linux-kernel - Re: net/ipv4: use-after-free in ipv4

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Message-ID: <CAM_iQpUk_=oeGoYLb4DGc2+mNw+3Z08mw4wVNKAaDZgsbB1i+g@mail.gmail.com>
Date:   Wed, 5 Apr 2017 15:33:40 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: net/ipv4: use-after-free in ipv4_mtu

On Tue, Apr 4, 2017 at 7:45 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-04-04 at 18:11 -0700, Cong Wang wrote:
>> On Tue, Apr 4, 2017 at 11:51 AM, Eric Dumazet <edumazet@...gle.com> wrote:
>> > Looking at fib->fib_metrics, I fail to understand how the following can work :
>> >
>> > dst_init_metrics(&rt->dst, fi->fib_metrics, true);
>> >
>> > In the cases fi->fib_metrics is _not_ dst_default_metrics,
>> > fi->fib_metrics can be freed when the fib is deleted,
>> > while dst(s) have still the 'read only pointer'.
>> >
>> > RCU grace period before fi->fib_metrics freeing does not help.
>> >
>> > Without refcounts, it looks like we need to copy the fib_metrics.
>>
>> The dst is obtained from sk_dst_cache which is cached for a fast
>> path where fib_info is obtained in fib_lookup() without refcnt:
>>
>>                 err = fib_table_lookup(tb, flp, res, flags | FIB_LOOKUP_NOREF);
>>
>>
>> ...
>>                         if (!(fib_flags & FIB_LOOKUP_NOREF))
>>                                 atomic_inc(&fi->fib_clntref);
>>
>>
>> This probably starts from:
>>
>> commit ebc0ffae5dfb4447e0a431ffe7fe1d467c48bbb9
>> Author: Eric Dumazet <eric.dumazet@...il.com>
>> Date:   Tue Oct 5 10:41:36 2010 +0000
>>
>>     fib: RCU conversion of fib_lookup()
>
> Interesting. I might had too many beers tonight, but ...
>
> refcount was removed in 2860583fe840 many months later

Good find! I missed the refcnt in rt_set_nexthop() before that commit.

We need to revert that commit to restore the refcnt for fib_info.

View attachment "ipv4-fib-info.diff" of type "text/plain" (1798 bytes)