bpf: Implement signature verification for BPF programs

This patch extends the BPF_PROG_LOAD command by adding three new fields
to `union bpf_attr` in the user-space API:

- signature: A pointer to the signature blob.
- signature_size: The size of the signature blob.
- keyring_id: The serial number of a loaded kernel keyring (e.g.,
the user or session keyring) containing the trusted public keys.

When a BPF program is loaded with a signature, the kernel:

1. Retrieves the trusted keyring using the provided `keyring_id`.
2. Verifies the supplied signature against the BPF program's
instruction buffer.
3. If the signature is valid and was generated by a key in the trusted
keyring, the program load proceeds.
4. If no signature is provided, the load proceeds as before, allowing
for backward compatibility. LSMs can chose to restrict unsigned
programs and implement a security policy.
5. If signature verification fails for any reason,
the program is not loaded.

Tested-by: [e-mail]
Signed-off-by: KP Singh <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

crypto/asymmetric_keys/pkcs7_verify.c

···
 		/* Authattr presence checked in parser */
 		break;
 	case VERIFYING_UNSPECIFIED_SIGNATURE:
+	case VERIFYING_BPF_SIGNATURE:
 		if (pkcs7->data_type != OID_data) {
 			pr_warn("Invalid unspecified sig (not pkcs7-data)\n");
 			return -EKEYREJECTED;

include/linux/verification.h

···
 	VERIFYING_KEY_SIGNATURE,
 	VERIFYING_KEY_SELF_SIGNATURE,
 	VERIFYING_UNSPECIFIED_SIGNATURE,
+	VERIFYING_BPF_SIGNATURE,
 	NR__KEY_BEING_USED_FOR
 };
 #ifdef CONFIG_SYSTEM_DATA_VERIFICATION

include/uapi/linux/bpf.h

···
 		 * continuous.
 		 */
 		__u32		fd_array_cnt;
+		/* Pointer to a buffer containing the signature of the BPF
+		 * program.
+		 */
+		__aligned_u64   signature;
+		/* Size of the signature buffer in bytes. */
+		__u32 		signature_size;
+		/* ID of the kernel keyring to be used for signature
+		 * verification.
+		 */
+		__s32		keyring_id;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_* commands */

kernel/bpf/helpers.c

···
 
 	return verify_pkcs7_signature(data, data_len, sig, sig_len,
 				      trusted_keyring->key,
-				      VERIFYING_UNSPECIFIED_SIGNATURE, NULL,
+				      VERIFYING_BPF_SIGNATURE, NULL,
 				      NULL);
 #else
 	return -EOPNOTSUPP;

kernel/bpf/syscall.c

···
 #include <linux/tracepoint.h>
 #include <linux/overflow.h>
 #include <linux/cookie.h>
+#include <linux/verification.h>
 
 #include <net/netfilter/nf_bpf_link.h>
 #include <net/netkit.h>
···
 	}
 }
 
+static int bpf_prog_verify_signature(struct bpf_prog *prog, union bpf_attr *attr,
+				     bool is_kernel)
+{
+	bpfptr_t usig = make_bpfptr(attr->signature, is_kernel);
+	struct bpf_dynptr_kern sig_ptr, insns_ptr;
+	struct bpf_key *key = NULL;
+	void *sig;
+	int err = 0;
+
+	if (system_keyring_id_check(attr->keyring_id) == 0)
+		key = bpf_lookup_system_key(attr->keyring_id);
+	else
+		key = bpf_lookup_user_key(attr->keyring_id, 0);
+
+	if (!key)
+		return -EINVAL;
+
+	sig = kvmemdup_bpfptr(usig, attr->signature_size);
+	if (IS_ERR(sig)) {
+		bpf_key_put(key);
+		return -ENOMEM;
+	}
+
+	bpf_dynptr_init(&sig_ptr, sig, BPF_DYNPTR_TYPE_LOCAL, 0,
+			attr->signature_size);
+	bpf_dynptr_init(&insns_ptr, prog->insnsi, BPF_DYNPTR_TYPE_LOCAL, 0,
+			prog->len * sizeof(struct bpf_insn));
+
+	err = bpf_verify_pkcs7_signature((struct bpf_dynptr *)&insns_ptr,
+					 (struct bpf_dynptr *)&sig_ptr, key);
+
+	bpf_key_put(key);
+	kvfree(sig);
+	return err;
+}
+
 /* last field in 'union bpf_attr' used by this command */
-#define BPF_PROG_LOAD_LAST_FIELD fd_array_cnt
+#define BPF_PROG_LOAD_LAST_FIELD keyring_id
 
 static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
 {
···
 
 	/* eBPF programs must be GPL compatible to use GPL-ed functions */
 	prog->gpl_compatible = license_is_gpl_compatible(license) ? 1 : 0;
+
+	if (attr->signature) {
+		err = bpf_prog_verify_signature(prog, attr, uattr.is_kernel);
+		if (err)
+			goto free_prog;
+	}
 
 	prog->orig_prog = NULL;
 	prog->jited = 0;

tools/include/uapi/linux/bpf.h

···
 		 * continuous.
 		 */
 		__u32		fd_array_cnt;
+		/* Pointer to a buffer containing the signature of the BPF
+		 * program.
+		 */
+		__aligned_u64   signature;
+		/* Size of the signature buffer in bytes. */
+		__u32 		signature_size;
+		/* ID of the kernel keyring to be used for signature
+		 * verification.
+		 */
+		__s32		keyring_id;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_* commands */

tools/lib/bpf/bpf.c

···
 		  const struct bpf_insn *insns, size_t insn_cnt,
 		  struct bpf_prog_load_opts *opts)
 {
-	const size_t attr_sz = offsetofend(union bpf_attr, fd_array_cnt);
+	const size_t attr_sz = offsetofend(union bpf_attr, keyring_id);
 	void *finfo = NULL, *linfo = NULL;
 	const char *func_info, *line_info;
 	__u32 log_size, log_level, attach_prog_fd, attach_btf_obj_fd;