linux-kernel - Re: [PATCH v2] net: ppp: Add bound checking for skb d on ppp_sync

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Message-ID: <09aeed01-405d-4eb7-9a12-297203f1edcc@redhat.com>
Date: Thu, 10 Apr 2025 11:30:56 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Arnaud Lecomte <contact@...aud-lcm.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>
Cc: linux-ppp@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, skhan@...uxfoundation.org,
 syzbot+29fc8991b0ecb186cf40@...kaller.appspotmail.com
Subject: Re: [PATCH v2] net: ppp: Add bound checking for skb d on
 ppp_sync_txmung

On 4/8/25 5:55 PM, Arnaud Lecomte wrote:
> Ensure we have enough data in linear buffer from skb before accessing
> initial bytes. This prevents potential out-of-bounds accesses
> when processing short packets.
> 
> When ppp_sync_txmung receives an incoming package with an empty
> payload:
> (remote) gef➤  p *(struct pppoe_hdr *) (skb->head + skb->network_header)
> $18 = {
> 	type = 0x1,
> 	ver = 0x1,
> 	code = 0x0,
> 	sid = 0x2,
>         length = 0x0,
> 	tag = 0xffff8880371cdb96
> }
> 
> from the skb struct (trimmed)
>       tail = 0x16,
>       end = 0x140,
>       head = 0xffff88803346f400 "4",
>       data = 0xffff88803346f416 ":\377",
>       truesize = 0x380,
>       len = 0x0,
>       data_len = 0x0,
>       mac_len = 0xe,
>       hdr_len = 0x0,
> 
> it is not safe to access data[2].
> 
> Reported-by: syzbot+29fc8991b0ecb186cf40@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
> Tested-by: syzbot+29fc8991b0ecb186cf40@...kaller.appspotmail.com
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>

A couple of notes for future submission: you should have included the
target tree in the subj prefix (in this case: "[PATCH net v2]..."), and
there is a small typo in the subjected (skb d -> skb data). The patch
looks good I'm applying it.

Thanks,

Paolo