On Sun, Oct 6, 2019 at 7:50 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> Out of those, only __copy_to_user_inatomic(), __copy_to_user(),
> _copy_to_user() and iov_iter.c:copyout() can be called on
> any architecture.
>
> The last two should just do user_access_begin()/user_access_end()
> instead of access_ok(). __copy_to_user_inatomic() has very few callers as well:
Yeah, good points.
It looks like it would be better to just change over semantics
entirely to the unsafe_copy_user() model.
> So few, in fact, that I wonder if we want to keep it at all; the only
> thing stopping me from "let's remove it" is that I don't understand
> the i915 side of things. Where does it do an equivalent of access_ok()?
Honestly, if you have to ask, I think the answer is: just add one.
Every single time we've had people who optimized things to try to
avoid the access_ok(), they just caused bugs and problems.
In this case, I think it's done a few callers up in i915_gem_pread_ioctl():
if (!access_ok(u64_to_user_ptr(args->data_ptr),
args->size))
return -EFAULT;
but honestly, trying to optimize away another "access_ok()" is just
not worth it. I'd rather have an extra one than miss one.
> And mm/maccess.c one is __probe_kernel_write(), so presumably we don't
> want stac/clac there at all...
Yup.
> So do we want to bother with separation between raw_copy_to_user() and
> unsafe_copy_to_user()? After all, __copy_to_user() also has only few
> callers, most of them in arch/*
No, you're right. Just switch over.
> I'll take a look into that tomorrow - half-asleep right now...
Thanks. No huge hurry.
Linus
