One of the desirable features in security is the ability to restrict import of data to a given system based on data authenticity. If data import can be restricted, it would be possible to enforce a system-wide policy based on the signing keys the system owner trusts.
This feature is widely used in the kernel. For example, if the restriction is enabled, kernel modules can be plugged in only if they are signed with a key whose public part is in the primary or secondary keyring.
For eBPF, it can be useful as well. For example, it might be useful to authenticate data an eBPF program makes security decisions on.
After a discussion in the eBPF mailing list, it was decided that the stated goal should be accomplished by introducing a new helper: bpf_verify_signature(). Its job is simply to call the signature verification function corresponding to the passed signature type, with the keyring selected through the passed keyring identifier.
Since verify_pkcs7_signature() is doing crypto operations, it must be called by a sleepable program. This restricts the set of functions that can call the associated helper (for example, lsm.s/bpf is suitable, fexit/array_map_update_elem is not).
The added test checks the ability of an eBPF program to verify module-style appended signatures, as produced by the kernel tool sign-file, currently used to sign kernel modules.
The patch set is organized as follows.
Patch 1 introduces the new helper. Patch 2 adds the test for the new helper.
Changelog
v1: - Don't define new map flag but introduce simple wrapper of verify_pkcs7_signature() (suggested by Alexei and KP)
v2: - Rename bpf_verify_pkcs7_signature() to a more generic bpf_verify_signature() and pass the signature type (suggested by KP) - Move the helper and prototype declaration under #ifdef so that user space can probe for support for the helper (suggested by Daniel) - Describe better the keyring types (suggested by Daniel) - Include linux/bpf.h instead of vmlinux.h to avoid implicit or redeclaration - Make the test selfcontained (suggested by Alexei)
Roberto Sassu (2): bpf: Add bpf_verify_signature() helper selftests/bpf: Add test for bpf_verify_signature() helper
include/uapi/linux/bpf.h | 17 ++ kernel/bpf/bpf_lsm.c | 46 ++++ tools/include/uapi/linux/bpf.h | 17 ++ tools/testing/selftests/bpf/Makefile | 11 +- tools/testing/selftests/bpf/config | 1 + .../selftests/bpf/prog_tests/verify_sig.c | 200 ++++++++++++++++++ .../selftests/bpf/progs/test_verify_sig.c | 160 ++++++++++++++ .../testing/selftests/bpf/verify_sig_setup.sh | 100 +++++++++ 8 files changed, 549 insertions(+), 3 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/verify_sig.c create mode 100644 tools/testing/selftests/bpf/progs/test_verify_sig.c create mode 100755 tools/testing/selftests/bpf/verify_sig_setup.sh