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_pkcs7_signature(), dedicated to verify PKCS#7 signatures. More helpers will be introduced later, as necessary. The job of bpf_verify_pkcs7_signature() is simply to call the corresponding signature verification function verify_pkcs7_signature(). Data and signature can be provided to the new helper with two dynamic pointers, to reduce the number of parameters. The keyring can be provided from its serial number, with the new helper bpf_lookup_user_key(). Since it acquires a reference of the found key, the corresponding release helper bpf_key_put() has been introduced to decrement the reference count. The eBPF verifier has been enhanced to ensure that the key reference count is always decreased, when incremented, or otherwise it refuses to load the program. This ability is being verified with the lookup_user_key_norelease test. While the new helpers provide great flexibility, they seem to be suboptimal in terms of security guarantees. If the goal is to do signature verification with system-provided keys (e.g. from the built-in keyring), the eBPF program would have to rely on the user space counterpart to search the correct keyring and to pass its serial. If only the eBPF program is signed and verified, there is not certainty that this operation is done correctly by unverified code. Instead, since verify_pkcs7_signature() understands a pre-determined set of struct key pointer values, which translates into the corresponding system keyrings, the keyring ID parameter has been added as well to the eBPF helper. It is considered only if the passed struct key pointer is NULL. That would guaranteed, assuming that the keyring ID is hardcoded, that signature verification is always done with the desired keys. The introduced helpers can be called only from sleepable programs, because of memory allocation (with key flag KEY_LOOKUP_CREATE) and crypto operations. For example, the lsm.s/bpf attach point is suitable, fexit/array_map_update_elem is not. A test was added to check the ability of bpf_verify_pkcs7_signature() to verify PKCS#7 signatures from the session keyring, a newly-created keyring, and from the secondary keyring (taking an existing kernel module for the verification). The test does not fail if a suitable kernel module is not found (needs support from the CI). The patch set is organized as follows. Patch 1 exports bpf_dynptr_get_size(), to obtain the real size of data carried by a dynamic pointer. Patch 2 introduces the bpf_lookup_user_key() and bpf_key_put() helpers. Patch 3 introduces the bpf_verify_pkcs7_signature() helper. Patches 4 and 5 respectively add the test for the first and the last helper. Changelog v4: - Remove bpf_request_key_by_id(), don't return an invalid pointer that other helpers can use - Pass the keyring ID (without ULONG_MAX, suggested by Alexei) to bpf_verify_pkcs7_signature() - Introduce bpf_lookup_user_key() and bpf_key_put() helpers (suggested by Alexei) - Add lookup_key_norelease test, to ensure that the verifier blocks eBPF programs which don't decrement the key reference count - Parse raw PKCS#7 signature instead of module-style signature in the verify_pkcs7_signature test (suggested by Alexei) - Parse kernel module in user space and pass raw PKCS#7 signature to the eBPF program for signature verification v3: - Rename bpf_verify_signature() back to bpf_verify_pkcs7_signature() to avoid managing different parameters for each signature verification function in one helper (suggested by Daniel) - Use dynamic pointers and export bpf_dynptr_get_size() (suggested by Alexei) - Introduce bpf_request_key_by_id() to give more flexibility to the caller of bpf_verify_pkcs7_signature() to retrieve the appropriate keyring (suggested by Alexei) - Fix test by reordering the gcc command line, always compile sign-file - Improve helper support check mechanism in the test 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) v1: - Don't define new map flag but introduce simple wrapper of verify_pkcs7_signature() (suggested by Alexei and KP) Roberto Sassu (5): bpf: Export bpf_dynptr_get_size() bpf: Add bpf_lookup_user_key() and bpf_key_put() helpers bpf: Add bpf_verify_pkcs7_signature() helper selftests/bpf: Add test for unreleased key references selftests/bpf: Add test for bpf_verify_pkcs7_signature() helper include/linux/bpf.h | 1 + include/uapi/linux/bpf.h | 33 ++ kernel/bpf/bpf_lsm.c | 85 +++++ kernel/bpf/helpers.c | 2 +- kernel/bpf/verifier.c | 6 +- scripts/bpf_doc.py | 2 + tools/include/uapi/linux/bpf.h | 33 ++ tools/testing/selftests/bpf/Makefile | 14 +- tools/testing/selftests/bpf/config | 2 + .../prog_tests/lookup_user_key_norelease.c | 52 +++ .../bpf/prog_tests/verify_pkcs7_sig.c | 341 ++++++++++++++++++ .../progs/test_lookup_user_key_norelease.c | 24 ++ .../bpf/progs/test_verify_pkcs7_sig.c | 90 +++++ .../testing/selftests/bpf/verify_sig_setup.sh | 104 ++++++ 14 files changed, 783 insertions(+), 6 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/lookup_user_key_norelease.c create mode 100644 tools/testing/selftests/bpf/prog_tests/verify_pkcs7_sig.c create mode 100644 tools/testing/selftests/bpf/progs/test_lookup_user_key_norelease.c create mode 100644 tools/testing/selftests/bpf/progs/test_verify_pkcs7_sig.c create mode 100755 tools/testing/selftests/bpf/verify_sig_setup.sh -- 2.25.1
