[PATCH net v2 1/3] netfilter: synproxy: Fix out of bounds when parsing TCP options

[Maxim Mikityanskiy <maximmi@nvidia.com>]

The TCP option parser in synproxy (synproxy_parse_options) could read
one byte out of bounds. When the length is 1, the execution flow gets
into the loop, reads one byte of the opcode, and if the opcode is
neither TCPOPT_EOL nor TCPOPT_NOP, it reads one more byte, which exceeds
the length of 1.

This fix is inspired by commit 9609dad263f8 ("ipv4: tcp_input: fix stack
out of bounds when parsing TCP options.").

v2 changes:

Added an early return when length < 0 to avoid calling
skb_header_pointer with negative length.

Cc: Young Xiao <92siuyang@gmail.com>
Fixes: 48b1de4c110a ("netfilter: add SYNPROXY core/target")
Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
---
 net/netfilter/nf_synproxy_core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/netfilter/nf_synproxy_core.c b/net/netfilter/nf_synproxy_core.c
index b100c04a0e43..3d6d49420db8 100644
--- a/net/netfilter/nf_synproxy_core.c
+++ b/net/netfilter/nf_synproxy_core.c
@@ -31,6 +31,9 @@ synproxy_parse_options(const struct sk_buff *skb, unsigned int doff,
 	int length = (th->doff * 4) - sizeof(*th);
 	u8 buf[40], *ptr;
 
+	if (unlikely(length < 0))
+		return false;
+
 	ptr = skb_header_pointer(skb, doff + sizeof(*th), length, buf);
 	if (ptr == NULL)
 		return false;
@@ -47,6 +50,8 @@ synproxy_parse_options(const struct sk_buff *skb, unsigned int doff,
 			length--;
 			continue;
 		default:
+			if (length < 2)
+				return true;
 			opsize = *ptr++;
 			if (opsize < 2)
 				return true;
-- 
2.25.1