[Patch v6 12/20] perf/x86/intel: Use a dedicated LBR stack for BPF snapshotting

[Dapeng Mi]

BPF LBR snapshotting currently reuses the default LBR stack
(cpuc->__lbr_stack) when storing sampled data. This is unsafe when the
BPF helper runs in PMI context via bpf_overflow_handler(): it can
overwrite the cached LBR state for active PEBS LBR events, causing
incorrect LBR data to be reported to userspace. When the PEBS and BPF
stacks have different depths, this can also lead to out-of-bounds reads
when the LBR stack is later exported.

Use a dedicated LBR stack for BPF snapshotting so the BPF path no
longer clobbers the shared LBR state used by other perf events.

Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Fixes: 425507fa5f32 ("perf/x86/intel/lbr: Optimize v4 LBR unfreezing")
Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
---
 arch/x86/events/intel/core.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index b7045c05b736..a3aaa98108cd 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -2819,17 +2819,25 @@ static void intel_pmu_enable_all(int added)
 	__intel_pmu_enable_all(false);
 }
 
+struct lbr_snapshot {
+	u64 __lbr_stack[LBR_STACK_SIZE / sizeof(u64)];
+	u64 counters[MAX_LBR_ENTRIES];
+};
+static DEFINE_PER_CPU(struct lbr_snapshot, lbr_snapshot);
+
 static noinline int
 __intel_pmu_snapshot_branch_stack(struct perf_branch_entry *entries,
 				  unsigned int cnt, unsigned long flags,
 				  int pmu_enabled, int lbr_enabled)
 {
-	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+	struct lbr_snapshot *bpf = this_cpu_ptr(&lbr_snapshot);
+	struct perf_branch_stack *lbr_stack =
+		(struct perf_branch_stack *)bpf->__lbr_stack;
 
-	intel_pmu_lbr_read(cpu_lbr_stack(cpuc), cpuc->lbr_counters);
-	cnt = min_t(unsigned int, cnt, cpu_lbr_stack(cpuc)->nr);
+	intel_pmu_lbr_read(lbr_stack, bpf->counters);
+	cnt = min_t(unsigned int, cnt, lbr_stack->nr);
 
-	memcpy(entries, cpu_lbr_stack(cpuc)->entries,
+	memcpy(entries, lbr_stack->entries,
 	       sizeof(struct perf_branch_entry) * cnt);
 
 	/*
-- 
2.34.1