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

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Date: Sat, 18 May 2024 10:32:07 -0700
From: Kees Cook <keescook@...omium.org>
To: Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc: Guenter Roeck <linux@...ck-us.net>, 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 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. :)

-Kees

-- 
Kees Cook