Re: [PATCH bpf-next v2] Update perf ring buffer to prevent corruption

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Nov 05, 2020 at 08:19:47PM -0800, Alexei Starovoitov wrote:

> > Subject: [PATCH] Update perf ring buffer to prevent corruption from
> >  bpf_perf_output_event()

$Subject is broken, it lacks subsystem prefix.

> >
> > The bpf_perf_output_event() helper takes a sample size parameter of u64, but
> > the underlying perf ring buffer uses a u16 internally. This 64KB maximum size
> > has to also accommodate a variable sized header. Failure to observe this
> > restriction can result in corruption of the perf ring buffer as samples
> > overlap.
> >
> > Track the sample size and return -E2BIG if too big to fit into the u16
> > size parameter.
> >
> > Signed-off-by: Kevin Sheldrake <kevin.sheldrake@microsoft.com>

> > ---
> >  include/linux/perf_event.h |  2 +-
> >  kernel/events/core.c       | 40 ++++++++++++++++++++++++++--------------
> >  2 files changed, 27 insertions(+), 15 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 0c19d27..b9802e5 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -1060,7 +1060,7 @@ extern void perf_output_sample(struct perf_output_handle *handle,
> >                                struct perf_event_header *header,
> >                                struct perf_sample_data *data,
> >                                struct perf_event *event);
> > -extern void perf_prepare_sample(struct perf_event_header *header,
> > +extern int perf_prepare_sample(struct perf_event_header *header,
> >                                 struct perf_sample_data *data,
> >                                 struct perf_event *event,
> >                                 struct pt_regs *regs);
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index da467e1..c6c4a3c 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -7016,15 +7016,17 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
> >         return callchain ?: &__empty_callchain;
> >  }
> >
> > -void perf_prepare_sample(struct perf_event_header *header,
> > +int perf_prepare_sample(struct perf_event_header *header,
> >                          struct perf_sample_data *data,
> >                          struct perf_event *event,
> >                          struct pt_regs *regs)

please re-align things.

> >  {
> >         u64 sample_type = event->attr.sample_type;
> > +       u32 header_size = header->size;
> > +
> >
> >         header->type = PERF_RECORD_SAMPLE;
> > -       header->size = sizeof(*header) + event->header_size;
> > +       header_size = sizeof(*header) + event->header_size;
> >
> >         header->misc = 0;
> >         header->misc |= perf_misc_flags(regs);
> > @@ -7042,7 +7044,7 @@ void perf_prepare_sample(struct perf_event_header *header,
> >
> >                 size += data->callchain->nr;
> >
> > -               header->size += size * sizeof(u64);
> > +               header_size += size * sizeof(u64);
> >         }
> >
> >         if (sample_type & PERF_SAMPLE_RAW) {
> > @@ -7067,7 +7069,7 @@ void perf_prepare_sample(struct perf_event_header *header,
> >                         size = sizeof(u64);
> >                 }
> >
> > -               header->size += size;
> > +               header_size += size;
> >         }

AFAICT perf_raw_frag::size is a u32, so the above addition can already
fully overflow. Best is probably to make header_size u64 and delay that
until the final tally below.

> >
> >         if (sample_type & PERF_SAMPLE_BRANCH_STACK) {

> > @@ -7162,14 +7164,20 @@ void perf_prepare_sample(struct perf_event_header *header,
> >                  * Make sure this doesn't happen by using up to U16_MAX bytes
> >                  * per sample in total (rounded down to 8 byte boundary).
> >                  */
> > -               size = min_t(size_t, U16_MAX - header->size,
> > +               size = min_t(size_t, U16_MAX - header_size,
> >                              event->attr.aux_sample_size);
> >                 size = rounddown(size, 8);
> >                 size = perf_prepare_sample_aux(event, data, size);
> >
> > -               WARN_ON_ONCE(size + header->size > U16_MAX);
> > -               header->size += size;
> > +               WARN_ON_ONCE(size + header_size > U16_MAX);
> > +               header_size += size;
> >         }
> > +
> > +       if (header_size > U16_MAX)
> > +               return -E2BIG;
> > +
> > +       header->size = header_size;
> > +
> >         /*
> >          * If you're adding more sample types here, you likely need to do
> >          * something about the overflowing header::size, like repurpose the
> > @@ -7179,6 +7187,8 @@ void perf_prepare_sample(struct perf_event_header *header,
> >          * do here next.
> >          */
> >         WARN_ON_ONCE(header->size & 7);
> > +
> > +       return 0;
> >  }
> >
> >  static __always_inline int
> > @@ -7196,7 +7206,9 @@ __perf_event_output(struct perf_event *event,
> >         /* protect the callchain buffers */
> >         rcu_read_lock();
> >
> > -       perf_prepare_sample(&header, data, event, regs);
> > +       err = perf_prepare_sample(&header, data, event, regs);
> > +       if (err)
> > +               goto exit;

This is wrong I think. The thing is that when output_begin() below
returns an error, there either is no buffer (in which case we can't do
anything much at all) or it will have incremented rb->lost.

This OTOH will completely fail to report the loss. The error case here
is to immediately try and emit a RECORD_LOST event, but then please also
consider these patches:

  https://lkml.kernel.org/r/20201030151345.540479897@infradead.org

(which I'll be pushing into tip/perf/urgent soonish)

> >
> >         err = output_begin(&handle, event, header.size);
> >         if (err)
> > --
> > 2.7.4
> >