[PATCH v2] tee: fix register_shm_helper()

[Jens Wiklander <jens.wiklander@linaro.org>]

In register_shm_helper(), fix incorrect error handling for a call to
iov_iter_extract_pages(). A case is missing for when
iov_iter_extract_pages() only got some pages and return a number larger
than 0, but not the requested amount.

This fixes a possible NULL pointer dereference following a bad input from
ioctl(TEE_IOC_SHM_REGISTER) where parts of the buffer isn't mapped.

Cc: stable@vger.kernel.org
Reported-by: Masami Ichikawa <masami256@gmail.com>
Closes: https://lore.kernel.org/op-tee/CACOXgS-Bo2W72Nj1_44c7bntyNYOavnTjJAvUbEiQfq=u9W+-g@mail.gmail.com/
Tested-by: Masami Ichikawa <masami256@gmail.com>
Fixes: 7bdee4157591 ("tee: Use iov_iter to better support shared buffer registration")
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
---
Changes from v1
- Refactor the if statement as requested by Sumit
- Adding Tested-by: Masami Ichikawa <masami256@gmail.com
- Link to v1:
  https://lore.kernel.org/op-tee/20250919124217.2934718-1-jens.wiklander@linaro.org/
---
 drivers/tee/tee_shm.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
index daf6e5cfd59a..76c54e1dc98c 100644
--- a/drivers/tee/tee_shm.c
+++ b/drivers/tee/tee_shm.c
@@ -319,6 +319,14 @@ register_shm_helper(struct tee_context *ctx, struct iov_iter *iter, u32 flags,
 	if (unlikely(len <= 0)) {
 		ret = len ? ERR_PTR(len) : ERR_PTR(-ENOMEM);
 		goto err_free_shm_pages;
+	} else if (DIV_ROUND_UP(len + off, PAGE_SIZE) != num_pages) {
+		/*
+		 * If we only got a few pages, update to release the
+		 * correct amount below.
+		 */
+		shm->num_pages = len / PAGE_SIZE;
+		ret = ERR_PTR(-ENOMEM);
+		goto err_put_shm_pages;
 	}
 
 	/*
-- 
2.43.0