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

[David Laight]

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'.

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

David

> Fixes: cf6e7bac6357 ("drm/i915: Add support for drm syncobjs")
> Cc: Jason Ekstrand 
> Cc: Chris Wilson 
> Signed-off-by: Kees Cook 
> ---
>  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(, user++, sizeof(fence))) {
> + if (copy_from_user(, user++, sizeof(fence))) {
>   err = -EFAULT;
>   goto err;
>   }
> --
> 2.7.4
> 
> 
> --
> Kees Cook
> Pixel Security
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Re: [PATCH] drm/i915: Use copy_from_user() in fence copying

[Kees Cook]

On Fri, Dec 8, 2017 at 2:17 AM, David Laight  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  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 
>> Cc: Chris Wilson 
>> Signed-off-by: Kees Cook 
>> ---
>>  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(, user++, sizeof(fence))) {
>> + if (copy_from_user(, user++, sizeof(fence))) {
>>   err = -EFAULT;
>>   goto err;
>>   }
>> --
>> 2.7.4
>>
>>
>> --
>> Kees Cook
>> Pixel Security



-- 
Kees Cook
Pixel Security
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RE: [PATCH] drm/i915: Use copy_from_user() in fence copying

[David Laight]

From: Kees Cook
> Sent: 08 December 2017 21:10
> >> 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

Not compared to get_user() :-)

David

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

[Joonas Lahtinen]

On Wed, 2017-12-06 at 12:28 -0800, Kees Cook wrote:
> 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.
> 
> Fixes: cf6e7bac6357 ("drm/i915: Add support for drm syncobjs")

There's been request to reduce the amount of Fixes: tags that are not
actually fixing bugs. This seems more like an optimization.

References: has been suggested for these cases instead.

Regards, Joonas
-- 
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Open Source Technology Center
Intel Corporation
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[PATCH] drm/i915: Use copy_from_user() in fence copying

[Kees Cook]

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.

Fixes: cf6e7bac6357 ("drm/i915: Add support for drm syncobjs")
Cc: Jason Ekstrand 
Cc: Chris Wilson 
Signed-off-by: Kees Cook 
---
 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(, user++, sizeof(fence))) {
+   if (copy_from_user(, user++, sizeof(fence))) {
err = -EFAULT;
goto err;
}
-- 
2.7.4


-- 
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Pixel Security
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