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    SubjectRE: [PATCH] drm/i915: Use copy_from_user() in fence copying
    Date
    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 &fence probably refers to something in the
    current stack frame.)

    David

    > Fixes: cf6e7bac6357 ("drm/i915: Add support for drm syncobjs")
    > Cc: Jason Ekstrand <jason@jlekstrand.net>
    > Cc: Chris Wilson <chris@chris-wilson.co.uk>
    > Signed-off-by: Kees Cook <keescook@chromium.org>
    > ---
    > 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

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