[GIT PULL] sigqueue cache fix

From: Ingo Molnar
Date: Thu Jun 24 2021 - 03:13:18 EST


Linus,

Please pull the latest core/urgent git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core-urgent-2021-06-24

# HEAD: 399f8dd9a866e107639eabd3c1979cd526ca3a98 signal: Prevent sigqueue caching after task got released

Fix a memory leak in the recently introduced sigqueue cache.

Thanks,

Ingo

------------------>
Thomas Gleixner (1):
signal: Prevent sigqueue caching after task got released


kernel/signal.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index f7c6ffcbd044..f1ecd8f0c11d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -435,6 +435,12 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t gfp_flags,
* Preallocation does not hold sighand::siglock so it can't
* use the cache. The lockless caching requires that only
* one consumer and only one producer run at a time.
+ *
+ * For the regular allocation case it is sufficient to
+ * check @q for NULL because this code can only be called
+ * if the target task @t has not been reaped yet; which
+ * means this code can never observe the error pointer which is
+ * written to @t->sigqueue_cache in exit_task_sigqueue_cache().
*/
q = READ_ONCE(t->sigqueue_cache);
if (!q || sigqueue_flags)
@@ -463,13 +469,18 @@ void exit_task_sigqueue_cache(struct task_struct *tsk)
struct sigqueue *q = tsk->sigqueue_cache;

if (q) {
- tsk->sigqueue_cache = NULL;
/*
* Hand it back to the cache as the task might
* be self reaping which would leak the object.
*/
kmem_cache_free(sigqueue_cachep, q);
}
+
+ /*
+ * Set an error pointer to ensure that @tsk will not cache a
+ * sigqueue when it is reaping it's child tasks
+ */
+ tsk->sigqueue_cache = ERR_PTR(-1);
}

static void sigqueue_cache_or_free(struct sigqueue *q)
@@ -481,6 +492,10 @@ static void sigqueue_cache_or_free(struct sigqueue *q)
* is intentional when run without holding current->sighand->siglock,
* which is fine as current obviously cannot run __sigqueue_free()
* concurrently.
+ *
+ * The NULL check is safe even if current has been reaped already,
+ * in which case exit_task_sigqueue_cache() wrote an error pointer
+ * into current->sigqueue_cache.
*/
if (!READ_ONCE(current->sigqueue_cache))
WRITE_ONCE(current->sigqueue_cache, q);