From: Jann Horn <jannh@google.com>
Date: Wed, 4 Apr 2018 19:03:21 +0000 (+0200)
Subject: tee: shm: fix use-after-free via temporarily dropped reference
X-Git-Tag: v4.17-rc6~12^2~9^2~1
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=bb765d1c331f62b59049d35607ed2e365802bef9;p=thirdparty%2Flinux.git

tee: shm: fix use-after-free via temporarily dropped reference

Bump the file's refcount before moving the reference into the fd table,
not afterwards. The old code could drop the file's refcount to zero for a
short moment before calling get_file() via get_dma_buf().

This code can only be triggered on ARM systems that use Linaro's OP-TEE.

Fixes: 967c9cca2cc5 ("tee: generic TEE subsystem")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
---

diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
index 556960a1bab3b..07d3be6f0780d 100644
--- a/drivers/tee/tee_shm.c
+++ b/drivers/tee/tee_shm.c
@@ -360,9 +360,10 @@ int tee_shm_get_fd(struct tee_shm *shm)
 	if (!(shm->flags & TEE_SHM_DMA_BUF))
 		return -EINVAL;
 
+	get_dma_buf(shm->dmabuf);
 	fd = dma_buf_fd(shm->dmabuf, O_CLOEXEC);
-	if (fd >= 0)
-		get_dma_buf(shm->dmabuf);
+	if (fd < 0)
+		dma_buf_put(shm->dmabuf);
 	return fd;
 }