Messages in this thread Patch in this message | | | Date | Wed, 5 Aug 2020 14:44:54 +0200 | From | peterz@infradea ... | Subject | Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples |
| |
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.
> 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.
> > > + 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; + + if (sample_regs(regs)) + return sample_regs; + + if (!(current->flags & PF_KTHREAD)) { + 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;
| |