linux-kernel - Re: [PATCH] mm/uaccess: Use 'unsigned long' to placate UBSAN warnings, again

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Message-ID: <202101061536.C4A93132@keescook>
Date:   Wed, 6 Jan 2021 15:37:53 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        aryabinin@...tuozzo.com, dvyukov@...gle.com
Subject: Re: [PATCH] mm/uaccess: Use 'unsigned long' to placate UBSAN
 warnings, again

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?

-- 
Kees Cook