[PATCH bpf-next v3] bpf: Fix stack out-of-bounds write in cgroup link update

[Sanghyun Park]

The cgroup link update path checks only the program type. Several cgroup
hooks share a type while using different runtime contexts or verifier
contracts. A UDP6 sock_addr program can therefore replace a UDP4 program
and write beyond the four-byte ipc.addr context into adjacent fields of
the stack-local struct ipcm_cookie. The same omission lets an LSM_MAC
program replace an LSM_CGROUP program despite the incompatible return
semantics.

Validate replacement programs against the link attach type. Use the
existing per-type rules where applicable, and compare LSM
expected_attach_type explicitly because both flavors share
BPF_PROG_TYPE_LSM. Preserve legacy non-enforcing CGROUP_SKB
ingress/egress updates.

CGROUP_SKB programs do not require CAP_NET_ADMIN when loaded. That
permission is checked when the program is attached. Once the link exists,
updates are controlled through its FD, so BPF_LINK_UPDATE does not check
CAP_NET_ADMIN again. Keep this behavior and only validate the attach type
during link update.

Fixes: 0c991ebc8c69 ("bpf: Implement bpf_prog replacement for an active bpf_cgroup_link")
Signed-off-by: Sanghyun Park <sanghyun.park.cnu@xxxxxxxxx>
---
v3:
  - Factor the CGROUP_SKB CAP_NET_ADMIN check into an attach-only helper.
v2: https://lore.kernel.org/r/20260818061021.2551771-2-sanghyun.park.cnu@xxxxxxxxx
  - Extend validation from cgroup sock_addr programs to all cgroup program
    types, including exact LSM attach flavors.
  - Preserve legacy CGROUP_SKB ingress/egress replacement compatibility.
  - Keep the CGROUP_SKB CAP_NET_ADMIN check on attach, not link update.
v1: https://lore.kernel.org/r/20260805052858.2390918-3-sanghyun.park.cnu@xxxxxxxxx
---
 kernel/bpf/syscall.c | 41 ++++++++++++++++++++++++++++++++---------
 1 file changed, 32 insertions(+), 9 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 6874ba1424af05..dfea337ff25ea5 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -4483,12 +4483,6 @@ static int bpf_prog_attach_check_attach_type(const struct bpf_prog *prog,
 	case BPF_PROG_TYPE_SK_LOOKUP:
 		return attach_type == prog->expected_attach_type ? 0 : -EINVAL;
 	case BPF_PROG_TYPE_CGROUP_SKB:
-		if (!bpf_token_capable(prog->aux->token, CAP_NET_ADMIN))
-			/* cg-skb progs can be loaded by unpriv user.
-			 * check permissions at attach time.
-			 */
-			return -EPERM;
-
 		ptype = attach_type_to_prog_type(attach_type);
 		if (prog->type != ptype)
 			return -EINVAL;
@@ -4542,6 +4536,19 @@ static int bpf_prog_attach_check_attach_type(const struct bpf_prog *prog,
 	}
 }
 
+static int bpf_prog_attach_check(const struct bpf_prog *prog,
+				 enum bpf_attach_type attach_type)
+{
+	if (prog->type == BPF_PROG_TYPE_CGROUP_SKB &&
+	    !bpf_token_capable(prog->aux->token, CAP_NET_ADMIN))
+		/* cg-skb progs can be loaded by unpriv user.
+		 * check permissions at attach time.
+		 */
+		return -EPERM;
+
+	return bpf_prog_attach_check_attach_type(prog, attach_type);
+}
+
 static bool is_cgroup_prog_type(enum bpf_prog_type ptype, enum bpf_attach_type atype,
 				bool check_atype)
 {
@@ -4606,7 +4613,7 @@ static int bpf_prog_attach(const union bpf_attr *attr)
 	if (IS_ERR(prog))
 		return PTR_ERR(prog);
 
-	if (bpf_prog_attach_check_attach_type(prog, attr->attach_type)) {
+	if (bpf_prog_attach_check(prog, attr->attach_type)) {
 		bpf_prog_put(prog);
 		return -EINVAL;
 	}
@@ -5805,8 +5812,7 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
 	if (IS_ERR(prog))
 		return PTR_ERR(prog);
 
-	ret = bpf_prog_attach_check_attach_type(prog,
-						attr->link_create.attach_type);
+	ret = bpf_prog_attach_check(prog, attr->link_create.attach_type);
 	if (ret)
 		goto out;
 
@@ -5931,6 +5937,7 @@ static int link_update(union bpf_attr *attr)
 {
 	struct bpf_prog *old_prog = NULL, *new_prog;
 	struct bpf_link *link;
+	enum bpf_attach_type atype;
 	u32 flags;
 	int ret;
 
@@ -5968,6 +5975,22 @@ static int link_update(union bpf_attr *attr)
 		goto out_put_progs;
 	}
 
+	if (link->type == BPF_LINK_TYPE_CGROUP) {
+		atype = link->attach_type;
+		/*
+		 * BPF_LSM_MAC and BPF_LSM_CGROUP share BPF_PROG_TYPE_LSM, so
+		 * the helper's default prog-type check cannot distinguish them.
+		 */
+		if (new_prog->type == BPF_PROG_TYPE_LSM &&
+		    new_prog->expected_attach_type != atype)
+			ret = -EINVAL;
+		else
+			ret = bpf_prog_attach_check_attach_type(new_prog,
+								atype);
+		if (ret)
+			goto out_put_progs;
+	}
+
 	if (link->ops->update_prog)
 		ret = link->ops->update_prog(link, new_prog, old_prog);
 	else