Re: perf: bug in rdpmc/mmap accounting after exec

[Peter Zijlstra]

On Wed, Jul 26, 2017 at 03:39:01PM -0400, Vince Weaver wrote:
> Hello
> 
> so one last bug found by the PAPI testsuite.
> 
> This one involves the rdpmc auto-disable on last unmap of an event 
> feature.
> 
> Failing test case:
> 
>       fd=perf_event_open();
>       addr=mmap(fd);
>       exec()  // without closing or unmapping the event
>       fd=perf_event_open();
>       addr=mmap(fd);
>       rdpmc() // GPFs due to rdpmc being disabled
> 
> I won't pretend to be able to follow the rdpmc disabling code, but if I 
> add some printks it looks like 
>       current->mm->context.perf_rdpmc_allowed
> isn't properly being reset on exec?
> 
> In fact, current->mm->context.perf_rdpmc_allowed goes negative which seems
> like it shouldn't happen?
> 
> Anyway, a test case for this can be found in the perf_event_tests,
>       tests/rdpmc/rdpmc_exec_papi

Good find that...

The below seems to fix that for me.

---
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 8e3db8f642a7..3186f6e081c4 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2114,11 +2114,14 @@ static void refresh_pce(void *ignored)
        load_mm_cr4(this_cpu_read(cpu_tlbstate.loaded_mm));
 }
 
-static void x86_pmu_event_mapped(struct perf_event *event)
+static void x86_pmu_event_mapped(struct perf_event *event, struct 
vm_area_struct *vma)
 {
+       struct mm_struct *mm;
+
        if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
                return;
 
+       mm = vma->vm_mm;
        /*
         * This function relies on not being called concurrently in two
         * tasks in the same mm.  Otherwise one task could observe
@@ -2129,22 +2132,21 @@ static void x86_pmu_event_mapped(struct perf_event 
*event)
         * For now, this can't happen because all callers hold mmap_sem
         * for write.  If this changes, we'll need a different solution.
         */
-       lockdep_assert_held_exclusive(&current->mm->mmap_sem);
+       lockdep_assert_held_exclusive(&mm->mmap_sem);
 
-       if (atomic_inc_return(&current->mm->context.perf_rdpmc_allowed) == 1)
-               on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1);
+       if (atomic_inc_return(&mm->context.perf_rdpmc_allowed) == 1)
+               on_each_cpu_mask(mm_cpumask(mm), refresh_pce, NULL, 1);
 }
 
-static void x86_pmu_event_unmapped(struct perf_event *event)
+static void x86_pmu_event_unmapped(struct perf_event *event, struct 
vm_area_struct *vma)
 {
-       if (!current->mm)
-               return;
+       struct mm_struct *mm = vma->vm_mm;
 
        if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
                return;
 
-       if (atomic_dec_and_test(&current->mm->context.perf_rdpmc_allowed))
-               on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1);
+       if (atomic_dec_and_test(&mm->context.perf_rdpmc_allowed))
+               on_each_cpu_mask(mm_cpumask(mm), refresh_pce, NULL, 1);
 }
 
 static int x86_pmu_event_idx(struct perf_event *event)
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index a3b873fc59e4..271a32a7e3e1 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -310,8 +310,8 @@ struct pmu {
         * Notification that the event was mapped or unmapped.  Called
         * in the context of the mapping task.
         */
-       void (*event_mapped)            (struct perf_event *event); /*optional*/
-       void (*event_unmapped)          (struct perf_event *event); /*optional*/
+       void (*event_mapped)            (struct perf_event *event, struct 
vm_area_struct *vma); /*optional*/
+       void (*event_unmapped)          (struct perf_event *event, struct 
vm_area_struct *vma); /*optional*/
 
        /*
         * Flags for ->add()/->del()/ ->start()/->stop(). There are
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 426c2ffba16d..f724348bba08 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5090,7 +5105,7 @@ static void perf_mmap_open(struct vm_area_struct *vma)
                atomic_inc(&event->rb->aux_mmap_count);
 
        if (event->pmu->event_mapped)
-               event->pmu->event_mapped(event);
+               event->pmu->event_mapped(event, vma);
 }
 
 static void perf_pmu_output_stop(struct perf_event *event);
@@ -5113,7 +5128,7 @@ static void perf_mmap_close(struct vm_area_struct *vma)
        unsigned long size = perf_data_size(rb);
 
        if (event->pmu->event_unmapped)
-               event->pmu->event_unmapped(event);
+               event->pmu->event_unmapped(event, vma);
 
        /*
         * rb->aux_mmap_count will always drop before rb->mmap_count and
@@ -5411,7 +5426,7 @@ static int perf_mmap(struct file *file, struct 
vm_area_struct *vma)
        vma->vm_ops = &perf_mmap_vmops;
 
        if (event->pmu->event_mapped)
-               event->pmu->event_mapped(event);
+               event->pmu->event_mapped(event, vma);
 
        return ret;
 }