ipv6: rpl: Fix Route of Death.

A remote DoS vulnerability of RPL Source Routing is assigned CVE-2023-2156.

The Source Routing Header (SRH) has the following format:

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Next Header  |  Hdr Ext Len  | Routing Type  | Segments Left |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | CmprI | CmprE |  Pad  |               Reserved                |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  .                                                               .
  .                        Addresses[1..n]                        .
  .                                                               .
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The originator of an SRH places the first hop's IPv6 address in the IPv6
header's IPv6 Destination Address and the second hop's IPv6 address as
the first address in Addresses[1..n].

The CmprI and CmprE fields indicate the number of prefix octets that are
shared with the IPv6 Destination Address.  When CmprI or CmprE is not 0,
Addresses[1..n] are compressed as follows:

  1..n-1 : (16 - CmprI) bytes
       n : (16 - CmprE) bytes

Segments Left indicates the number of route segments remaining.  When the
value is not zero, the SRH is forwarded to the next hop.  Its address
is extracted from Addresses[n - Segment Left + 1] and swapped with IPv6
Destination Address.

When Segment Left is greater than or equal to 2, the size of SRH is not
changed because Addresses[1..n-1] are decompressed and recompressed with
CmprI.

OTOH, when Segment Left changes from 1 to 0, the new SRH could have a
different size because Addresses[1..n-1] are decompressed with CmprI and
recompressed with CmprE.

Let's say CmprI is 15 and CmprE is 0.  When we receive SRH with Segment
Left >= 2, Addresses[1..n-1] have 1 byte for each, and Addresses[n] has
16 bytes.  When Segment Left is 1, Addresses[1..n-1] is decompressed to
16 bytes and not recompressed.  Finally, the new SRH will need more room
in the header, and the size is (16 - 1) * (n - 1) bytes.

Here the max value of n is 255 as Segment Left is u8, so in the worst case,
we have to allocate 3825 bytes in the skb headroom.  However, now we only
allocate a small fixed buffer that is IPV6_RPL_SRH_WORST_SWAP_SIZE (16 + 7
bytes).  If the decompressed size overflows the room, skb_push() hits BUG()
below [0].

Instead of allocating the fixed buffer for every packet, let's allocate
enough headroom only when we receive SRH with Segment Left 1.

[0]:
skbuff: skb_under_panic: text:ffffffff81c9f6e2 len:576 put:576 head:ffff8880070b5180 data:ffff8880070b4fb0 tail:0x70 end:0x140 dev:lo
kernel BUG at net/core/skbuff.c:200!
invalid opcode: 0000 [#1] PREEMPT SMP PTI
CPU: 0 PID: 154 Comm: python3 Not tainted 6.4.0-rc4-00190-gc308e9ec0047 #7
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
RIP: 0010:skb_panic (net/core/skbuff.c:200)
Code: 4f 70 50 8b 87 bc 00 00 00 50 8b 87 b8 00 00 00 50 ff b7 c8 00 00 00 4c 8b 8f c0 00 00 00 48 c7 c7 80 6e 77 82 e8 ad 8b 60 ff <0f> 0b 66 66 2e 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc90000003da0 EFLAGS: 00000246
RAX: 0000000000000085 RBX: ffff8880058a6600 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff88807dc1c540 RDI: ffff88807dc1c540
RBP: ffffc90000003e48 R08: ffffffff82b392c8 R09: 00000000ffffdfff
R10: ffffffff82a592e0 R11: ffffffff82b092e0 R12: ffff888005b1c800
R13: ffff8880070b51b8 R14: ffff888005b1ca18 R15: ffff8880070b5190
FS:  00007f4539f0b740(0000) GS:ffff88807dc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055670baf3000 CR3: 0000000005b0e000 CR4: 00000000007506f0
PKRU: 55555554
Call Trace:
 <IRQ>
 skb_push (net/core/skbuff.c:210)
 ipv6_rthdr_rcv (./include/linux/skbuff.h:2880 net/ipv6/exthdrs.c:634 net/ipv6/exthdrs.c:718)
 ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:437 (discriminator 5))
 ip6_input_finish (./include/linux/rcupdate.h:805 net/ipv6/ip6_input.c:483)
 __netif_receive_skb_one_core (net/core/dev.c:5494)
 process_backlog (./include/linux/rcupdate.h:805 net/core/dev.c:5934)
 __napi_poll (net/core/dev.c:6496)
 net_rx_action (net/core/dev.c:6565 net/core/dev.c:6696)
 __do_softirq (./arch/x86/include/asm/jump_label.h:27 ./include/linux/jump_label.h:207 ./include/trace/events/irq.h:142 kernel/softirq.c:572)
 do_softirq (kernel/softirq.c:472 kernel/softirq.c:459)
 </IRQ>
 <TASK>
 __local_bh_enable_ip (kernel/softirq.c:396)
 __dev_queue_xmit (net/core/dev.c:4272)
 ip6_finish_output2 (./include/net/neighbour.h:544 net/ipv6/ip6_output.c:134)
 rawv6_sendmsg (./include/net/dst.h:458 ./include/linux/netfilter.h:303 net/ipv6/raw.c:656 net/ipv6/raw.c:914)
 sock_sendmsg (net/socket.c:724 net/socket.c:747)
 __sys_sendto (net/socket.c:2144)
 __x64_sys_sendto (net/socket.c:2156 net/socket.c:2152 net/socket.c:2152)
 do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120)
RIP: 0033:0x7f453a138aea
Code: d8 64 89 02 48 c7 c0 ff ff ff ff eb b8 0f 1f 00 f3 0f 1e fa 41 89 ca 64 8b 04 25 18 00 00 00 85 c0 75 15 b8 2c 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 7e c3 0f 1f 44 00 00 41 54 48 83 ec 30 44 89
RSP: 002b:00007ffcc212a1c8 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007ffcc212a288 RCX: 00007f453a138aea
RDX: 0000000000000060 RSI: 00007f4539084c20 RDI: 0000000000000003
RBP: 00007f4538308e80 R08: 00007ffcc212a300 R09: 000000000000001c
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: ffffffffc4653600 R14: 0000000000000001 R15: 00007f4539712d1b
 </TASK>
Modules linked in:

Fixes: 8610c7c6e3 ("net: ipv6: add support for rpl sr exthdr")
Reported-by: Max VA
Closes: https://www.interruptlabs.co.uk/articles/linux-ipv6-route-of-death
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230605180617.67284-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/rpl.h

@@ -23,9 +23,6 @@ static inline int rpl_init(void)
 static inline void rpl_exit(void) {}
 #endif
 
-/* Worst decompression memory usage ipv6 address (16) + pad 7 */
-#define IPV6_RPL_SRH_WORST_SWAP_SIZE (sizeof(struct in6_addr) + 7)
-
 size_t ipv6_rpl_srh_size(unsigned char n, unsigned char cmpri,
 			 unsigned char cmpre);
 

net/ipv6/exthdrs.c

@@ -569,24 +569,6 @@ looped_back:
 		return -1;
 	}
 
-	if (skb_cloned(skb)) {
-		if (pskb_expand_head(skb, IPV6_RPL_SRH_WORST_SWAP_SIZE, 0,
-				     GFP_ATOMIC)) {
-			__IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
-					IPSTATS_MIB_OUTDISCARDS);
-			kfree_skb(skb);
-			return -1;
-		}
-	} else {
-		err = skb_cow_head(skb, IPV6_RPL_SRH_WORST_SWAP_SIZE);
-		if (unlikely(err)) {
-			kfree_skb(skb);
-			return -1;
-		}
-	}
-
-	hdr = (struct ipv6_rpl_sr_hdr *)skb_transport_header(skb);
-
 	if (!pskb_may_pull(skb, ipv6_rpl_srh_size(n, hdr->cmpri,
 						  hdr->cmpre))) {
 		kfree_skb(skb);
@@ -630,6 +612,17 @@ looped_back:
 	skb_pull(skb, ((hdr->hdrlen + 1) << 3));
 	skb_postpull_rcsum(skb, oldhdr,
 			   sizeof(struct ipv6hdr) + ((hdr->hdrlen + 1) << 3));
+	if (unlikely(!hdr->segments_left)) {
+		if (pskb_expand_head(skb, sizeof(struct ipv6hdr) + ((chdr->hdrlen + 1) << 3), 0,
+				     GFP_ATOMIC)) {
+			__IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)), IPSTATS_MIB_OUTDISCARDS);
+			kfree_skb(skb);
+			kfree(buf);
+			return -1;
+		}
+
+		oldhdr = ipv6_hdr(skb);
+	}
 	skb_push(skb, ((chdr->hdrlen + 1) << 3) + sizeof(struct ipv6hdr));
 	skb_reset_network_header(skb);
 	skb_mac_header_rebuild(skb);