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

[George Kennedy]

On 11/19/2021 9:25 AM, Jani Nikula wrote:
> On Fri, 19 Nov 2021, Daniel Vetter <daniel at ffwll.ch> wrote:
>> On Fri, Nov 19, 2021 at 12:03:00PM +0200, Ville Syrjälä wrote:
>>> On Fri, Nov 19, 2021 at 10:40:38AM +0100, Daniel Vetter wrote:
>>>> On Thu, Oct 28, 2021 at 05:04:19PM +0300, 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 at 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.
>>>> Yeah maybe even a compile-time check of the format table(s) to validate
>>>> them properly and scream ... Or at least a selftest.
>>> I really wish C had (even very limited) compile time evaluation
>>> so one could actually loop over arrays like at compile time to
>>> check each element. As it stands you either have to check each
>>> array element by hand, or you do some cpp macro horrors to
>>> pretend you're iterating the array.
>> Python preprocess or so seems to be the usual answer, and that then just
>> generates the C table after it's all checked.
>>
>> Or a post-processor which fishes the table out from the .o (or just links
>> against it).
>>
>> But yeah doing this in cpp isn't going to work, aside from it'd be really
>> ugly.
> Kbuild does have support for hostprogs which are typically used in the
> build. The obvious idea is to use that for code generation, but it would
> also be interesting to see how that could be used for compile-time
> evaluation of sorts. Kind of like compile-time selftests? And, of
> course, how badly that would be frowned upon.
>
> git grep says there are only four hostprogs users in drivers/, so it
> certainly isn't a popularity contest winner. (One of them is
> "mkregtable" in radeon.)

So, can someone suggest a fix? A cpp type of approach does not seem 
feasible.

Adding the sanity checks that are in the patch, which are similar to the 
sanity checks preceding them in framebuffer_check(), along with a 
self-test that ran through all the table entries, might address all the 
concerns brought up in this thread.

Thank you,
George
>
>
> BR,
> Jani.
>