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charset="utf-8" Consider the following race: -------- o OP_CTL contains stale value: OP_CTL[Val]=3D1, OP_CTL[En]=3D0 o A new IBS OP event is being added o [P]: Process context, [N]: NMI context [P] perf_ibs_add(event) { [P] if (test_and_set_bit(IBS_ENABLED, pcpu->state)) [P] return; [P] /* pcpu->state =3D IBS_ENABLED */ [P] [P] pcpu->event =3D event; [P] [P] perf_ibs_start(event) { [P] set_bit(IBS_STARTED, pcpu->state); [P] /* pcpu->state =3D IBS_ENABLED | IBS_STARTED */ [P] clear_bit(IBS_STOPPING, pcpu->state); [P] /* pcpu->state =3D IBS_ENABLED | IBS_STARTED */ [N] --> NMI due to genuine FETCH event. perf_ibs_handle_irq() [N] called for OP PMU as well. [N] [N] perf_ibs_handle_irq(perf_ibs) { [N] event =3D pcpu->event; /* See line 6 */ [N] [N] if (!test_bit(IBS_STARTED, pcpu->state)) /* false */ [N] return 0; [N] [N] if (WARN_ON_ONCE(!event)) /* false */ [N] goto fail; [N] [N] if (!(*buf++ & perf_ibs->valid_mask)) /* false due to stale [N] * IBS_OP_CTL value */ [N] goto fail; [N] [N] ... [N] [N] perf_ibs_enable_event() // *Accidentally* enable the event. [N] } [N] [N] /* [N] * Repeated NMIs may follow due to accidentally enabled IBS OP [N] * event if the sample period is very low. It could also lead [N] * to pcpu->state corruption if the event gets throttled due [N] * to too frequent NMIs. [N] */ [P] perf_ibs_enable_event(); [P] } [P] } -------- We cannot safely clear IBS_{FETCH|OP}_CTL while disabling the event, because the register might be read again later. So, clear the register in the enable path - before we update pcpu->state and enable the event. This guarantees that any NMI that lands in the gap finds Val=3D0 and bails out cleanly. Signed-off-by: Ravi Bangoria --- arch/x86/events/amd/ibs.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c index 004226b52ac7..32e6456cb5e5 100644 --- a/arch/x86/events/amd/ibs.c +++ b/arch/x86/events/amd/ibs.c @@ -493,6 +493,14 @@ static void perf_ibs_start(struct perf_event *event, i= nt flags) } config |=3D period >> 4; =20 + /* + * Reset the IBS_{FETCH|OP}_CTL MSR before updating pcpu->state. + * Doing so prevents a race condition in which an NMI due to other + * source might accidentally activate the event before we enable + * it ourselves. + */ + perf_ibs_disable_event(perf_ibs, hwc, 0); + /* * Set STARTED before enabling the hardware, such that a subsequent NMI * must observe it. --=20 2.43.0