Re: [PATCH] drm: check drm_format_info hsub and vsub to avoid divide by zero

[George Kennedy <george.kennedy@oracle.com>]

On 10/28/2021 10:04 AM, Ville Syrjälä wrote:
> On Thu, Oct 28, 2021 at 08:57:17AM -0500, George Kennedy wrote:
>> Do a sanity check on struct drm_format_info hsub and vsub values to
>> avoid divide by zero.
>>
>> Syzkaller reported a divide error in framebuffer_check() when the
>> DRM_FORMAT_Q410 or DRM_FORMAT_Q401 pixel_format is passed in via
>> the DRM_IOCTL_MODE_ADDFB2 ioctl. The drm_format_info struct for
>> the DRM_FORMAT_Q410 pixel_pattern has ".hsub = 0" and ".vsub = 0".
>> fb_plane_width() uses hsub as a divisor and fb_plane_height() uses
>> vsub as a divisor. These divisors need to be sanity checked for
>> zero before use.
>>
>> divide error: 0000 [#1] SMP KASAN NOPTI
>> CPU: 0 PID: 14995 Comm: syz-executor709 Not tainted 5.15.0-rc6-syzk #1
>> Hardware name: Red Hat KVM, BIOS 1.13.0-2
>> RIP: 0010:framebuffer_check drivers/gpu/drm/drm_framebuffer.c:199 [inline]
>> RIP: 0010:drm_internal_framebuffer_create+0x604/0xf90
>> drivers/gpu/drm/drm_framebuffer.c:317
>>
>> Call Trace:
>>   drm_mode_addfb2+0xdc/0x320 drivers/gpu/drm/drm_framebuffer.c:355
>>   drm_mode_addfb2_ioctl+0x2a/0x40 drivers/gpu/drm/drm_framebuffer.c:391
>>   drm_ioctl_kernel+0x23a/0x2e0 drivers/gpu/drm/drm_ioctl.c:795
>>   drm_ioctl+0x589/0xac0 drivers/gpu/drm/drm_ioctl.c:898
>>   vfs_ioctl fs/ioctl.c:51 [inline]
>>   __do_sys_ioctl fs/ioctl.c:874 [inline]
>>   __se_sys_ioctl fs/ioctl.c:860 [inline]
>>   __x64_sys_ioctl+0x19d/0x220 fs/ioctl.c:860
>>   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>   do_syscall_64+0x3a/0x80 arch/x86/entry/common.c:80
>>   entry_SYSCALL_64_after_hwframe+0x44/0xae
>>
>> Signed-off-by: George Kennedy <george.kennedy@oracle.com>
>> ---
>>   drivers/gpu/drm/drm_framebuffer.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
>> index 07f5abc..a146e4b 100644
>> --- a/drivers/gpu/drm/drm_framebuffer.c
>> +++ b/drivers/gpu/drm/drm_framebuffer.c
>> @@ -195,6 +195,16 @@ static int framebuffer_check(struct drm_device *dev,
>>   	/* now let the driver pick its own format info */
>>   	info = drm_get_format_info(dev, r);
>>   
>> +	if (info->hsub == 0) {
>> +		DRM_DEBUG_KMS("bad horizontal chroma subsampling factor %u\n", info->hsub);
>> +		return -EINVAL;
>> +	}
>> +
>> +	if (info->vsub == 0) {
>> +		DRM_DEBUG_KMS("bad vertical chroma subsampling factor %u\n", info->vsub);
>> +		return -EINVAL;
>> +	}
> Looks like duct tape to me. I think we need to either fix those formats
> to have valid format info, or just revert the whole patch that added such
> broken things.

Adding the authors and reviewer of the patch in question to CC.

94b292b277343190175d39172c903c0c5fb814f1 drm: drm_fourcc: add NV15, 
Q410, Q401 YUV formats

Asking if you have any input on how to deal with hsub and vsub = zero?

The sanity checks of hsub and vsub in the proposed patch are similar to 
other error checking done in framebuffer_check() preceding the proposed 
patch.

Thank you,
George
>
>> +
>>   	for (i = 0; i < info->num_planes; i++) {
>>   		unsigned int width = fb_plane_width(r->width, info, i);
>>   		unsigned int height = fb_plane_height(r->height, info, i);
>> -- 
>> 1.8.3.1