selftests/bpf: Add test that uses fsverity and xattr to sign a file

This selftests shows a proof of concept method to use BPF LSM to enforce
file signature. This test is added to verify_pkcs7_sig, so that some
existing logic can be reused.

This file signature method uses fsverity, which provides reliable and
efficient hash (known as digest) of the file. The file digest is signed
with asymmetic key, and the signature is stored in xattr. At the run time,
BPF LSM reads file digest and the signature, and then checks them against
the public key.

Note that this solution does NOT require FS_VERITY_BUILTIN_SIGNATURES.
fsverity is only used to provide file digest. The signature verification
and access control is all implemented in BPF LSM.

Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231129234417.856536-7-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Fix verify_pkcs7_sig on s390x

Use bpf_probe_read_kernel() instead of bpf_probe_read(), which is not
defined on all architectures.

While at it, improve the error handling: do not hide the verifier log,
and check the return values of bpf_probe_read_kernel() and
bpf_copy_from_user().

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-10-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add test for bpf_verify_pkcs7_signature() kfunc

Perform several tests to ensure the correct implementation of the
bpf_verify_pkcs7_signature() kfunc.

Do the tests with data signed with a generated testing key (by using
sign-file from scripts/) and with the tcp_bic.ko kernel module if it is
found in the system. The test does not fail if tcp_bic.ko is not found.

First, perform an unsuccessful signature verification without data.

Second, perform a successful signature verification with the session
keyring and a new one created for testing.

Then, ensure that permission and validation checks are done properly on the
keyring provided to bpf_verify_pkcs7_signature(), despite those checks were
deferred at the time the keyring was retrieved with bpf_lookup_user_key().
The tests expect to encounter an error if the Search permission is removed
from the keyring, or the keyring is expired.

Finally, perform a successful and unsuccessful signature verification with
the keyrings with pre-determined IDs (the last test fails because the key
is not in the platform keyring).

The test is currently in the deny list for s390x (JIT does not support
calling kernel function).

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Link: https://lore.kernel.org/r/20220920075951.929132-13-roberto.sassu@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>