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Date:   Tue, 22 Dec 2020 23:07:19 +0100
From:   Florian Westphal <fw@strlen.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     syzbot <syzbot+e86f7c428c8c50db65b4@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        coreteam@netfilter.org, David Miller <davem@davemloft.net>,
        Florian Westphal <fw@strlen.de>,
        Jakub Jelinek <jakub@redhat.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Jozsef Kadlecsik <kadlec@netfilter.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>,
        NetFilter <netfilter-devel@vger.kernel.org>,
        Pablo Neira Ayuso <pablo@netfilter.org>,
        Peter Zijlstra <peterz@infradead.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Tejun Heo <tj@kernel.org>
Subject: Re: kernel BUG at lib/string.c:LINE! (6)
Message-ID: <20201222220719.GB9639@breakpoint.cc>
References: <000000000000fcbe0705b70e9bd9@google.com>
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Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Tue, Dec 22, 2020 at 6:44 AM syzbot
> <syzbot+e86f7c428c8c50db65b4@syzkaller.appspotmail.com> wrote:
> >
> > The issue was bisected to:
> >
> > commit 2f78788b55ba ("ilog2: improve ilog2 for constant arguments")
> 
> That looks unlikely, although possibly some constant folding
> improvement might make the fortify code notice something with it.
> 
> > detected buffer overflow in strlen
> > ------------[ cut here ]------------
> > kernel BUG at lib/string.c:1149!
> > Call Trace:
> >  strlen include/linux/string.h:325 [inline]
> >  strlcpy include/linux/string.h:348 [inline]
> >  xt_rateest_tg_checkentry+0x2a5/0x6b0 net/netfilter/xt_RATEEST.c:143
> 
> Honestly, this just looks like the traditional bug in "strlcpy()".

Yes, thats exactly what this is, no idea why the bisection points
at ilog2 changes.

> That BSD function is complete garbage, exactly because it doesn't
> limit the source length. People tend to _think_ it does ("what's that
> size_t argument for?") but strlcpy() only limits the *destination*
> size, and the source is always read fully.

Right, I'll send a patch shortly.