[PATCH v2 4/5] perf/amd/ibs: Avoid calling perf_allow_kernel() from the IBS NMI handler

[Ravi Bangoria]

Calling perf_allow_kernel() from the NMI context is unsafe and could be
fatal. Capture the permission at event-initialization time by storing it
in event->hw.flags, and have the NMI handler rely on that cached flag
instead of making the call directly.

Reported-by: Sadasivan Shaiju <sadasivan.shaiju2@xxxxxxx>
Fixes: 50a53b60e141d ("perf/amd/ibs: Prevent leaking sensitive data to userspace")
Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
---
 arch/x86/events/amd/ibs.c          | 5 ++++-
 arch/x86/events/perf_event_flags.h | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
index 05b7c9f2ec33..004226b52ac7 100644
--- a/arch/x86/events/amd/ibs.c
+++ b/arch/x86/events/amd/ibs.c
@@ -313,6 +313,9 @@ static int perf_ibs_init(struct perf_event *event)
 	if (ret)
 		return ret;
 
+	if (perf_allow_kernel())
+		hwc->flags |= PERF_X86_EVENT_UNPRIVILEGED;
+
 	if (hwc->sample_period) {
 		if (config & perf_ibs->cnt_mask)
 			/* raw max_cnt may not be set */
@@ -1349,7 +1352,7 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
 	 * unprivileged users.
 	 */
 	if ((event->attr.sample_type & PERF_SAMPLE_RAW) &&
-	    perf_allow_kernel()) {
+	    (hwc->flags & PERF_X86_EVENT_UNPRIVILEGED)) {
 		perf_ibs_phyaddr_clear(perf_ibs, &ibs_data);
 	}
 
diff --git a/arch/x86/events/perf_event_flags.h b/arch/x86/events/perf_event_flags.h
index 70078334e4a3..47f84ee8f540 100644
--- a/arch/x86/events/perf_event_flags.h
+++ b/arch/x86/events/perf_event_flags.h
@@ -23,3 +23,4 @@ PERF_ARCH(PEBS_LAT_HYBRID,	0x0020000) /* ld and st lat for hybrid */
 PERF_ARCH(NEEDS_BRANCH_STACK,	0x0040000) /* require branch stack setup */
 PERF_ARCH(BRANCH_COUNTERS,	0x0080000) /* logs the counters in the extra space of each branch */
 PERF_ARCH(ACR,			0x0100000) /* Auto counter reload */
+PERF_ARCH(UNPRIVILEGED,		0x0200000) /* Unprivileged event (wrt perf_allow_kernel()) */
-- 
2.43.0