__ip_options_echo() re-reads each option length byte (RR/TS/SRR/CIPSO)
from skb->data when building the echoed options into a 40-byte
__data[] buffer.  __ip_options_compile() saved only the option offset
into IPCB(skb)->opt, not the length.  An nftables LOCAL_IN payload
write reachable from an unprivileged user namespace can mutate the
length byte between parse and recvmsg, turning a parse-time validated
7-byte option into a 255-byte read.

  unsigned char optbuf[sizeof(struct ip_options) + 40];
  /* in __ip_options_echo: */
  optlen = sptr[sopt->rr + 1];        /* re-read; nft can mutate */
  memcpy(dptr, sptr + sopt->rr, optlen); /* into 40-byte buffer */

The destination is a stack buffer in ip_cmsg_recv_retopts() and a
DEFINE_RAW_FLEX() buffer in icmp.c / ip_output.c sized
IP_OPTIONS_DATA_FIXED_SIZE (40).  KASAN reports a stack-out-of-bounds
write of size 255:

  BUG: KASAN: stack-out-of-bounds in __ip_options_echo+0x7fc/0x1310
  Write of size 255 at addr ffff88800a657950
   __asan_memcpy+0x3c/0x60
   __ip_options_echo+0x7fc/0x1310
   ip_cmsg_recv_offset+0x58b/0xd10
   udp_recvmsg+0x8da/0xc20
   ____sys_recvmsg+0x1b1/0x620

Validate that each re-read option length stays within
skb_tail_pointer(skb) before the memcpy.

Cc: stable@vger.kernel.org
Reported-by: Qi Tang <redacted>
Reported-by: Tong Liu <redacted>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Qi Tang <redacted>
---
 net/ipv4/ip_options.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/ipv4/ip_options.c b/net/ipv4/ip_options.c
index be8815ce3ac24..1cc6096e6dd9d 100644
--- a/net/ipv4/ip_options.c
+++ b/net/ipv4/ip_options.c

@@ -91,6 +91,8 @@ int __ip_options_echo(struct net *net, struct ip_options *dopt,
 
 	if (sopt->rr) {
 		optlen  = sptr[sopt->rr+1];
+		if (sptr + sopt->rr + optlen > skb_tail_pointer(skb))
+			return -EINVAL;
 		soffset = sptr[sopt->rr+2];
 		dopt->rr = dopt->optlen + sizeof(struct iphdr);
 		memcpy(dptr, sptr+sopt->rr, optlen);

@@ -105,6 +107,8 @@ int __ip_options_echo(struct net *net, struct ip_options *dopt,
 	}
 	if (sopt->ts) {
 		optlen = sptr[sopt->ts+1];
+		if (sptr + sopt->ts + optlen > skb_tail_pointer(skb))
+			return -EINVAL;
 		soffset = sptr[sopt->ts+2];
 		dopt->ts = dopt->optlen + sizeof(struct iphdr);
 		memcpy(dptr, sptr+sopt->ts, optlen);

@@ -145,6 +149,8 @@ int __ip_options_echo(struct net *net, struct ip_options *dopt,
 		__be32 faddr;
 
 		optlen  = start[1];
+		if (start + optlen > skb_tail_pointer(skb))
+			return -EINVAL;
 		soffset = start[2];
 		doffset = 0;
 		if (soffset > optlen)

@@ -174,6 +180,8 @@ int __ip_options_echo(struct net *net, struct ip_options *dopt,
 	}
 	if (sopt->cipso) {
 		optlen  = sptr[sopt->cipso+1];
+		if (sptr + sopt->cipso + optlen > skb_tail_pointer(skb))
+			return -EINVAL;
 		dopt->cipso = dopt->optlen+sizeof(struct iphdr);
 		memcpy(dptr, sptr+sopt->cipso, optlen);
 		dptr += optlen;

-- 
2.47.3