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

[George Kennedy]

On 11/25/2021 10:27 AM, Daniel Vetter wrote:
> On Mon, Nov 22, 2021 at 10:29:05AM -0500, George Kennedy wrote:
>>
>> 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.
> drm selftest sounds like a reasonable approach to me.
In the meantime, should a bugzilla bug be opened to track the issue? 
 From this thread it does not seem as though there is a drm selftest in 
the works.

Thanks,
George
> -Daniel