perf: Fix NULL deref in inheritance code

Liming found a NULL deref when a task has a perf context but no
counters  when it forks.

This can occur in two cases, a race during construction where
the fork hits after installing the context but before the first
counter gets inserted, or more reproducably, a fork after the
last counter is closed (which leaves the context around).

Reported-by: Wang Liming <liming.wang@windriver.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
CC: <stable@kernel.org>
LKML-Reference: <1262185684.7135.222.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 2 insertions, 3 deletions

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 03cc061398d1..58ed1dae5875 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -5148,7 +5148,7 @@ int perf_event_init_task(struct task_struct *child)
                                             GFP_KERNEL);
                         if (!child_ctx) {
                                 ret = -ENOMEM;
-                                goto exit;
+                                break;
                         }
 
                         __perf_event_init_context(child_ctx, child);
@@ -5164,7 +5164,7 @@ int perf_event_init_task(struct task_struct *child)
                 }
         }
 
-        if (inherited_all) {
+        if (child_ctx && inherited_all) {
                 /*
                  * Mark the child context as a clone of the parent
                  * context, or of whatever the parent is a clone of.
@@ -5184,7 +5184,6 @@ int perf_event_init_task(struct task_struct *child)
                 get_ctx(child_ctx->parent_ctx);
         }
 
-exit:
         mutex_unlock(&parent_ctx->mutex);
 
         perf_unpin_context(parent_ctx);