On Tue, Aug 09, 2022 at 03:45:53PM +0200, Roberto Sassu wrote: > 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. Security decision in LSM BPF? > After a discussion in the eBPF mailing list, it was decided that the stated > goal should be accomplished by introducing four new kfuncs: > bpf_lookup_user_key() and bpf_lookup_system_key(), for retrieving a keyring > with keys trusted for signature verification, respectively from its serial > and from a pre-determined ID; bpf_key_put(), to release the reference > obtained with the former two kfuncs, bpf_verify_pkcs7_signature(), for > verifying PKCS#7 signatures. > > Other than the key serial, bpf_lookup_user_key() also accepts key lookup > flags, that influence the behavior of the lookup. bpf_lookup_system_key() > accepts pre-determined IDs defined in include/linux/verification.h. > > bpf_key_put() accepts the new bpf_key structure, introduced to tell whether > the other structure member, a key pointer, is valid or not. The reason is > that verify_pkcs7_signature() also accepts invalid pointers, set with the > pre-determined ID, to select a system-defined keyring. key_put() must be > called only for valid key pointers. > > Since the two key lookup functions allocate memory and one increments a key > reference count, they must be used in conjunction with bpf_key_put(). The > latter must be called only if the lookup functions returned a non-NULL > pointer. The verifier denies the execution of eBPF programs that don't > respect this rule. > > The two key lookup functions should be used in alternative, depending on > the use case. While bpf_lookup_user_key() provides great flexibility, it > seems suboptimal in terms of security guarantees, as even if the eBPF > program is assumed to be trusted, the serial used to obtain the key pointer > might come from untrusted user space not choosing one that the system > administrator approves to enforce a mandatory policy. > > bpf_lookup_system_key() instead provides much stronger guarantees, > especially if the pre-determined ID is not passed by user space but is > hardcoded in the eBPF program, and that program is signed. In this case, > bpf_verify_pkcs7_signature() will always perform signature verification > with a key that the system administrator approves, i.e. the primary, > secondary or platform keyring. > > Nevertheless, key permission checks need to be done accurately. Since > bpf_lookup_user_key() cannot determine how a key will be used by other > kfuncs, it has to defer the permission check to the actual kfunc using the > key. It does it by calling lookup_user_key() with KEY_DEFER_PERM_CHECK as > needed permission. Later, bpf_verify_pkcs7_signature(), if called, > completes the permission check by calling key_validate(). It does not need > to call key_task_permission() with permission KEY_NEED_SEARCH, as it is > already done elsewhere by the key subsystem. Future kfuncs using the > bpf_key structure need to implement the proper checks as well. > > Finally, the last kfunc, bpf_verify_pkcs7_signature(), accepts the data and > signature to verify as eBPF dynamic pointers, to minimize the number of > kfunc parameters, and the keyring with keys for signature verification as a > bpf_key structure, returned by one of the two key lookup functions. > > All kfuncs except bpf_key_put() can be called only from sleepable programs, > because of memory allocation and crypto operations. For example, the > lsm.s/bpf attach point is suitable, fexit/array_map_update_elem is not. > > The correctness of implementation of the new kfuncs and of their usage is > checked with the introduced tests. > > The patch set includes patches from other authors (dependencies) for sake > of completeness. It is organized as follows. > > Patch 1 from Benjamin Tissoires introduces the new KF_SLEEPABLE kfunc flag. > Patch 2 from KP Singh allows kfuncs to be used by LSM programs. Patch 3 > allows dynamic pointers to be used as kfunc parameters. Patch 4 exports > bpf_dynptr_get_size(), to obtain the real size of data carried by a dynamic > pointer. Patch 5 makes available for new eBPF kfuncs some key-related > definitions. Patch 6 introduces the bpf_lookup_*_key() and bpf_key_put() > kfuncs. Patch 7 introduces the bpf_verify_pkcs7_signature() kfunc. Finally, > patches 8-10 introduce the tests. > > Changelog > > v8: > - Define the new bpf_key structure to carry the key pointer and whether > that pointer is valid or not (suggested by Daniel) > - Drop patch to mark a kfunc parameter with the __maybe_null suffix > - Improve documentation of kfuncs > - Introduce bpf_lookup_system_key() to obtain a key pointer suitable for > verify_pkcs7_signature() (suggested by Daniel) > - Use the new kfunc registration API > - Drop patch to test the __maybe_null suffix > - Add tests for bpf_lookup_system_key() > > v7: > - Add support for using dynamic and NULL pointers in kfunc (suggested by > Alexei) > - Add new kfunc-related tests > > v6: > - Switch back to key lookup helpers + signature verification (until v5), > and defer permission check from bpf_lookup_user_key() to > bpf_verify_pkcs7_signature() > - Add additional key lookup test to illustrate the usage of the > KEY_LOOKUP_CREATE flag and validate the flags (suggested by Daniel) > - Make description of flags of bpf_lookup_user_key() more user-friendly > (suggested by Daniel) > - Fix validation of flags parameter in bpf_lookup_user_key() (reported by > Daniel) > - Rename bpf_verify_pkcs7_signature() keyring-related parameters to > user_keyring and system_keyring to make their purpose more clear > - Accept keyring-related parameters of bpf_verify_pkcs7_signature() as > alternatives (suggested by KP) > - Replace unsigned long type with u64 in helper declaration (suggested by > Daniel) > - Extend the bpf_verify_pkcs7_signature() test by calling the helper > without data, by ensuring that the helper enforces the keyring-related > parameters as alternatives, by ensuring that the helper rejects > inaccessible and expired keyrings, and by checking all system keyrings > - Move bpf_lookup_user_key() and bpf_key_put() usage tests to > ref_tracking.c (suggested by John) > - Call bpf_lookup_user_key() and bpf_key_put() only in sleepable programs > > v5: > - Move KEY_LOOKUP_ to include/linux/key.h > for validation of bpf_verify_pkcs7_signature() parameter > - Remove bpf_lookup_user_key() and bpf_key_put() helpers, and the > corresponding tests > - Replace struct key parameter of bpf_verify_pkcs7_signature() with the > keyring serial and lookup flags > - Call lookup_user_key() and key_put() in bpf_verify_pkcs7_signature() > code, to ensure that the retrieved key is used according to the > permission requested at lookup time > - Clarified keyring precedence in the description of > bpf_verify_pkcs7_signature() (suggested by John) > - Remove newline in the second argument of ASSERT_ > - Fix helper prototype regular expression in bpf_doc.py > > 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) > > Benjamin Tissoires (1): > btf: Add a new kfunc flag which allows to mark a function to be > sleepable > > KP Singh (1): > bpf: Allow kfuncs to be used in LSM programs > > Roberto Sassu (8): > btf: Handle dynamic pointer parameter in kfuncs > bpf: Export bpf_dynptr_get_size() > KEYS: Move KEY_LOOKUP_ to include/linux/key.h > bpf: Add bpf_lookup_*_key() and bpf_key_put() kfuncs > bpf: Add bpf_verify_pkcs7_signature() kfunc > selftests/bpf: Add verifier tests for bpf_lookup_*_key() and > bpf_key_put() > selftests/bpf: Add additional tests for bpf_lookup_*_key() > selftests/bpf: Add test for bpf_verify_pkcs7_signature() kfunc > > Documentation/bpf/kfuncs.rst | 6 + > include/linux/bpf.h | 7 + > include/linux/bpf_verifier.h | 3 + > include/linux/btf.h | 1 + > include/linux/key.h | 3 + > kernel/bpf/btf.c | 27 ++ > kernel/bpf/helpers.c | 2 +- > kernel/bpf/verifier.c | 4 +- > kernel/trace/bpf_trace.c | 207 +++++++++ > security/keys/internal.h | 2 - > tools/testing/selftests/bpf/Makefile | 14 +- > tools/testing/selftests/bpf/config | 2 + > .../selftests/bpf/prog_tests/lookup_key.c | 112 +++++ > .../bpf/prog_tests/verify_pkcs7_sig.c | 399 ++++++++++++++++++ > .../selftests/bpf/progs/test_lookup_key.c | 46 ++ > .../bpf/progs/test_verify_pkcs7_sig.c | 100 +++++ > tools/testing/selftests/bpf/test_verifier.c | 3 +- > .../selftests/bpf/verifier/ref_tracking.c | 139 ++++++ > .../testing/selftests/bpf/verify_sig_setup.sh | 104 +++++ > 19 files changed, 1172 insertions(+), 9 deletions(-) > create mode 100644 tools/testing/selftests/bpf/prog_tests/lookup_key.c > create mode 100644 tools/testing/selftests/bpf/prog_tests/verify_pkcs7_sig.c > create mode 100644 tools/testing/selftests/bpf/progs/test_lookup_key.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 BR, Jarkko
