drm: udl: Properly check framebuffer mmap offsets

Commit Message

Greg KH March 21, 2018, 3:45 p.m. UTC

The memmap options sent to the udl framebuffer driver were not being
checked for all sets of possible crazy values.  Fix this up by properly
bounding the allowed values.

Reported-by: Eyal Itkin <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Comments

Daniel Vetter March 22, 2018, 6:59 a.m. UTC | #1

On Wed, Mar 21, 2018 at 04:45:53PM +0100, Greg Kroah-Hartman wrote:
> The memmap options sent to the udl framebuffer driver were not being
> checked for all sets of possible crazy values.  Fix this up by properly
> bounding the allowed values.
> 
> Reported-by: Eyal Itkin <[e-mail]>
> Cc: stable <[e-mail]>
> Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Applied to drm-misc-fixes, thanks for the patch.

Does anyone working on overflow-proof integers? That would make a lot of
this code so much simpler if we could just ask the compiler to carry the
oferflow bit around for a given expression and then check that and bail
with -EINVAL.
-Daniel

> 
> diff --git a/drivers/gpu/drm/udl/udl_fb.c b/drivers/gpu/drm/udl/udl_fb.c
> index b5b335c9b2bb..2ebdc6d5a76e 100644
> --- a/drivers/gpu/drm/udl/udl_fb.c
> +++ b/drivers/gpu/drm/udl/udl_fb.c
> @@ -159,10 +159,15 @@ static int udl_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)
>  {
>  	unsigned long start = vma->vm_start;
>  	unsigned long size = vma->vm_end - vma->vm_start;
> -	unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
> +	unsigned long offset;
>  	unsigned long page, pos;
>  
> -	if (offset + size > info->fix.smem_len)
> +	if (vma->vm_pgoff > (~0UL >> PAGE_SHIFT))
> +		return -EINVAL;
> +
> +	offset = vma->vm_pgoff << PAGE_SHIFT;
> +
> +	if (offset > info->fix.smem_len || size > info->fix.smem_len - offset)
>  		return -EINVAL;
>  
>  	pos = (unsigned long)info->fix.smem_start + offset;

Greg KH March 22, 2018, 8:03 a.m. UTC | #2

On Thu, Mar 22, 2018 at 07:59:59AM +0100, Daniel Vetter wrote:
> Does anyone working on overflow-proof integers? That would make a lot of
> this code so much simpler if we could just ask the compiler to carry the
> oferflow bit around for a given expression and then check that and bail
> with -EINVAL.

That would be nice, but no, I don't think that's part of any C standard
work that I have heard of :(

greg k-h

Daniel Vetter March 22, 2018, 9:54 a.m. UTC | #3

On Thu, Mar 22, 2018 at 9:03 AM, Greg Kroah-Hartman
<[e-mail]> wrote:
> On Thu, Mar 22, 2018 at 07:59:59AM +0100, Daniel Vetter wrote:
>> Does anyone working on overflow-proof integers? That would make a lot of
>> this code so much simpler if we could just ask the compiler to carry the
>> oferflow bit around for a given expression and then check that and bail
>> with -EINVAL.
>
> That would be nice, but no, I don't think that's part of any C standard
> work that I have heard of :(

Well we have refcount_t already, stitching something together that
would work and not suck too badly with performance should be possible.
But yeah direct compiler support would be better (and would allow
optimizing the carry flag checks I guess). I kinda hoped Kees&team
would be working on this eventually.

Adding Kees+kernel-hardening, maybe he'll grow fond of this :-)
-Daniel

Kees Cook March 23, 2018, 12:14 a.m. UTC | #4

On Thu, Mar 22, 2018 at 2:54 AM, Daniel Vetter <[e-mail]> wrote:
> On Thu, Mar 22, 2018 at 9:03 AM, Greg Kroah-Hartman
> <[e-mail]> wrote:
>> On Thu, Mar 22, 2018 at 07:59:59AM +0100, Daniel Vetter wrote:
>>> Does anyone working on overflow-proof integers? That would make a lot of
>>> this code so much simpler if we could just ask the compiler to carry the
>>> oferflow bit around for a given expression and then check that and bail
>>> with -EINVAL.
>>
>> That would be nice, but no, I don't think that's part of any C standard
>> work that I have heard of :(
>
> Well we have refcount_t already, stitching something together that
> would work and not suck too badly with performance should be possible.
> But yeah direct compiler support would be better (and would allow
> optimizing the carry flag checks I guess). I kinda hoped Kees&team
> would be working on this eventually.

refcount_t could be used if it happens to match the needed semantics.

> Adding Kees+kernel-hardening, maybe he'll grow fond of this :-)

Yeah, general integer overflow is on the list of things to get fixed
in the kernel. It's a bit of a long road, though.

Clang has -fsanitize=integer (and sub-options) which could be added
for specific object or trees:
https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html#ubsan-checks

GCC seems to only support manual marking of overflow detections:
https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html

grsecurity/PaX has a gcc plugin for overflow detection, though it
hasn't been upstreamed and comes with various caveats:
http://forums.grsecurity.net/viewtopic.php?f=7&t=3043
https://github.com/ephox-gcc-plugins/size_overflow

-Kees

Daniel Vetter March 26, 2018, 8:26 a.m. UTC | #5

On Thu, Mar 22, 2018 at 05:14:40PM -0700, Kees Cook wrote:
> On Thu, Mar 22, 2018 at 2:54 AM, Daniel Vetter <[e-mail]> wrote:
> > On Thu, Mar 22, 2018 at 9:03 AM, Greg Kroah-Hartman
> > <[e-mail]> wrote:
> >> On Thu, Mar 22, 2018 at 07:59:59AM +0100, Daniel Vetter wrote:
> >>> Does anyone working on overflow-proof integers? That would make a lot of
> >>> this code so much simpler if we could just ask the compiler to carry the
> >>> oferflow bit around for a given expression and then check that and bail
> >>> with -EINVAL.
> >>
> >> That would be nice, but no, I don't think that's part of any C standard
> >> work that I have heard of :(
> >
> > Well we have refcount_t already, stitching something together that
> > would work and not suck too badly with performance should be possible.
> > But yeah direct compiler support would be better (and would allow
> > optimizing the carry flag checks I guess). I kinda hoped Kees&team
> > would be working on this eventually.
> 
> refcount_t could be used if it happens to match the needed semantics.
> 
> > Adding Kees+kernel-hardening, maybe he'll grow fond of this :-)
> 
> Yeah, general integer overflow is on the list of things to get fixed
> in the kernel. It's a bit of a long road, though.
> 
> Clang has -fsanitize=integer (and sub-options) which could be added
> for specific object or trees:
> https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html#ubsan-checks

I expect that a bunch of folks will scream about checking all interger
math for overflows

> GCC seems to only support manual marking of overflow detections:
> https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html

And this sounds _very_ verbose. What I have in mind is something like

	result = overflow_checked(expr, &overflow)
	if (overflow)
		return -EINVAL;

and expr could be any expression at all (except calling functions ofc, or
whatever the correct verbiage from the C standard for this is). I don't
want to annotate each compuation individually at least, that's too much
typing :-) Even neater would be an entire block:

	while {
		/* compute stuff except function calls, but allow inlining */
	} has_overflow {
		return -EINVAL;
	}

Anyway, yay for this being somewhere on the future plans!

> grsecurity/PaX has a gcc plugin for overflow detection, though it
> hasn't been upstreamed and comes with various caveats:
> http://forums.grsecurity.net/viewtopic.php?f=7&t=3043
> https://github.com/ephox-gcc-plugins/size_overflow

Yeah this looks fairly fragile to me, also can't use the overflow bits
instructions sets have I think.
-Daniel

Patch

diff --git a/drivers/gpu/drm/udl/udl_fb.c b/drivers/gpu/drm/udl/udl_fb.c
index b5b335c9b2bb..2ebdc6d5a76e 100644
--- a/drivers/gpu/drm/udl/udl_fb.c
+++ b/drivers/gpu/drm/udl/udl_fb.c
@@ -159,10 +159,15 @@  static int udl_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)
 {
 	unsigned long start = vma->vm_start;
 	unsigned long size = vma->vm_end - vma->vm_start;
-	unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
+	unsigned long offset;
 	unsigned long page, pos;
 
-	if (offset + size > info->fix.smem_len)
+	if (vma->vm_pgoff > (~0UL >> PAGE_SHIFT))
+		return -EINVAL;
+
+	offset = vma->vm_pgoff << PAGE_SHIFT;
+
+	if (offset > info->fix.smem_len || size > info->fix.smem_len - offset)
 		return -EINVAL;
 
 	pos = (unsigned long)info->fix.smem_start + offset;