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

[Peter Zijlstra]

On Fri, Aug 04, 2017 at 11:17:02AM +0200, Peter Zijlstra wrote:
> Yeah, I'll make it pretty, write a changelog and things like that :-)

Slightly different from the earlier one in that I now pass @mm along
instead of @vma. Same thing otherwise.

---
Subject: perf,x86: Fix RDPMC vs mm_struct tracking
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Wed, 2 Aug 2017 19:39:30 +0200

Vince reported:

> 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

The problem is of course that exec() plays tricks with what is
current->mm, only destroying the old mappings after having installed
the new mm.

Fix this confusion by passing along vma->vm_mm instead of relying on
current->mm.

Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Vince Weaver <vincent.weaver@xxxxxxxxx>
Tested-by: Vince Weaver <vincent.weaver@xxxxxxxxx>
Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
Fixes: 1e0fb9ec679c ("perf: Add pmu callbacks to track event mapping and unmapping")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20170802173930.cstykcqefmqt7jau@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 arch/x86/events/core.c     |   16 +++++++---------
 include/linux/perf_event.h |    4 ++--
 kernel/events/core.c       |    6 +++---
 3 files changed, 12 insertions(+), 14 deletions(-)

--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2114,7 +2114,7 @@ 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 mm_struct *mm)
 {
 	if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
 		return;
@@ -2129,22 +2129,20 @@ static void x86_pmu_event_mapped(struct
 	 * 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 mm_struct *mm)
 {
-	if (!current->mm)
-		return;
 
 	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)
--- 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 mm_struct *mm); /*optional*/
+	void (*event_unmapped)		(struct perf_event *event, struct mm_struct *mm); /*optional*/
 
 	/*
 	 * Flags for ->add()/->del()/ ->start()/->stop(). There are
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5090,7 +5090,7 @@ static void perf_mmap_open(struct vm_are
 		atomic_inc(&event->rb->aux_mmap_count);
 
 	if (event->pmu->event_mapped)
-		event->pmu->event_mapped(event);
+		event->pmu->event_mapped(event, vma->vm_mm);
 }
 
 static void perf_pmu_output_stop(struct perf_event *event);
@@ -5113,7 +5113,7 @@ static void perf_mmap_close(struct vm_ar
 	unsigned long size = perf_data_size(rb);
 
 	if (event->pmu->event_unmapped)
-		event->pmu->event_unmapped(event);
+		event->pmu->event_unmapped(event, vma->vm_mm);
 
 	/*
 	 * rb->aux_mmap_count will always drop before rb->mmap_count and
@@ -5411,7 +5411,7 @@ static int perf_mmap(struct file *file,
 	vma->vm_ops = &perf_mmap_vmops;
 
 	if (event->pmu->event_mapped)
-		event->pmu->event_mapped(event);
+		event->pmu->event_mapped(event, vma->vm_mm);
 
 	return ret;
 }