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

peterz
2020-08-05T19:50:23Z

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;