Re: [PATCH] tee: fix register_shm_helper()

[Sumit Garg <sumit.garg@kernel.org>]

On Fri, Sep 19, 2025 at 02:40:16PM +0200, Jens Wiklander wrote:
> 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/
> Fixes: 7bdee4157591 ("tee: Use iov_iter to better support shared buffer registration")
> Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
> ---
>  drivers/tee/tee_shm.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
> index daf6e5cfd59a..6ed7d030f4ed 100644
> --- a/drivers/tee/tee_shm.c
> +++ b/drivers/tee/tee_shm.c
> @@ -316,7 +316,16 @@ register_shm_helper(struct tee_context *ctx, struct iov_iter *iter, u32 flags,
>  
>  	len = iov_iter_extract_pages(iter, &shm->pages, LONG_MAX, num_pages, 0,
>  				     &off);
> -	if (unlikely(len <= 0)) {
> +	if (DIV_ROUND_UP(len + off, PAGE_SIZE) != num_pages) {
> +		if (len > 0) {
> +			/*
> +			 * 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;
> +		}
>  		ret = len ? ERR_PTR(len) : ERR_PTR(-ENOMEM);
>  		goto err_free_shm_pages;
>  	}

Rather than operating directly on "len" without checking for error code
first doesn't seems appropriate to me. How about following diff instead?

diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
index daf6e5cfd59a..cb52bc51943e 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;
        }
 
        /*

-Sumit

> -- 
> 2.43.0
>