RE: [PATCH] HID: hyperv: avoid struct memcpy overrun warning

[Michael Kelley (LINUX)]

From: Arnd Bergmann <arnd@xxxxxxxxxx> Sent: Wednesday, July 5, 2023 7:02 AM
> 
> A previous patch addressed the fortified memcpy warning for most
> builds, but I still see this one with gcc-9:
> 
> In file included from include/linux/string.h:254,
>                  from drivers/hid/hid-hyperv.c:8:
> In function 'fortify_memcpy_chk',
>     inlined from 'mousevsc_on_receive' at drivers/hid/hid-hyperv.c:272:3:
> include/linux/fortify-string.h:583:4: error: call to '__write_overflow_field' declared with
> attribute warning: detected write beyond size of field (1st parameter); maybe use
> struct_group()? [-Werror=attribute-warning]
>   583 |    __write_overflow_field(p_size_field, size);
>       |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> My guess is that the WARN_ON() itself is what confuses gcc, so it no
> longer sees that there is a correct range check. Rework the code in a
> way that helps readability and avoids the warning.
> 
> Fixes: 542f25a944715 ("HID: hyperv: Replace one-element array with flexible-array member")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  drivers/hid/hid-hyperv.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/hid/hid-hyperv.c b/drivers/hid/hid-hyperv.c
> index 49d4a26895e76..f33485d83d24f 100644
> --- a/drivers/hid/hid-hyperv.c
> +++ b/drivers/hid/hid-hyperv.c
> @@ -258,19 +258,17 @@ static void mousevsc_on_receive(struct hv_device *device,
> 
>  	switch (hid_msg_hdr->type) {
>  	case SYNTH_HID_PROTOCOL_RESPONSE:
> +		len = struct_size(pipe_msg, data, pipe_msg->size);
> +
>  		/*
>  		 * While it will be impossible for us to protect against
>  		 * malicious/buggy hypervisor/host, add a check here to
>  		 * ensure we don't corrupt memory.
>  		 */
> -		if (struct_size(pipe_msg, data, pipe_msg->size)
> -			> sizeof(struct mousevsc_prt_msg)) {
> -			WARN_ON(1);
> +		if (WARN_ON(len > sizeof(struct mousevsc_prt_msg)))
>  			break;
> -		}
> 
> -		memcpy(&input_dev->protocol_resp, pipe_msg,
> -				struct_size(pipe_msg, data, pipe_msg->size));
> +		memcpy(&input_dev->protocol_resp, pipe_msg, len);
>  		complete(&input_dev->wait_event);
>  		break;
> 
> --
> 2.39.2

Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>