linux-hardening - Re: [PATCH] drm/nouveau/nvif: Avoid build error due to potential integer overflows

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Message-ID: <a912c2d1-9008-410a-92cc-912e17c63695@roeck-us.net>
Date: Sat, 18 May 2024 11:23:44 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Kees Cook <keescook@...omium.org>,
 Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc: airlied@...il.com, dakr@...hat.com, daniel@...ll.ch,
 dri-devel@...ts.freedesktop.org, jani.nikula@...el.com, javierm@...hat.com,
 kherbst@...hat.com, linux-kernel@...r.kernel.org, lyude@...hat.com,
 mripard@...nel.org, nouveau@...ts.freedesktop.org, tzimmermann@...e.de,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH] drm/nouveau/nvif: Avoid build error due to potential
 integer overflows

On 5/18/24 10:32, Kees Cook wrote:
> On Sat, May 18, 2024 at 06:54:36PM +0200, Christophe JAILLET wrote:
>> (adding linux-hardening@...r.kernel.org)
>>
>>
>> Le 18/05/2024 à 16:37, Guenter Roeck a écrit :
>>> Trying to build parisc:allmodconfig with gcc 12.x or later results
>>> in the following build error.
>>>
>>> drivers/gpu/drm/nouveau/nvif/object.c: In function 'nvif_object_mthd':
>>> drivers/gpu/drm/nouveau/nvif/object.c:161:9: error:
>>> 	'memcpy' accessing 4294967264 or more bytes at offsets 0 and 32 overlaps 6442450881 bytes at offset -2147483617 [-Werror=restrict]
>>>     161 |         memcpy(data, args->mthd.data, size);
>>>         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> drivers/gpu/drm/nouveau/nvif/object.c: In function 'nvif_object_ctor':
>>> drivers/gpu/drm/nouveau/nvif/object.c:298:17: error:
>>> 	'memcpy' accessing 4294967240 or more bytes at offsets 0 and 56 overlaps 6442450833 bytes at offset -2147483593 [-Werror=restrict]
>>>     298 |                 memcpy(data, args->new.data, size);
>>>
>>> gcc assumes that 'sizeof(*args) + size' can overflow, which would result
>>> in the problem.
>>>
>>> The problem is not new, only it is now no longer a warning but an error since W=1
>>> has been enabled for the drm subsystem and since Werror is enabled for test builds.
>>>
>>> Rearrange arithmetic and add extra size checks to avoid the overflow.
>>>
>>> Fixes: a61ddb4393ad ("drm: enable (most) W=1 warnings by default across the subsystem")
>>> Cc: Javier Martinez Canillas <javierm-H+wXaHxf7aLQT0dZR+AlfA@...lic.gmane.org>
>>> Cc: Jani Nikula <jani.nikula-ral2JQCrhuEAvxtiuMwx3w@...lic.gmane.org>
>>> Cc: Thomas Zimmermann <tzimmermann-l3A5Bk7waGM@...lic.gmane.org>
>>> Cc: Danilo Krummrich <dakr-H+wXaHxf7aLQT0dZR+AlfA@...lic.gmane.org>
>>> Cc: Maxime Ripard <mripard-DgEjT+Ai2ygdnm+yROfE0A@...lic.gmane.org>
>>> Signed-off-by: Guenter Roeck <linux-0h96xk9xTtrk1uMJSBkQmQ@...lic.gmane.org>
>>> ---
>>> checkpatch complains about the line length in the description and the (pre-existing)
>>> assignlemts in if conditions, but I did not want to split lines in the description
>>> or rearrange the code further.
>>>
>>> I don't know why I only see the problem with parisc builds (at least so far).
>>>
>>>    drivers/gpu/drm/nouveau/nvif/object.c | 8 +++++---
>>>    1 file changed, 5 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/nouveau/nvif/object.c b/drivers/gpu/drm/nouveau/nvif/object.c
>>> index 4d1aaee8fe15..baf623a48874 100644
>>> --- a/drivers/gpu/drm/nouveau/nvif/object.c
>>> +++ b/drivers/gpu/drm/nouveau/nvif/object.c
>>> @@ -145,8 +145,9 @@ nvif_object_mthd(struct nvif_object *object, u32 mthd, void *data, u32 size)
>>>    	u8 stack[128];
>>>    	int ret;
>>> -	if (sizeof(*args) + size > sizeof(stack)) {
>>> -		if (!(args = kmalloc(sizeof(*args) + size, GFP_KERNEL)))
>>> +	if (size > sizeof(stack) - sizeof(*args)) {
>>> +		if (size > INT_MAX ||
>>> +		    !(args = kmalloc(sizeof(*args) + size, GFP_KERNEL)))
>>
>> Hi,
>>
>> Would it be cleaner or better to use size_add(sizeof(*args), size)?
> 
> I think the INT_MAX test is actually better in this case because
> nvif_object_ioctl()'s size argument is u32:
> 
> ret = nvif_object_ioctl(object, args, sizeof(*args) + size, NULL);
>                                        ^^^^^^^^^^^^^^^^^^^^
> 
> So that could wrap around, even though the allocation may not.
> 
> Better yet, since "sizeof(*args) + size" is repeated 3 times in the
> function, I'd recommend:
> 
> 	...
> 	u32 args_size;
> 
> 	if (check_add_overflow(sizeof(*args), size, &args_size))
> 		return -ENOMEM;
> 	if (args_size > sizeof(stack)) {
> 		if (!(args = kmalloc(args_size, GFP_KERNEL)))
> 			return -ENOMEM;
>          } else {
>                  args = (void *)stack;
>          }
> 	...
>          ret = nvif_object_ioctl(object, args, args_size, NULL);
> 
> This will catch the u32 overflow to nvif_object_ioctl(), catch an
> allocation underflow on 32-bits systems, and make the code more
> readable. :)
> 

Makes sense. I'll change that and send v2.

Thanks,
Guenter