[PATCH v6 4/5] bpf: Add bpf_verify_pkcs7_signature() helper

[Roberto Sassu]

Add the bpf_verify_pkcs7_signature() helper, to give eBPF security modules
the ability to check the validity of a signature against supplied data, by
using user-provided or system-provided keys as trust anchor.

The new helper makes it possible to enforce mandatory policies, as eBPF
programs might be allowed to make security decisions only based on data
sources the system administrator approves.

The caller should provide both the data to be verified and the signature as
eBPF dynamic pointers (to minimize the number of parameters).

The caller should also provide a trusted keyring serial, together with key
lookup-specific flags, to determine which keys can be used for signature
verification. Alternatively, the caller could specify zero as serial value
(not valid, serials must be positive), and provide instead a special
keyring ID.

Key lookup flags are defined in include/linux/key.h and can be: 1, to
request that special keyrings be created if referred to directly; 2 to
permit partially constructed keys to be found.

Special IDs are defined in include/linux/verification.h and can be: 0 for
the primary keyring (immutable keyring of system keys); 1 for both the
primary and secondary keyring (where keys can be added only if they are
vouched for by existing keys in those keyrings); 2 for the platform keyring
(primarily used by the integrity subsystem to verify a kexec'ed kerned
image and, possibly, the initramfs signature).

Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx> (cast warning)
---
 include/uapi/linux/bpf.h       | 24 +++++++++++++
 kernel/bpf/bpf_lsm.c           | 63 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h | 24 +++++++++++++
 3 files changed, 111 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index e81362891596..b4f5ad863281 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5325,6 +5325,29 @@ union bpf_attr {
  *		**-EACCES** if the SYN cookie is not valid.
  *
  *		**-EPROTONOSUPPORT** if CONFIG_IPV6 is not builtin.
+ *
+ * long bpf_verify_pkcs7_signature(struct bpf_dynptr *data_ptr, struct bpf_dynptr *sig_ptr, u32 trusted_keyring_serial, unsigned long lookup_flags, unsigned long trusted_keyring_id)
+ *	Description
+ *		Verify the PKCS#7 signature *sig_ptr* against the supplied
+ *		*data_ptr* with keys in a keyring with serial
+ *		*trusted_keyring_serial*, searched with *lookup_flags*, if the
+ *		parameter value is positive, or alternatively in a keyring with
+ *		special ID *trusted_keyring_id* if *trusted_keyring_serial* is
+ *		zero.
+ *
+ *		*lookup_flags* are defined in include/linux/key.h and can be: 1,
+ *		to request that special keyrings be created if referred to
+ *		directly; 2 to permit partially constructed keys to be found.
+ *
+ *		Special IDs are defined in include/linux/verification.h and can
+ *		be: 0 for the primary keyring (immutable keyring of system
+ *		keys); 1 for both the primary and secondary keyring (where keys
+ *		can be added only if they are vouched for by existing keys in
+ *		those keyrings); 2 for the platform keyring (primarily used by
+ *		the integrity subsystem to verify a kexec'ed kerned image and,
+ *		possibly, the initramfs signature).
+ *	Return
+ *		0 on success, a negative value on error.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5535,6 +5558,7 @@ union bpf_attr {
 	FN(tcp_raw_gen_syncookie_ipv6),	\
 	FN(tcp_raw_check_syncookie_ipv4),	\
 	FN(tcp_raw_check_syncookie_ipv6),	\
+	FN(verify_pkcs7_signature),	\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
index c1351df9f7ee..401bda01ad84 100644
--- a/kernel/bpf/bpf_lsm.c
+++ b/kernel/bpf/bpf_lsm.c
@@ -16,6 +16,8 @@
 #include <linux/bpf_local_storage.h>
 #include <linux/btf_ids.h>
 #include <linux/ima.h>
+#include <linux/verification.h>
+#include <linux/key.h>
 
 /* For every LSM hook that allows attachment of BPF programs, declare a nop
  * function where a BPF program can be attached.
@@ -132,6 +134,62 @@ static const struct bpf_func_proto bpf_get_attach_cookie_proto = {
 	.arg1_type	= ARG_PTR_TO_CTX,
 };
 
+#ifdef CONFIG_SYSTEM_DATA_VERIFICATION
+BPF_CALL_5(bpf_verify_pkcs7_signature, struct bpf_dynptr_kern *, data_ptr,
+	   struct bpf_dynptr_kern *, sig_ptr, u32, trusted_keyring_serial,
+	   unsigned long, lookup_flags, unsigned long, trusted_keyring_id)
+{
+	key_ref_t trusted_keyring_ref;
+	struct key *trusted_keyring;
+	int ret;
+
+	/* Keep in sync with defs in include/linux/key.h. */
+	if (lookup_flags > KEY_LOOKUP_PARTIAL)
+		return -EINVAL;
+
+	/* Keep in sync with defs in include/linux/verification.h. */
+	if (trusted_keyring_id > (unsigned long)VERIFY_USE_PLATFORM_KEYRING)
+		return -EINVAL;
+
+	if (trusted_keyring_serial) {
+		trusted_keyring_ref = lookup_user_key(trusted_keyring_serial,
+						      lookup_flags,
+						      KEY_NEED_SEARCH);
+		if (IS_ERR(trusted_keyring_ref))
+			return PTR_ERR(trusted_keyring_ref);
+
+		trusted_keyring = key_ref_to_ptr(trusted_keyring_ref);
+		goto verify;
+	}
+
+	trusted_keyring = (struct key *)trusted_keyring_id;
+verify:
+	ret = verify_pkcs7_signature(data_ptr->data,
+				     bpf_dynptr_get_size(data_ptr),
+				     sig_ptr->data,
+				     bpf_dynptr_get_size(sig_ptr),
+				     trusted_keyring,
+				     VERIFYING_UNSPECIFIED_SIGNATURE, NULL,
+				     NULL);
+	if (trusted_keyring_serial)
+		key_put(trusted_keyring);
+
+	return ret;
+}
+
+static const struct bpf_func_proto bpf_verify_pkcs7_signature_proto = {
+	.func		= bpf_verify_pkcs7_signature,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_DYNPTR | DYNPTR_TYPE_LOCAL,
+	.arg2_type	= ARG_PTR_TO_DYNPTR | DYNPTR_TYPE_LOCAL,
+	.arg3_type	= ARG_ANYTHING,
+	.arg4_type	= ARG_ANYTHING,
+	.arg5_type	= ARG_ANYTHING,
+	.allowed	= bpf_ima_inode_hash_allowed,
+};
+#endif /* CONFIG_SYSTEM_DATA_VERIFICATION */
+
 static const struct bpf_func_proto *
 bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
@@ -158,6 +216,11 @@ bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return prog->aux->sleepable ? &bpf_ima_file_hash_proto : NULL;
 	case BPF_FUNC_get_attach_cookie:
 		return bpf_prog_has_trampoline(prog) ? &bpf_get_attach_cookie_proto : NULL;
+#ifdef CONFIG_SYSTEM_DATA_VERIFICATION
+	case BPF_FUNC_verify_pkcs7_signature:
+		return prog->aux->sleepable ?
+		       &bpf_verify_pkcs7_signature_proto : NULL;
+#endif /* CONFIG_SYSTEM_DATA_VERIFICATION */
 	default:
 		return tracing_prog_func_proto(func_id, prog);
 	}
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index e81362891596..b4f5ad863281 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5325,6 +5325,29 @@ union bpf_attr {
  *		**-EACCES** if the SYN cookie is not valid.
  *
  *		**-EPROTONOSUPPORT** if CONFIG_IPV6 is not builtin.
+ *
+ * long bpf_verify_pkcs7_signature(struct bpf_dynptr *data_ptr, struct bpf_dynptr *sig_ptr, u32 trusted_keyring_serial, unsigned long lookup_flags, unsigned long trusted_keyring_id)
+ *	Description
+ *		Verify the PKCS#7 signature *sig_ptr* against the supplied
+ *		*data_ptr* with keys in a keyring with serial
+ *		*trusted_keyring_serial*, searched with *lookup_flags*, if the
+ *		parameter value is positive, or alternatively in a keyring with
+ *		special ID *trusted_keyring_id* if *trusted_keyring_serial* is
+ *		zero.
+ *
+ *		*lookup_flags* are defined in include/linux/key.h and can be: 1,
+ *		to request that special keyrings be created if referred to
+ *		directly; 2 to permit partially constructed keys to be found.
+ *
+ *		Special IDs are defined in include/linux/verification.h and can
+ *		be: 0 for the primary keyring (immutable keyring of system
+ *		keys); 1 for both the primary and secondary keyring (where keys
+ *		can be added only if they are vouched for by existing keys in
+ *		those keyrings); 2 for the platform keyring (primarily used by
+ *		the integrity subsystem to verify a kexec'ed kerned image and,
+ *		possibly, the initramfs signature).
+ *	Return
+ *		0 on success, a negative value on error.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5535,6 +5558,7 @@ union bpf_attr {
 	FN(tcp_raw_gen_syncookie_ipv6),	\
 	FN(tcp_raw_check_syncookie_ipv4),	\
 	FN(tcp_raw_check_syncookie_ipv6),	\
+	FN(verify_pkcs7_signature),	\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
-- 
2.25.1