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

On Sun, May 24, 2026 at 12:15 AM Qi Tang [off-list ref] wrote:

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 <redacted>
Reported-by: Tong Liu <redacted>
Fixes: 2917f57b6bc1 ("calipso: Allow the lsm to label the skbuff directly.")
Signed-off-by: Qi Tang <redacted>
---
 net/netlabel/netlabel_kapi.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

Thanks for the fix.

Acked-by: Paul Moore <paul@paul-moore.com>

-- 
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