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
- In snapshot/overwrite mode, stop() without PERF_EF_UPDATE leaves the
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.