Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20191006222046.GA18027@roeck-us.net> <CAHk-=wgrqwuZJmwbrjhjCFeSUu2i57unaGOnP4qZAmSyuGwMZA@mail.gmail.com>
 <CAHk-=wjRPerXedTDoBbJL=tHBpH+=sP6pX_9NfgWxpnmHC5RtQ@mail.gmail.com>
 <5f06c138-d59a-d811-c886-9e73ce51924c@roeck-us.net> <CAHk-=whAQWEMADgxb_qAw=nEY4OnuDn6HU4UCSDMNT5ULKvg3g@mail.gmail.com>
 <20191007012437.GK26530@ZenIV.linux.org.uk> <CAHk-=whKJfX579+2f-CHc4_YmEmwvMe_Csr0+CPfLAsSAdfDoA@mail.gmail.com>
 <20191007025046.GL26530@ZenIV.linux.org.uk>
In-Reply-To: <20191007025046.GL26530@ZenIV.linux.org.uk>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 6 Oct 2019 20:11:42 -0700
Message-ID: <CAHk-=whraNSys_Lj=Ut1EA=CJEfw2Uothh+5-WL+7nDJBegWcQ@mail.gmail.com>
Subject: Re: [PATCH] Convert filldir[64]() from __put_user() to unsafe_put_user()
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sun, Oct 6, 2019 at 7:50 PM Al Viro <viro@zeniv.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