[v2] net: drop bogus skb with CHECKSUM_PARTIAL and offset beyond end of trimmed packet

Commit Message

Vasily Averin Dec. 14, 2020, 7:07 p.m. UTC

syzbot reproduces BUG_ON in skb_checksum_help():
tun creates (bogus) skb with huge partial-checksummed area and
small ip packet inside. Then ip_rcv trims the skb based on size
of internal ip packet, after that csum offset points beyond of
trimmed skb. Then checksum_tg() called via netfilter hook
triggers BUG_ON:

        offset = skb_checksum_start_offset(skb);
        BUG_ON(offset >= skb_headlen(skb));

To work around the problem this patch forces pskb_trim_rcsum_slow()
to return -EINVAL in described scenario. It allows its callers to
drop such kind of packets.

Link: https://syzkaller.appspot.com/bug?id=b419a5ca95062664fe1a60b764621eb4526e2cd0
Reported-by: [e-mail]
Signed-off-by: Vasily Averin <[e-mail]>
---
v2: drop bogus packets instead change its CHECKSUM_PARTIAL to CHECKSUM_NONE 

 net/core/skbuff.c | 6 ++++++
 1 file changed, 6 insertions(+)

Comments

Willem de Bruijn Dec. 14, 2020, 8:59 p.m. UTC | #1

On Mon, Dec 14, 2020 at 2:21 PM Vasily Averin <[e-mail]> wrote:
>
> syzbot reproduces BUG_ON in skb_checksum_help():
> tun creates (bogus) skb with huge partial-checksummed area and
> small ip packet inside. Then ip_rcv trims the skb based on size
> of internal ip packet, after that csum offset points beyond of
> trimmed skb. Then checksum_tg() called via netfilter hook
> triggers BUG_ON:
>
>         offset = skb_checksum_start_offset(skb);
>         BUG_ON(offset >= skb_headlen(skb));
>
> To work around the problem this patch forces pskb_trim_rcsum_slow()
> to return -EINVAL in described scenario. It allows its callers to
> drop such kind of packets.
>
> Link: https://syzkaller.appspot.com/bug?id=b419a5ca95062664fe1a60b764621eb4526e2cd0
> Reported-by: [e-mail]
> Signed-off-by: Vasily Averin <[e-mail]>
> ---
> v2: drop bogus packets instead change its CHECKSUM_PARTIAL to CHECKSUM_NONE

Thanks for revising.

As far as I can tell, this goes back to the original introduction of
that user interface to set checksum offload, so

Fixes: 296f96fcfc16 ("Net driver using virtio")

For next time, please also mark network fixes as [PATCH net]. With that

Acked-by: Willem de Bruijn <[e-mail]>

[e-mail] Dec. 15, 2020, 3:10 a.m. UTC | #2

Hello:

This patch was applied to netdev/net-next.git (refs/heads/master):

On Mon, 14 Dec 2020 22:07:39 +0300 you wrote:
> syzbot reproduces BUG_ON in skb_checksum_help():
> tun creates (bogus) skb with huge partial-checksummed area and
> small ip packet inside. Then ip_rcv trims the skb based on size
> of internal ip packet, after that csum offset points beyond of
> trimmed skb. Then checksum_tg() called via netfilter hook
> triggers BUG_ON:
> 
> [...]

Here is the summary with links:
  - [v2] net: drop bogus skb with CHECKSUM_PARTIAL and offset beyond end of trimmed packet
    https://git.kernel.org/netdev/net-next/c/54970a2fbb67

You are awesome, thank you!
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Deet-doot-dot, I am a bot.
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Patch

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index e578544..fbadd93 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2011,6 +2011,12 @@  int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
 		skb->csum = csum_block_sub(skb->csum,
 					   skb_checksum(skb, len, delta, 0),
 					   len);
+	} else if (skb->ip_summed == CHECKSUM_PARTIAL) {
+		int hdlen = (len > skb_headlen(skb)) ? skb_headlen(skb) : len;
+		int offset = skb_checksum_start_offset(skb) + skb->csum_offset;
+
+		if (offset + sizeof(__sum16) > hdlen)
+			return -EINVAL;
 	}
 	return __pskb_trim(skb, len);
 }