xfrm/compat: memset(0) 64-bit padding at right place

32-bit messages translated by xfrm_compat can have attributes attached.
For all, but XFRMA_SA, XFRMA_POLICY the size of payload is the same
in 32-bit UABI and 64-bit UABI. For XFRMA_SA (struct xfrm_usersa_info)
and XFRMA_POLICY (struct xfrm_userpolicy_info) it's only tail-padding
that is present in 64-bit payload, but not in 32-bit.
The proper size for destination nlattr is already calculated by
xfrm_user_rcv_calculate_len64() and allocated with kvmalloc().

xfrm_attr_cpy32() copies 32-bit copy_len into 64-bit attribute
translated payload, zero-filling possible padding for SA/POLICY.
Due to a typo, *pos already has 64-bit payload size, in a result next
memset(0) is called on the memory after the translated attribute, not on
the tail-padding of it.

Fixes: 5106f4a8 ("xfrm/compat: Add 32=>64-bit messages translator")
Reported-by: syzbot+c43831072e7df506a646@syzkaller.appspotmail.com
Signed-off-by: NDmitry Safonov <dima@arista.com>
Signed-off-by: NSteffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_compat.c

@@ -388,7 +388,7 @@ static int xfrm_attr_cpy32(void *dst, size_t *pos, const struct nlattr *src,
 
 	memcpy(nla, src, nla_attr_size(copy_len));
 	nla->nla_len = nla_attr_size(payload);
-	*pos += nla_attr_size(payload);
+	*pos += nla_attr_size(copy_len);
 	nlmsg->nlmsg_len += nla->nla_len;
 
 	memset(dst + *pos, 0, payload - copy_len);