perf_counter: Close race in perf_lock_task_context()
perf_lock_task_context() is buggy because it can return a dead
context.
the RCU read lock in perf_lock_task_context() only guarantees
the memory won't get freed, it doesn't guarantee the object is
valid (in our case refcount > 0).
Therefore we can return a locked object that can get freed the
moment we release the rcu read lock.
perf_pin_task_context() then increases the refcount and does an
unlock on freed memory.
That increased refcount will cause a double free, in case it
started out with 0.
Ammend this by including the get_ctx() functionality in
perf_lock_task_context() (all users already did this later
anyway), and return a NULL context when the found one is
already dead.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 8d4f0dd41..adb6ae5 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -175,6 +175,11 @@
spin_unlock_irqrestore(&ctx->lock, *flags);
goto retry;
}
+
+ if (!atomic_inc_not_zero(&ctx->refcount)) {
+ spin_unlock_irqrestore(&ctx->lock, *flags);
+ ctx = NULL;
+ }
}
rcu_read_unlock();
return ctx;
@@ -193,7 +198,6 @@
ctx = perf_lock_task_context(task, &flags);
if (ctx) {
++ctx->pin_count;
- get_ctx(ctx);
spin_unlock_irqrestore(&ctx->lock, flags);
}
return ctx;
@@ -1459,11 +1463,6 @@
put_ctx(parent_ctx);
ctx->parent_ctx = NULL; /* no longer a clone */
}
- /*
- * Get an extra reference before dropping the lock so that
- * this context won't get freed if the task exits.
- */
- get_ctx(ctx);
spin_unlock_irqrestore(&ctx->lock, flags);
}