[PATCH net v2 3/4] netlabel: validate CALIPSO option against skb tail in netlbl_skbuff_getattr

[Qi Tang <tpluszz77@gmail.com>]

netlbl_skbuff_getattr() locates the CALIPSO option in the IPv6 HBH
header via calipso_optptr() and hands the bare pointer to
calipso_getattr() -> calipso_opt_getattr().  The consumer re-reads
calipso[1] (option data length) and calipso[6] (cat_len/4) and walks
calipso + 10 for cat_len bytes via netlbl_bitmap_walk().

ipv6_hop_calipso() validates these bytes only at parse time inside
ipv6_parse_hopopts().  An nftables PRE_ROUTING payload write reachable
from an unprivileged user namespace can rewrite both bytes between
parse and the SELinux peer-label consume path
(selinux_sock_rcv_skb_compat -> selinux_netlbl_sock_rcv_skb ->
netlbl_skbuff_getattr).  The self-consistency check
(cat_len + 8 > len) inside calipso_opt_getattr() is defeated by
mutating both bytes consistently, allowing a ~232-byte
slab-out-of-bounds read from calipso + 10 whose set bits become MLS
categories driving the access decision.

netlbl_skbuff_getattr() has the skb; gate the consume on the option
fitting within skb_tail_pointer().  The IPv6 option layout is
type(1) + length(1) + length bytes of data, so requiring
ptr + 2 + ptr[1] <= skb_tail covers the option and its embedded
bitmap.  When the bounds check fails the packet has been mutated
after parse, so return -EINVAL rather than fall through to the
unlabeled path.

Runtime confirmation (SELinux compat path with selinux=1 enforcing=0
and a CALIPSO DOI added via netlabelctl): Udp6InDatagrams increments
to 1 with the mutated cat_len, showing
selinux_socket_sock_rcv_skb -> netlbl_skbuff_getattr ->
calipso_opt_getattr -> netlbl_bitmap_walk runs end-to-end past the
option's true bound; with this patch the consume path returns
-EINVAL at the bounds check and the counter stays 0.

Cc: stable@vger.kernel.org
Reported-by: Qi Tang <tpluszz77@gmail.com>
Reported-by: Tong Liu <lyutoon@gmail.com>
Fixes: 2917f57b6bc1 ("calipso: Allow the lsm to label the skbuff directly.")
Signed-off-by: Qi Tang <tpluszz77@gmail.com>
---
 net/netlabel/netlabel_kapi.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/net/netlabel/netlabel_kapi.c b/net/netlabel/netlabel_kapi.c
index 3583fa63dd01f..d0d6220b8d59d 100644
--- a/net/netlabel/netlabel_kapi.c
+++ b/net/netlabel/netlabel_kapi.c
@@ -1399,11 +1399,22 @@ int netlbl_skbuff_getattr(const struct sk_buff *skb,
 			return 0;
 		break;
 #if IS_ENABLED(CONFIG_IPV6)
-	case AF_INET6:
+	case AF_INET6: {
+		const unsigned char *tail = skb_tail_pointer(skb);
+		u8 opt_data_len;
+
 		ptr = calipso_optptr(skb);
-		if (ptr && calipso_getattr(ptr, secattr) == 0)
+		if (!ptr)
+			break;
+		if (ptr + 2 > tail)
+			return -EINVAL;
+		opt_data_len = ptr[1];	/* IPv6 option data length */
+		if (ptr + 2 + opt_data_len > tail)
+			return -EINVAL;
+		if (calipso_getattr(ptr, secattr) == 0)
 			return 0;
 		break;
+	}
 #endif /* IPv6 */
 	}
 
-- 
2.47.3