Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 13 Jan 2021 17:27:04 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     Randy Dunlap <rdunlap@infradead.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org, aryabinin@virtuozzo.com,
        dvyukov@google.com
Subject: Re: [PATCH] mm/uaccess: Use 'unsigned long' to placate UBSAN
 warnings, again
Message-ID: <20210113232704.66hqq4o5oes4up76@treble>
References: <590998aa9cc50f431343f76cae72b2abf8ac1fdd.1608699683.git.jpoimboe@redhat.com>
 <20210104151317.GR3021@hirez.programming.kicks-ass.net>
 <202101061536.C4A93132@keescook>
 <d5b192b2-b216-57d1-4505-06233ae2b882@infradead.org>
 <202101071306.5A39AAC3A7@keescook>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <202101071306.5A39AAC3A7@keescook>
Precedence: bulk

On Thu, Jan 07, 2021 at 01:07:21PM -0800, Kees Cook wrote:
> On Wed, Jan 06, 2021 at 04:06:57PM -0800, Randy Dunlap wrote:
> > On 1/6/21 3:37 PM, Kees Cook wrote:
> > > On Mon, Jan 04, 2021 at 04:13:17PM +0100, Peter Zijlstra wrote:
> > >> On Tue, Dec 22, 2020 at 11:04:54PM -0600, Josh Poimboeuf wrote:
> > >>> GCC 7 has a known bug where UBSAN ignores '-fwrapv' and generates false
> > >>> signed-overflow-UB warnings.  The type mismatch between 'i' and
> > >>> 'nr_segs' in copy_compat_iovec_from_user() is causing such a warning,
> > >>> which also happens to violate uaccess rules:
> > >>>
> > >>>   lib/iov_iter.o: warning: objtool: iovec_from_user()+0x22d: call to __ubsan_handle_add_overflow() with UACCESS enabled
> > >>>
> > >>> Fix it by making the variable types match.
> > >>>
> > >>> This is similar to a previous commit:
> > >>>
> > >>>   29da93fea3ea ("mm/uaccess: Use 'unsigned long' to placate UBSAN warnings on older GCC versions")
> > >>
> > >> Maybe it's time we make UBSAN builds depend on GCC-8+ ?
> > > 
> > > I would be totally fine with that. The only thing I can think of that
> > > might care is syzbot. Dmitry, does syzbot use anything older than gcc 8?
> > 
> > I use UBSAN successfully with GCC 7.5.0.
> > However, I can revert whatever future patch someone adds for this...
> 
> Peter, which GCC version specifically are you seeing this on? (i.e. can
> I just make in 7.5+ instead of 8+ to make Randy's life easier?)

I don't think that would help, we saw this bug on GCC 7.5.

-- 
Josh