linux-kernel - Re: [PATCH] Convert filldir[64]() from __put_user() to unsafe_put

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Date:   Sun, 6 Oct 2019 20:11:42 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] Convert filldir[64]() from __put_user() to unsafe_put_user()

On Sun, Oct 6, 2019 at 7:50 PM Al Viro <viro@...iv.linux.org.uk> 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