[PATCH ipsec] xfrm: account XFRMA_MARK in xfrm_sa_len()

[Yasuaki Torimaru <yasuakitorimaru@gmail.com>]

xfrm_notify_sa() sizes its netlink message with xfrm_sa_len(), while
copy_to_user_state_extra() appends XFRMA_MARK whenever the state carries
a mark.  xfrm_sa_len() does not count that attribute.  Only the
XFRM_MSG_DELSA path adds it to the length separately, so for a marked
state the XFRM_MSG_NEWSA and XFRM_MSG_UPDSA messages are sized short by
nla_total_size(sizeof(struct xfrm_mark)), 12 bytes.  The other length
calculations in this file already account for the mark; xfrm_sa_len() is
the only one that does not.

Instrumenting xfrm_notify_sa() on an x86-64 KASAN build under qemu to
print the accounted length while adding a marked state confirms the gap:
accounting the mark raises the computed length by exactly 12 bytes
(348 -> 360), matching nla_total_size(sizeof(struct xfrm_mark)).

Whether the shortfall is reached depends on the skb allocation, which is
rounded up: xfrm_notify_sa() fails with -EMSGSIZE only when that slack is
exhausted.  km_state_notify() discards the return value, so such a
failure would be silent.  This was not reproduced.

Count the mark in xfrm_sa_len() for every event and drop the now
redundant addition in the DELSA path.  The message content is unchanged;
only the computed length changes.

Fixes: 6f26b61e177e ("xfrm: Allow user space config of SAD mark")
Signed-off-by: Yasuaki Torimaru <yasuakitorimaru@gmail.com>
---
 net/xfrm/xfrm_user.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index a2587c7e796b..5c6ee35bc012 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -4046,6 +4046,8 @@ static inline unsigned int xfrm_sa_len(struct xfrm_state *x)
 		l += nla_total_size(sizeof(*x->encap));
 	if (x->tfcpad)
 		l += nla_total_size(sizeof(x->tfcpad));
+	if (x->mark.v | x->mark.m)
+		l += nla_total_size(sizeof(struct xfrm_mark));
 	if (x->replay_esn)
 		l += nla_total_size(xfrm_replay_state_esn_len(x->replay_esn));
 	else
@@ -4101,7 +4103,6 @@ static int xfrm_notify_sa(struct xfrm_state *x, const struct km_event *c)
 	if (c->event == XFRM_MSG_DELSA) {
 		len += nla_total_size(headlen);
 		headlen = sizeof(*id);
-		len += nla_total_size(sizeof(struct xfrm_mark));
 	}
 	len += NLMSG_ALIGN(headlen);
 
-- 
2.54.0