In the Linux kernel, the following vulnerability has been resolved: perf/x86/intel/pt: Fix stop/start with no update If pt_event_stop() is

In the Linux kernel, the following vulnerability has been resolved:

perf/x86/intel/pt: Fix stop/start with no update

If pt_event_stop() is called without PERF_EF_UPDATE flag, then
perf_aux_output_end() is not called. A subsequent call to pt_event_start()
will call perf_aux_output_begin() again which violates the rule against
nesting and triggers a WARNING in perf_aux_output_begin().

Originally, pt_event_stop() was never called without PERF_EF_UPDATE,
because the only code paths to do so are from event overflow, and Intel PT
does not do that.

However the introduction of group throttling by commit 9734e25fbf5ae
("perf: Fix the throttle logic for a group") meant that an Intel PT event
could be throttled if it was part of a group. Throttling calls PMU
->stop() / ->start() callbacks without flags.

An example is when AUX area sampling is used. The following commands
hit the issue:

echo 10000 > /proc/sys/kernel/perf_event_max_sample_rate

perf record -F32000 --aux-sample -e '{intel_pt//u,cycles:u}'
-- bash -c 'for i in seq 1 100000 ; do true ; done'

Use PERF_HES_UPTODATE to track whether perf_aux_output_begin() and
perf_aux_output_end() are balanced. A cleared PERF_HES_UPTODATE bit
indicates that an AUX output context is still open.

Amend pt_event_start() / pt_event_stop() accordingly so that begin/end
stay balanced:

• In non-snapshot mode, stop() always closes the buffer (the buffer may
  have run out of space, and that accounting is done by the update), so
  a following start() opens a fresh one as before.

• In snapshot/overwrite mode, stop() without PERF_EF_UPDATE leaves the
  buffer open so that pt_event_snapshot_aux() can still copy from it,
  and start() then only re-enables tracing instead of calling
  perf_aux_output_begin() again.

Note that pt_event_del() calls pt_event_stop() with PERF_EF_UPDATE flag set
(as is required by the documentation), so a final call to
perf_aux_output_end() is assured.