[PATCH bpf-next 3/5] bpftool: Cover loader metadata with the program signature

[Daniel Borkmann]

bpftool_prog_sign() signed only the loader instructions. The metadata
blob the loader installs was left to an in-loader hash check, which
the kernel now performs at load time over insns || metadata.

Sign that same concatenation: pass the metadata blob (gen_loader_opts
data) through to bpftool_prog_sign() and feed insns || metadata to
CMS_final(). The excl_prog_hash stays a digest of the instructions
alone; it binds the metadata map to the loader and is matched against
prog->digest by the verifier, independent of what the signature covers.

The signed artifact is now plain data: both bytes the signature
covers are embedded verbatim in the generated skeleton, so signing
and verifying an lskel is an ordinary CMS operation that a signer or
auditor can perform (or reproduce) offline, without analyzing loader
bytecode to establish what the signature actually attests to [0].

Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
Link: https://lore.kernel.org/lkml/ecf0521ed302db672672ebfbc670ecfba36a6e00.camel@HansenPartnership.com [0]
---
 tools/bpf/bpftool/gen.c  |  2 ++
 tools/bpf/bpftool/sign.c | 15 ++++++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 6ae7262ebe0c..a01d06d22d1a 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -793,6 +793,8 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 	if (sign_progs) {
 		sopts.insns = opts.insns;
 		sopts.insns_sz = opts.insns_sz;
+		sopts.data = opts.data;
+		sopts.data_sz = opts.data_sz;
 		sopts.excl_prog_hash = prog_sha;
 		sopts.excl_prog_hash_sz = sizeof(prog_sha);
 		sopts.signature = sig_buf;
diff --git a/tools/bpf/bpftool/sign.c b/tools/bpf/bpftool/sign.c
index f9b742f4bb10..4ce020a141dc 100644
--- a/tools/bpf/bpftool/sign.c
+++ b/tools/bpf/bpftool/sign.c
@@ -135,9 +135,21 @@ int bpftool_prog_sign(struct bpf_load_and_run_opts *opts)
 	CMS_ContentInfo *cms = NULL;
 	long actual_sig_len = 0;
 	X509 *x509 = NULL;
+	void *data = NULL;
+	size_t data_sz;
 	int err = 0;
 
-	bd_in = BIO_new_mem_buf(opts->insns, opts->insns_sz);
+	data_sz = (size_t)opts->insns_sz + opts->data_sz;
+	data = malloc(data_sz);
+	if (!data) {
+		err = -ENOMEM;
+		goto cleanup;
+	}
+	memcpy(data, opts->insns, opts->insns_sz);
+	if (opts->data_sz)
+		memcpy((char *)data + opts->insns_sz, opts->data, opts->data_sz);
+
+	bd_in = BIO_new_mem_buf(data, data_sz);
 	if (!bd_in) {
 		err = -ENOMEM;
 		goto cleanup;
@@ -212,6 +224,7 @@ int bpftool_prog_sign(struct bpf_load_and_run_opts *opts)
 	X509_free(x509);
 	EVP_PKEY_free(private_key);
 	BIO_free(bd_in);
+	free(data);
 	DISPLAY_OSSL_ERR(err < 0);
 	return err;
 }
-- 
2.43.0