netdev - Re: [PATCH net] net: icmp: zero-out cb in icmp{,v6}_ndo

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Message-ID: <20210218110820.18f29ee5@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 18 Feb 2021 11:08:20 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     netdev@...r.kernel.org, SinYu <liuxyon@...il.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] net: icmp: zero-out cb in icmp{,v6}_ndo_send before
 sending

On Thu, 18 Feb 2021 13:30:53 +0100 Jason A. Donenfeld wrote:
> The icmp{,v6}_send functions make all sorts of use of skb->cb, assuming
> the skb to have come directly from the inet layer. But when the packet
> comes from the ndo layer, especially when forwarded, there's no telling
> what might be in skb->cb at that point. So, icmp{,v6}_ndo_send must zero
> out its skb->cb before passing the packet off to icmp{,v6}_send.
> Otherwise the icmp sending code risks reading bogus memory contents,
> which can result in nasty stack overflows such as this one reported by a
> user:
> 
>     panic+0x108/0x2ea
>     __stack_chk_fail+0x14/0x20
>     __icmp_send+0x5bd/0x5c0
>     icmp_ndo_send+0x148/0x160
> 
> This is easy to simulate by doing a `memset(skb->cb, 0x41,
> sizeof(skb->cb));` before calling icmp{,v6}_ndo_send, and it's only by
> good fortune and the rarity of icmp sending from that context that we've
> avoided reports like this until now. For example, in KASAN:
> 
>     BUG: KASAN: stack-out-of-bounds in __ip_options_echo+0xa0e/0x12b0
>     Write of size 38 at addr ffff888006f1f80e by task ping/89
>     CPU: 2 PID: 89 Comm: ping Not tainted 5.10.0-rc7-debug+ #5
>     Call Trace:
>      dump_stack+0x9a/0xcc
>      print_address_description.constprop.0+0x1a/0x160
>      __kasan_report.cold+0x20/0x38
>      kasan_report+0x32/0x40
>      check_memory_region+0x145/0x1a0
>      memcpy+0x39/0x60
>      __ip_options_echo+0xa0e/0x12b0
>      __icmp_send+0x744/0x1700
> 
> Actually, out of the 4 drivers that do this, only gtp zeroed the cb for
> the v4 case, while the rest did not. So this commit actually removes the
> gtp-specific zeroing, while putting the code where it belongs in the
> shared infrastructure of icmp{,v6}_ndo_send.
> 
> Fixes: a2b78e9b2cac ("sunvnet: generate ICMP PTMUD messages for smaller port MTUs")

nit: please make sure you CC the authors of the commits you're blaming.