Message-ID: <9f549c7b-bc72-8037-abfc-fe2f146ff82c@ouvaton.org>
Date:   Tue, 7 Feb 2023 10:06:14 +0100
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Subject: Re: [PATCH] uaccess: Add minimum bounds check on kernel buffer size
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To:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Arnd Bergmann <arnd@arndb.de>
Cc:     Kees Cook <keescook@chromium.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Dinh Nguyen <dinguyen@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        Christian Brauner <brauner@kernel.org>,
        Stafford Horne <shorne@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
References: <20230203193523.never.667-kees@kernel.org>
 <6c728dfc-d777-4beb-b463-649704c81a5e@app.fastmail.com>
 <CAMuHMdXXSwYYoUMskhcgjF9SVjraZC-UsBT3sN+xkcUAYmJj4Q-2143@mail.gmail.com>
From:   Yann Droneaud <ydroneaud@ouvaton.org>
In-Reply-To: <CAMuHMdXXSwYYoUMskhcgjF9SVjraZC-UsBT3sN+xkcUAYmJj4Q-2143@mail.gmail.com>
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Hi,

Le 06/02/2023 à 21:03, Geert Uytterhoeven a écrit :

> Hi Arnd,
>
> On Fri, Feb 3, 2023 at 10:23 PM Arnd Bergmann <arnd@arndb.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?

It should be a compile-time check, because one would not want __builtin_object_size(dst, 1) to return -1 if dst' size is not known at compile-time.

Regards.

-- 
Yann Droneaud
OPTEYA