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 556960a..07d3be6 100644 (file)
--- 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;
 }