linux-kernel - Re: [PATCH] uaccess: Add minimum bounds check on kernel buffer size

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Message-ID: <63e171e4.630a0220.b52f4.b63f@mx.google.com>
Date:   Mon, 6 Feb 2023 13:32:19 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Aleksa Sarai <cyphar@...har.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Christian Brauner <brauner@...nel.org>,
        Stafford Horne <shorne@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] uaccess: Add minimum bounds check on kernel buffer size

On Mon, Feb 06, 2023 at 09:03:19PM +0100, Geert Uytterhoeven wrote:
> Hi Arnd,
> 
> On Fri, Feb 3, 2023 at 10:23 PM Arnd Bergmann <arnd@...db.de> wrote:
> > On Fri, Feb 3, 2023, at 20:35, Kees Cook wrote:
> > > --- a/include/linux/uaccess.h
> > > +++ b/include/linux/uaccess.h
> > > @@ -329,6 +329,10 @@ copy_struct_from_user(void *dst, size_t ksize,
> > > const void __user *src,
> > >       size_t size = min(ksize, usize);
> > >       size_t rest = max(ksize, usize) - size;
> > >
> > > +     /* Double check if ksize is larger than a known object size. */
> > > +     if (WARN_ON_ONCE(ksize > __builtin_object_size(dst, 1)))
> > > +             return -E2BIG;
> > > +
> >
> > WARN_ON_ONCE() may be a little expensive since that adds two
> > comparisons and a static variable to each copy, but it's probably
> > fine.
> 
> When seeing this, I was a bit worried about the size increase.
> Hence I gave it a try on atari_defconfig and ran bloat-o-meter.
> Surprisingly, there was no size increase at all, as all checks
> were optimized away.
> 
> Hence perhaps this can become a compile-time check?

Normally it should optimize away, yes.

-- 
Kees Cook