netdev - Re: kernel BUG at lib/string.c:LINE! (6)

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Date:   Tue, 22 Dec 2020 14:01:21 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     syzbot <syzbot+e86f7c428c8c50db65b4@...kaller.appspotmail.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, coreteam@...filter.org,
        David Miller <davem@...emloft.net>,
        Florian Westphal <fw@...len.de>,
        Jakub Jelinek <jakub@...hat.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        NetFilter <netfilter-devel@...r.kernel.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: kernel BUG at lib/string.c:LINE! (6)

On Tue, Dec 22, 2020 at 6:44 AM syzbot
<syzbot+e86f7c428c8c50db65b4@...kaller.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