linux-kernel - Re: [PATCH] uvc: fix missing check to determine if element is found in list

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Date:   Tue, 22 Mar 2022 13:40:07 +0100
From:   Ricardo Ribalda <ribalda@...omium.org>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     laurent.pinchart@...asonboard.com, mchehab@...nel.org,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [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@...il.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@...il.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