[PATCH v4] perf: Fix null pointer access in is_include_guest_event()

[Vinay Belgaumkar <vinay.belgaumkar@intel.com>]

A kernel panic is observed when an application has a perf connection open
and the driver managing the pmu event unbinds. The issue occurs when
perf_pmu_unregister() is called while there is an active perf session. The
null pointer error is seen when the event is freed through __free_event()
and mediated_pmu_unaccount_event is called. There is a missing null check
inside is_include_guest_event(). Also call mediated_pmu_unaccount_event()
inside __pmu_detach_event() similar to __free_event().

Signature of the failure:

 Oops: general protection fault, kernel NULL pointer dereference
 ...
 RIP: 0010:_free_event+0x3e/0x370
 ...
 Call Trace:
 ...
 perf_event_release_kernel+0x260/0x2d0
 perf_release+0x12/0x20

While here, also add a null check inside perf_event_read_local() which can
cause a similar crash. Sashiko pointed out a possible race between when we
check for revoked state here and then actually access event-pmu inside
__perf_event_read_cpu(). So, add another null check there.

v2: Add another check to prevent null access (sashiko)
v3: Use READ_ONCE for checking pmu state (sashiko)
v4: READ_ONCE for the pmu check as well (sashiko)

Fixes: eff95e170275 ("perf: Add APIs to create/release mediated guest vPMUs")
Cc: Alexander Kanevskiy <alexander.kanevskiy@intel.com>
Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
Assisted-by: Claude:Claude-Sonnet-5
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
---
 kernel/events/core.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 94ca277e0819..4f8c5191ef2c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4794,6 +4794,9 @@ static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
 	if ((unsigned)event_cpu >= nr_cpu_ids)
 		return event_cpu;
 
+	if (!READ_ONCE(event->pmu))
+		return -ENODEV;
+
 	if (event->group_caps & PERF_EV_CAP_READ_SCOPE) {
 		const struct cpumask *cpumask = perf_scope_cpu_topology_cpumask(event->pmu->scope, event_cpu);
 
@@ -4917,6 +4920,11 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
 		goto out;
 	}
 
+	if (READ_ONCE(event->state) <= PERF_EVENT_STATE_REVOKED) {
+		ret = -ENODEV;
+		goto out;
+	}
+
 	/*
 	 * Get the event CPU numbers, and adjust them to local if the event is
 	 * a per-package event that can be read locally
@@ -6350,6 +6358,9 @@ static DEFINE_MUTEX(perf_mediated_pmu_mutex);
 /* !exclude_guest event of PMU with PERF_PMU_CAP_MEDIATED_VPMU */
 static inline bool is_include_guest_event(struct perf_event *event)
 {
+	if (!event->pmu)
+		return false;
+
 	if ((event->pmu->capabilities & PERF_PMU_CAP_MEDIATED_VPMU) &&
 	    !event->attr.exclude_guest)
 		return true;
@@ -12977,6 +12988,7 @@ static void __pmu_detach_event(struct pmu *pmu, struct perf_event *event,
 	exclusive_event_destroy(event);
 	module_put(pmu->module);
 
+	mediated_pmu_unaccount_event(event);
 	event->pmu = NULL; /* force fault instead of UAF */
 }
 
-- 
2.38.1