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    SubjectRe: [PATCH v9 00/10] bpf: Add kfuncs for PKCS#7 signature verification
    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

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