Re: [PATCH] drm/i915: Use copy_from_user() in fence copying

[Kees Cook]

On Fri, Dec 8, 2017 at 2:17 AM, David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: Kees Cook
>> Sent: 06 December 2017 20:29
>>
>> There's no good reason to separate the access_ok() from the copy,
>> especially since the access_ok() size is hard-coded instead of using
>> sizeof(). Instead, just use copy_from_user() directly.
>
> Looks like an optimisation to save doing the access_ok() check
> for every 'fence'.

If it really makes a difference, okay, but access_ok() checks are fast. :P

> OTOH 'user copy hardening' probably makes a much larger performance
> degradation on this kind of code.
> (Might be ok here because &fence probably refers to something in the
> current stack frame.)

Well, the good news there is that it's using sizeof(fence), so no
hardening check is done (it's not a size that changes at runtime).
What I didn't like is that the access_ok() doesn't use sizeof(fence)
(it is currently correct: 2 u32s == sizeof(fence)) but that kind of
fragility keeps me up at night. ;)

So, fixing either would be fine, but if we're going to touch it, it
seems best to just do away with the __copy_*() usage instead.

-Kees


>
>         David
>
>> Fixes: cf6e7bac6357 ("drm/i915: Add support for drm syncobjs")
>> Cc: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
>> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 4 +---
>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
>> index 435ed95df144..1da703213b17 100644
>> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
>> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
>> @@ -2087,8 +2087,6 @@ get_fence_array(struct drm_i915_gem_execbuffer2 *args,
>>               return ERR_PTR(-EINVAL);
>>
>>       user = u64_to_user_ptr(args->cliprects_ptr);
>> -     if (!access_ok(VERIFY_READ, user, nfences * 2 * sizeof(u32)))
>> -             return ERR_PTR(-EFAULT);
>>
>>       fences = kvmalloc_array(args->num_cliprects, sizeof(*fences),
>>                               __GFP_NOWARN | GFP_KERNEL);
>> @@ -2099,7 +2097,7 @@ get_fence_array(struct drm_i915_gem_execbuffer2 *args,
>>               struct drm_i915_gem_exec_fence fence;
>>               struct drm_syncobj *syncobj;
>>
>> -             if (__copy_from_user(&fence, user++, sizeof(fence))) {
>> +             if (copy_from_user(&fence, user++, sizeof(fence))) {
>>                       err = -EFAULT;
>>                       goto err;
>>               }
>> --
>> 2.7.4
>>
>>
>> --
>> Kees Cook
>> Pixel Security



-- 
Kees Cook
Pixel Security