linux-kernel - Re: [PATCH 1/2] Input; Sanitize event code before modifying bitmaps

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Message-ID: <20200824195102.GY1665100@dtor-ws>
Date:   Mon, 24 Aug 2020 12:51:02 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/2] Input; Sanitize event code before modifying bitmaps

Hi Marc,

On Mon, Aug 17, 2020 at 12:26:59PM +0100, Marc Zyngier wrote:
> When calling into input_set_capability(), the passed event code is
> blindly used to set a bit in a number of bitmaps, without checking
> whether this actually fits the expected size of the bitmap.
> 
> This event code can come from a variety of sources, including devices
> masquerading as input devices, only a bit more "programmable".
> 
> Instead of taking the raw event code, sanitize it to the actual bitmap
> size and output a warning to let the user know.
> 
> These checks are, at least in spirit, in keeping with cb222aed03d7
> ("Input: add safety guards to input_set_keycode()").
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> ---
>  drivers/input/input.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/input/input.c b/drivers/input/input.c
> index 3cfd2c18eebd..1e77cf47aa44 100644
> --- a/drivers/input/input.c
> +++ b/drivers/input/input.c
> @@ -1974,14 +1974,18 @@ EXPORT_SYMBOL(input_get_timestamp);
>   * In addition to setting up corresponding bit in appropriate capability
>   * bitmap the function also adjusts dev->evbit.
>   */
> -void input_set_capability(struct input_dev *dev, unsigned int type, unsigned int code)
> +void input_set_capability(struct input_dev *dev, unsigned int type, unsigned int raw_code)
>  {
> +	unsigned int code = raw_code;
> +
>  	switch (type) {
>  	case EV_KEY:
> +		code &= KEY_MAX;
>  		__set_bit(code, dev->keybit);


I would much rather prefer we did not simply set some random bits in
this case, but instead complained loudly and refused to alter anything.

The function is not exported directly to userspace, so we expect callers
to give us sane inputs, and I believe WARN() splash in case of bad
inputs would be the best solution here.

Thanks.

-- 
Dmitry