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

[Jarkko Sakkinen]

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