions "!attr.exclude_kernel || !attr.exclude_callchain_kernel".

But I'm not very sure about the "!attr.exclude_hv || !attr.exclude_host || !attr.exclude_guest".

On host, exclude_hv = 1, exclude_guest = 1 and exclude_host = 0, right?

So even exclude_kernel = 1 but exclude_host = 0, we will still go perf_allow_kernel path. Please
correct me if my understanding is wrong.

Thanks
Jin Yao