[net,v2] gso: fix udp gso fraglist segmentation after pull from frag_list

Commit Message

Willem de Bruijn Oct. 1, 2024, 5:17 p.m. UTC

From: Willem de Bruijn <[e-mail]>

Detect gso fraglist skbs with corrupted geometry (see below) and
pass these to skb_segment instead of skb_segment_list, as the first
can segment them correctly.

Valid SKB_GSO_FRAGLIST skbs
- consist of two or more segments
- the head_skb holds the protocol headers plus first gso_size
- one or more frag_list skbs hold exactly one segment
- all but the last must be gso_size

Optional datapath hooks such as NAT and BPF (bpf_skb_pull_data) can
modify these skbs, breaking these invariants.

In extreme cases they pull all data into skb linear. For UDP, this
causes a NULL ptr deref in __udpv4_gso_segment_list_csum at
udp_hdr(seg->next)->dest.

Detect invalid geometry due to pull, by checking head_skb size.
Don't just drop, as this may blackhole a destination. Convert to be
able to pass to regular skb_segment.

Link: https://lore.kernel.org/netdev/[e-mail]/
Fixes: 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.")
Signed-off-by: Willem de Bruijn <[e-mail]>
Cc: [e-mail]

---

v1->v2
  - update Fixes tag to point to udp specific patch
  - reinit uh->check to pseudo csum as required by CHECKSUM_PARTIAL
---
 net/ipv4/udp_offload.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

Comments

[e-mail] Oct. 3, 2024, 12:40 a.m. UTC | #1

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <[e-mail]>:

On Tue,  1 Oct 2024 13:17:46 -0400 you wrote:
> From: Willem de Bruijn <[e-mail]>
> 
> Detect gso fraglist skbs with corrupted geometry (see below) and
> pass these to skb_segment instead of skb_segment_list, as the first
> can segment them correctly.
> 
> Valid SKB_GSO_FRAGLIST skbs
> - consist of two or more segments
> - the head_skb holds the protocol headers plus first gso_size
> - one or more frag_list skbs hold exactly one segment
> - all but the last must be gso_size
> 
> [...]

Here is the summary with links:
  - [net,v2] gso: fix udp gso fraglist segmentation after pull from frag_list
    https://git.kernel.org/netdev/net/c/a1e40ac5b5e9

You are awesome, thank you!

Patch

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index d842303587af..a5be6e4ed326 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -296,8 +296,26 @@  struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 		return NULL;
 	}
 
-	if (skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST)
-		return __udp_gso_segment_list(gso_skb, features, is_ipv6);
+	if (skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST) {
+		 /* Detect modified geometry and pass those to skb_segment. */
+		if (skb_pagelen(gso_skb) - sizeof(*uh) == skb_shinfo(gso_skb)->gso_size)
+			return __udp_gso_segment_list(gso_skb, features, is_ipv6);
+
+		 /* Setup csum, as fraglist skips this in udp4_gro_receive. */
+		gso_skb->csum_start = skb_transport_header(gso_skb) - gso_skb->head;
+		gso_skb->csum_offset = offsetof(struct udphdr, check);
+		gso_skb->ip_summed = CHECKSUM_PARTIAL;
+
+		uh = udp_hdr(gso_skb);
+		if (is_ipv6)
+			uh->check = ~udp_v6_check(gso_skb->len,
+						  &ipv6_hdr(gso_skb)->saddr,
+						  &ipv6_hdr(gso_skb)->daddr, 0);
+		else
+			uh->check = ~udp_v4_check(gso_skb->len,
+						  ip_hdr(gso_skb)->saddr,
+						  ip_hdr(gso_skb)->daddr, 0);
+	}
 
 	skb_pull(gso_skb, sizeof(*uh));