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From: Linus Torvalds <torvalds@linux-foundation.org>
To: syzbot <syzbot+e86f7c428c8c50db65b4@syzkaller.appspotmail.com>
Cc: 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)
Date: Tue, 22 Dec 2020 14:01:21 -0800
Message-ID: <CAHk-=wgfRnXz0W3D37d01q3JFkr_i_uTL=V6A6G1oUZcprmknw@mail.gmail.com> (raw)
In-Reply-To: <000000000000fcbe0705b70e9bd9@google.com>

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()".

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.

So it's a completely useless function if you can't implicitly trust
the source string - but that is almost always why people think they
should use it!

Nobody should use it. I really would like to remove it, and let
everybody know how incredibly broken sh*t that function is.

Can we please have everybody stop using strlcpy(). But in this
particular case, it's that xt_rateest_tg_checkentry() in
net/netfilter/xt_RATEEST.c

That said, this may be a real FORTIFY report if that info->name is
*supposed* to be trustworthy? The xt_RATETEST code does use
"info->name" a few lines earlier when it does

        est = __xt_rateest_lookup(xn, info->name);

or maybe the bisection is right, and this points to some problem with
__builtin_clzll?

                 Linus

  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-22 14:44 syzbot
2020-12-22 22:01 ` Linus Torvalds [this message]
2020-12-22 22:07   ` Florian Westphal
2020-12-23 11:25     ` Dmitry Vyukov
2020-12-22 22:23 ` [PATCH nf] netfilter: xt_RATEEST: reject non-null terminated string from userspace Florian Westphal
2020-12-22 22:29   ` Linus Torvalds
2020-12-22 22:50     ` Florian Westphal
2020-12-27 10:53   ` Pablo Neira Ayuso

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