[PATCH v6 0/2] perf: Add some null ptr checks

[PATCH v6 0/2] perf: Add some null ptr checks

[Vinay Belgaumkar]

This series fixes a crash seen when the PMU ptr is freed while
there is an open perf connection.

Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>

Vinay Belgaumkar (2):
  perf: Fix null pointer access in is_include_guest_event()
  perf: Add checks to prevent null ptr access

 kernel/events/core.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

-- 
2.38.1

[PATCH v6 1/2] perf: Fix null pointer access in is_include_guest_event()

[Vinay Belgaumkar]

A typical module unload occurring event when there is an active perf
connection leads to freeing of the pmu pointer. The call log is something
like:
 ..
 __pmu_detach_event
 pmu_detach_event
 pmu_detach_events
 perf_pmu_unregister
 ..

__pmu_detach_event() sets event->pmu to null. When the perf connection
finally is closed, the following stack trace is observed:

 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

A call to mediated_pmu_unaccount_event() inside _free_event() is the root
cause of this crash. Adding a check inside is_include_guest_event() ensures
we don't accidentally access a null pmu ptr. In addition to this, we will
now call mediated_pmu_unaccount_event() before clearing the pmu ptr so that
nr_include_guest_events counts are maintained correctly.

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)
v5: Improve commit message and split patch into 2 (Dapeng)

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 | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 94ca277e0819..f19d27ac411d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6350,6 +6350,9 @@ 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 +12980,7 @@ 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

[PATCH v6 2/2] perf: Add checks to prevent null ptr access

[Vinay Belgaumkar]

Sashiko recommended some additional checks to prevent null pointer
access. Check for revoked states inside perf_event_read_local(), as
the pmu event may have already been freed at this point. Add a null
check inside __perf_event_read_cpu() as well before accessing the pmu
ptr.

Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
---
 kernel/events/core.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f19d27ac411d..bb90034a2054 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4788,14 +4788,19 @@ static inline const struct cpumask *perf_scope_cpu_topology_cpumask(unsigned int
 
 static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
 {
+	struct pmu *pmu = READ_ONCE(event->pmu);
 	int local_cpu = smp_processor_id();
 	u16 local_pkg, event_pkg;
 
 	if ((unsigned)event_cpu >= nr_cpu_ids)
 		return event_cpu;
 
+	if (!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);
+		const struct cpumask *cpumask = perf_scope_cpu_topology_cpumask(pmu->scope,
+										event_cpu);
 
 		if (cpumask && cpumask_test_cpu(local_cpu, cpumask))
 			return local_cpu;
@@ -4917,6 +4922,11 @@ 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
-- 
2.38.1

Re: [PATCH v6 1/2] perf: Fix null pointer access in is_include_guest_event()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The commit message claims that `READ_ONCE` was added to `is_include_guest_event()` for checking the `pmu` state, but the code lacks this instruction.
--

commit 7c0504a46d493eec58eae8ca000ce324320508fd
Author: Vinay Belgaumkar <vinay.belgaumkar@intel.com>

perf: Fix null pointer access in is_include_guest_event()

This commit addresses a null pointer dereference in is_include_guest_event()
caused by a module unload occurring while a perf connection is active. It adds
a null check for the pmu pointer and calls mediated_pmu_unaccount_event()
before clearing the pointer to maintain the event counts correctly.

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a6c8e38a31104..7777e82aad5e7 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6350,6 +6350,9 @@ 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) &&

[Severity: Medium]
The commit message claims that READ_ONCE was added for checking the pmu state,
but the code lacks this instruction here.

Without READ_ONCE(), the compiler is free to issue two independent memory loads
of event->pmu: one for the null check and one for accessing capabilities.

Could this leave a potential double-read hazard if event->pmu is concurrently
nullified by __pmu_detach_event() while is_include_guest_event() is executing?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260902141824.733112-1-vinay.belgaumkar@intel.com?part=1

Re: [PATCH v6 2/2] perf: Add checks to prevent null ptr access

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The lockless checks for revoked state and NULL PMU pointers are insufficient to prevent a Use-After-Free of the PMU structure or a NULL pointer dereference during concurrent unregistration.
--

commit 4a985bf8ccfd6853c89a279d4b7181e83c33bcef
Author: Vinay Belgaumkar <vinay.belgaumkar@intel.com>

perf: Add checks to prevent null ptr access

This commit adds a check for revoked states in perf_event_read_local() and
a null pointer check for the PMU in __perf_event_read_cpu().

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 7777e82aad5e7..059f82f0cadd4 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4788,14 +4788,19 @@ static inline const struct cpumask *perf_scope_cpu_topology_cpumask(unsigned int
>  
>  static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
>  {
> +	struct pmu *pmu = READ_ONCE(event->pmu);
>  	int local_cpu = smp_processor_id();
>  	u16 local_pkg, event_pkg;
>  
>  	if ((unsigned)event_cpu >= nr_cpu_ids)
>  		return event_cpu;
>  
> +	if (!pmu)
> +		return -ENODEV;
> +

[Severity: High]
This is a pre-existing issue, but does this lockless check provide a false
sense of security against concurrent PMU unregistration? A concurrent
perf_pmu_unregister() can set event->pmu = NULL and free the PMU memory. Since
this check relies on a cached pointer without holding locks that serialize
against unregistration (e.g., ctx->lock), the PMU can be freed immediately
after this check passes.

>  	if (event->group_caps & PERF_EV_CAP_READ_SCOPE) {
> -		const struct cpumask *cpumask = perf_scope_cpu_topology_cpumask(event->pmu->scope, event_cpu);
> +		const struct cpumask *cpumask = perf_scope_cpu_topology_cpumask(pmu->scope,
> +										event_cpu);

[Severity: High]
This is a pre-existing issue, but can this lead to a Use-After-Free when
accessing pmu->scope? If the PMU is unregistered and freed by another CPU
after we cache it but before we dereference it here, pmu->scope will read from
freed memory.

[ ... ]

> @@ -4917,6 +4922,11 @@ 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;
> +	}
> +

[Severity: High]
This is a pre-existing issue, but is this check vulnerable to a TOCTOU race?
The event state can change immediately after this check if the event context is
inactive or belongs to a different CPU, allowing a concurrent unregistration to
detach the event and free the PMU while perf_event_read_local() continues
executing. This leaves the subsequent call to event->pmu->read(event)
unprotected, potentially resulting in a NULL pointer dereference or
Use-After-Free.

>  	/*
>  	 * Get the event CPU numbers, and adjust them to local if the event is
>  	 * a per-package event that can be read locally

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260902141824.733112-1-vinay.belgaumkar@intel.com?part=2