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SubjectRe: [PATCH] uvc: fix missing check to determine if element is found in list
Hi Xiaomeng

Thanks for the patch. Maybe it would be better to just make a function
to find the ITERM entity with a given id?

Regards!

On Mon, 21 Mar 2022 at 16:33, Xiaomeng Tong <xiam0nd.tong@gmail.com> wrote:
>
> The list iterator will point to a bogus position containing HEAD if
> the list is empty or the element is not found in list. This case
> should be checked before any use of the iterator, otherwise it will
> lead to a invalid memory access. The missing check here is before
> "pin = iterm->id;", just add check here to fix the security bug.
>
> In addition, the list iterator value will *always* be set and non-NULL
> by list_for_each_entry(), so it is incorrect to assume that the iterator
> value will be NULL if the element is not found in list, considering
> the (mis)use here: "if (iterm == NULL".
>
> Use a new value 'it' as the list iterator, while use the old value
> 'iterm' as a dedicated pointer to point to the found element, which
> 1. can fix this bug, due to 'iterm' is NULL only if it's not found.
> 2. do not need to change all the uses of 'iterm' after the loop.
> 3. can also limit the scope of the list iterator 'it' *only inside*
> the traversal loop by simply declaring 'it' inside the loop in the
> future, as usage of the iterator outside of the list_for_each_entry
> is considered harmful. https://lkml.org/lkml/2022/2/17/1032
>
> Fixes: d5e90b7a6cd1c ("[media] uvcvideo: Move to video_ioctl2")
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
> ---
> drivers/media/usb/uvc/uvc_v4l2.c | 20 +++++++++++++-------
> 1 file changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/media/usb/uvc/uvc_v4l2.c b/drivers/media/usb/uvc/uvc_v4l2.c
> index 711556d13d03..e7cdc01ad277 100644
> --- a/drivers/media/usb/uvc/uvc_v4l2.c
> +++ b/drivers/media/usb/uvc/uvc_v4l2.c
> @@ -871,6 +871,7 @@ static int uvc_ioctl_enum_input(struct file *file, void *fh,
> struct uvc_video_chain *chain = handle->chain;
> const struct uvc_entity *selector = chain->selector;
> struct uvc_entity *iterm = NULL;
> + struct uvc_entity *it;
> u32 index = input->index;
> int pin = 0;
>
> @@ -878,22 +879,27 @@ static int uvc_ioctl_enum_input(struct file *file, void *fh,
> (chain->dev->quirks & UVC_QUIRK_IGNORE_SELECTOR_UNIT)) {
> if (index != 0)
> return -EINVAL;
> - list_for_each_entry(iterm, &chain->entities, chain) {
> - if (UVC_ENTITY_IS_ITERM(iterm))
> + list_for_each_entry(it, &chain->entities, chain) {
> + if (UVC_ENTITY_IS_ITERM(it)) {
> + iterm = it;
> break;
> + }
> }
> - pin = iterm->id;
> + if (iterm)
> + pin = iterm->id;
> } else if (index < selector->bNrInPins) {
> pin = selector->baSourceID[index];
> - list_for_each_entry(iterm, &chain->entities, chain) {
> - if (!UVC_ENTITY_IS_ITERM(iterm))
> + list_for_each_entry(it, &chain->entities, chain) {
> + if (!UVC_ENTITY_IS_ITERM(it))
> continue;
> - if (iterm->id == pin)
> + if (it->id == pin) {
> + iterm = it;
> break;
> + }
> }
> }
>
> - if (iterm == NULL || iterm->id != pin)
> + if (iterm == NULL)
> return -EINVAL;
>
> memset(input, 0, sizeof(*input));
> --
> 2.17.1
>


--
Ricardo Ribalda

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