[Patch v4 4/5] perf/x86/intel/lbr: Limit branch snapshots to valid entries

[Dapeng Mi]

The branch snapshot callback used by bpf_get_branch_snapshot() always
copies x86_pmu.lbr_nr entries from cpuc->lbr_entries[], even when the
last LBR read kept fewer branches. The entries past lbr_stack.nr hold
invalid branches which end up reaching BPF programs.

Limit copying to lbr_stack.nr entries and also reset lbr_stack.nr when
no active LBR events or LBR hardware is owned by guest. So the stale or
guest LBR stack data can't be leaked.

For the case where all LBR events are PEBS events, hardware LBR reads are
currently skipped; that can leave stale LBR stack data from previous PEBS
sampling in place and may be inconsistent with the intended semantics of
bpf_get_branch_snapshot(). This will be addressed in a follow-up patch.

Fixes: c22ac2a3d4bd ("perf: Enable branch record for software events")
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/sashiko-outbox-163130@xxxxxxxxxx/
Co-developed-by: Sandipan Das <sandipan.das@xxxxxxx>
Signed-off-by: Sandipan Das <sandipan.das@xxxxxxx>
Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
---
 arch/x86/events/intel/core.c | 2 +-
 arch/x86/events/intel/lbr.c  | 9 +++++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 0a34d674df59..5dfb9e9156e8 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -2822,7 +2822,7 @@ __intel_pmu_snapshot_branch_stack(struct perf_branch_entry *entries,
 	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
 
 	intel_pmu_lbr_read();
-	cnt = min_t(unsigned int, cnt, x86_pmu.lbr_nr);
+	cnt = min_t(unsigned int, cnt, cpuc->lbr_stack.nr);
 
 	memcpy(entries, cpuc->lbr_entries, sizeof(struct perf_branch_entry) * cnt);
 	intel_pmu_enable_all(0);
diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index c7e295db9410..ac1759a71288 100644
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -1024,14 +1024,19 @@ void intel_pmu_lbr_read(void)
 {
 	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
 
+	if (!cpuc->lbr_users || vlbr_exclude_host()) {
+		/* Reset lbr_stack.nr to prevent stale data leakage. */
+		cpuc->lbr_stack.nr = 0;
+		return;
+	}
+
 	/*
 	 * Don't read when all LBRs users are using adaptive PEBS.
 	 *
 	 * This could be smarter and actually check the event,
 	 * but this simple approach seems to work for now.
 	 */
-	if (!cpuc->lbr_users || vlbr_exclude_host() ||
-	    cpuc->lbr_users == cpuc->lbr_pebs_users)
+	if (cpuc->lbr_users == cpuc->lbr_pebs_users)
 		return;
 
 	x86_pmu.lbr_read(cpuc);
-- 
2.34.1