| Message ID | 20241017155516.2582369-6-eric.snowberg@oracle.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Clavis LSM | expand |
On Thu, Oct 17, 2024 at 09:55:08 -0600, Eric Snowberg wrote: > Introduce a new key type for keyring access control. The new key type > is called clavis_key_acl. The clavis_key_acl contains the subject key > identifier along with the allowed usage type for the key. > > The format is as follows: > > XX:YYYYYYYYYYY > > XX - Single byte of the key type > VERIFYING_MODULE_SIGNATURE 00 > VERIFYING_FIRMWARE_SIGNATURE 01 > VERIFYING_KEXEC_PE_SIGNATURE 02 > VERIFYING_KEY_SIGNATURE 03 > VERIFYING_KEY_SELF_SIGNATURE 04 > VERIFYING_UNSPECIFIED_SIGNATURE 05 > : - ASCII colon > YY - Even number of hexadecimal characters representing the key id This is expected to be *lowercase* hexadecimal characters in the code; can that restriction please be documented? (Coming back here, there is a `tolower` pass performed when copying from userspace, so this seems to be an internal requirement, not userspace. Might be worth documenting somewhere in case the kernel wants to make such a key internally.) I also see a 32-byte (64 hex characters) limit in the code; that should also be documented somewhere. > This key type will be used in the clavis keyring for access control. To > be added to the clavis keyring, the clavis_key_acl must be S/MIME signed > by the sole asymmetric key contained within it. > > Below is an example of how this could be used. Within the example, the > key (b360d113c848ace3f1e6a80060b43d1206f0487d) is already in the machine > keyring. The intended usage for this key is to validate a signed kernel > for kexec: > > echo "02:b360d113c848ace3f1e6a80060b43d1206f0487d" > kernel-acl.txt > > The next step is to sign it: > > openssl smime -sign -signer clavis-lsm.x509 -inkey clavis-lsm.priv -in \ > kernel-acl.txt -out kernel-acl.pkcs7 -binary -outform DER \ > -nodetach -noattr > > The final step is how to add the acl to the .clavis keyring: > > keyctl padd clavis_key_acl "" %:.clavis < kernel-acl.pkcs7 > > Afterwards the new clavis_key_acl can be seen in the .clavis keyring: > > keyctl show %:.clavis > Keyring > keyring: .clavis > \_ asymmetric: Clavis LSM key: 4a00ab9f35c9dc3aed7c225d22bafcbd9285e1e8 > \_ clavis_key_acl: 02:b360d113c848ace3f1e6a80060b43d1206f0487d Can this be committed to `Documentation/` and not just the Git history please? Code comments inline below. > Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com> > --- > security/clavis/clavis.h | 1 + > security/clavis/clavis_keyring.c | 173 +++++++++++++++++++++++++++++++ > 2 files changed, 174 insertions(+) > > diff --git a/security/clavis/clavis.h b/security/clavis/clavis.h > index 5e397b55a60a..7b55a6050440 100644 > --- a/security/clavis/clavis.h > +++ b/security/clavis/clavis.h > @@ -5,6 +5,7 @@ > > /* Max length for the asymmetric key id contained on the boot param */ > #define CLAVIS_BIN_KID_MAX 32 > +#define CLAVIS_ASCII_KID_MAX 64 > > struct asymmetric_setup_kid { > struct asymmetric_key_id id; > diff --git a/security/clavis/clavis_keyring.c b/security/clavis/clavis_keyring.c > index 400ed455a3a2..00163e7f0fe9 100644 > --- a/security/clavis/clavis_keyring.c > +++ b/security/clavis/clavis_keyring.c > @@ -2,8 +2,12 @@ > > #include <linux/security.h> > #include <linux/integrity.h> > +#include <linux/ctype.h> > #include <keys/asymmetric-type.h> > +#include <keys/asymmetric-subtype.h> > #include <keys/system_keyring.h> > +#include <keys/user-type.h> > +#include <crypto/pkcs7.h> > #include "clavis.h" > > static struct key *clavis_keyring; > @@ -11,10 +15,173 @@ static struct asymmetric_key_id *clavis_boot_akid; > static struct asymmetric_setup_kid clavis_setup_akid; > static bool clavis_enforced; > > +static int pkcs7_preparse_content(void *ctx, const void *data, size_t len, size_t asn1hdrlen) > +{ > + struct key_preparsed_payload *prep = ctx; > + const void *saved_prep_data; > + size_t saved_prep_datalen; > + char *desc; > + int ret, i; > + > + /* key_acl_free_preparse will free this */ > + desc = kmemdup(data, len, GFP_KERNEL); > + if (!desc) > + return -ENOMEM; > + > + /* Copy the user supplied contents and remove any white space. */ > + for (i = 0; i < len; i++) { > + desc[i] = tolower(desc[i]); Ah, looking here it seems that userspace can provide upper or lowercase. THat this is being performed should be added to the comment here. > + if (isspace(desc[i])) > + desc[i] = 0; How is setting a space to `0` *removing* it? Surely the `isxdigit` check internally is going to reject this. Perhaps you meant to have two indices into `desc`, one read and one write and to stall the write index as long as we're reading whitespace? Also, that whitespace is stripped is a userspace-relevant detail that should be documented. > +static void key_acl_destroy(struct key *key) > +{ > + /* It should not be possible to get here */ > + pr_info("destroy clavis_key_acl denied\n"); > +} > + > +static void key_acl_revoke(struct key *key) > +{ > + /* It should not be possible to get here */ > + pr_info("revoke clavis_key_acl denied\n"); > +} These keys cannot be destroyed or revoked? This seems…novel to me. What if there's a timeout on the key? If such keys are immortal, timeouts should also be refused? > +static int key_acl_vet_description(const char *desc) > +{ > + int i, desc_len; > + s16 ktype; > + > + if (!desc) > + goto invalid; > + > + desc_len = sizeof(desc); This should be `strlen`, no? > + /* > + * clavis_acl format: > + * xx:yyyy... > + * > + * xx - Single byte of the key type > + * : - Ascii colon > + * yyyy.. - Even number of hexadecimal characters representing the keyid > + */ > + > + /* The min clavis acl is 7 characters. */ > + if (desc_len < 7) > + goto invalid; > + > + /* Check the first byte is a valid key type. */ > + if (sscanf(desc, "%2hx", &ktype) != 1) > + goto invalid; > + > + if (ktype >= VERIFYING_CLAVIS_SIGNATURE) > + goto invalid; > + > + /* Check that there is a colon following the key type */ > + if (desc[2] != ':') > + goto invalid; > + > + /* Move past the colon. */ > + desc += 3; > + > + for (i = 0; *desc && i < CLAVIS_ASCII_KID_MAX; desc++, i++) { > + /* Check if lowercase hex number */ > + if (!isxdigit(*desc) || isupper(*desc)) > + goto invalid; > + } > + > + /* Check if the has is greater than CLAVIS_ASCII_KID_MAX. */ > + if (*desc) > + goto invalid; > + > + /* Check for even number of hex characters. */ > + if (i == 0 || i & 1) FWIW< the `i == 0` is impossible due to the `desc_len < 7` check above (well, once `strlen` is used…). Thanks, --Ben
diff --git a/security/clavis/clavis.h b/security/clavis/clavis.h index 5e397b55a60a..7b55a6050440 100644 --- a/security/clavis/clavis.h +++ b/security/clavis/clavis.h @@ -5,6 +5,7 @@ /* Max length for the asymmetric key id contained on the boot param */ #define CLAVIS_BIN_KID_MAX 32 +#define CLAVIS_ASCII_KID_MAX 64 struct asymmetric_setup_kid { struct asymmetric_key_id id; diff --git a/security/clavis/clavis_keyring.c b/security/clavis/clavis_keyring.c index 400ed455a3a2..00163e7f0fe9 100644 --- a/security/clavis/clavis_keyring.c +++ b/security/clavis/clavis_keyring.c @@ -2,8 +2,12 @@ #include <linux/security.h> #include <linux/integrity.h> +#include <linux/ctype.h> #include <keys/asymmetric-type.h> +#include <keys/asymmetric-subtype.h> #include <keys/system_keyring.h> +#include <keys/user-type.h> +#include <crypto/pkcs7.h> #include "clavis.h" static struct key *clavis_keyring; @@ -11,10 +15,173 @@ static struct asymmetric_key_id *clavis_boot_akid; static struct asymmetric_setup_kid clavis_setup_akid; static bool clavis_enforced; +static int pkcs7_preparse_content(void *ctx, const void *data, size_t len, size_t asn1hdrlen) +{ + struct key_preparsed_payload *prep = ctx; + const void *saved_prep_data; + size_t saved_prep_datalen; + char *desc; + int ret, i; + + /* key_acl_free_preparse will free this */ + desc = kmemdup(data, len, GFP_KERNEL); + if (!desc) + return -ENOMEM; + + /* Copy the user supplied contents and remove any white space. */ + for (i = 0; i < len; i++) { + desc[i] = tolower(desc[i]); + if (isspace(desc[i])) + desc[i] = 0; + } + + prep->description = desc; + saved_prep_data = prep->data; + saved_prep_datalen = prep->datalen; + prep->data = desc; + prep->datalen = len; + ret = user_preparse(prep); + prep->data = saved_prep_data; + prep->datalen = saved_prep_datalen; + return ret; +} + +static void key_acl_free_preparse(struct key_preparsed_payload *prep) +{ + kfree(prep->description); + user_free_preparse(prep); +} + +static struct key *clavis_keyring_get(void) +{ + return clavis_keyring; +} + static bool clavis_acl_enforced(void) { return clavis_enforced; } + +static int key_acl_preparse(struct key_preparsed_payload *prep) +{ + /* + * Only allow the description to be set via the pkcs7 data contents. + * The exception to this rule is if the entry was builtin, it will have + * the original_description set. Since we don't have access to the key + * within the preparse step to determine if the entity is builtin, let + * it through now and this will be checked in the instantiate step. + */ + if (prep->orig_description) + return 0; + + return verify_pkcs7_signature(NULL, 0, prep->data, prep->datalen, clavis_keyring_get(), + VERIFYING_CLAVIS_SIGNATURE, pkcs7_preparse_content, + prep); +} + +static int key_acl_instantiate(struct key *key, struct key_preparsed_payload *prep) +{ + /* + * The orig_description may only be used for builtin entities. All + * other entries must have been validated through the pkcs7 signature + * within the preparse stage. + */ + if (prep->orig_description && !(key->flags & (1 << KEY_FLAG_BUILTIN))) + return -EINVAL; + + key->perm = KEY_POS_SEARCH | KEY_POS_VIEW | KEY_USR_SEARCH | + KEY_USR_VIEW; + set_bit(KEY_FLAG_KEEP, &key->flags); + return generic_key_instantiate(key, prep); +} + +static void key_acl_destroy(struct key *key) +{ + /* It should not be possible to get here */ + pr_info("destroy clavis_key_acl denied\n"); +} + +static void key_acl_revoke(struct key *key) +{ + /* It should not be possible to get here */ + pr_info("revoke clavis_key_acl denied\n"); +} + +static int key_acl_update(struct key *key, struct key_preparsed_payload *prep) +{ + return -EPERM; +} + +static int key_acl_vet_description(const char *desc) +{ + int i, desc_len; + s16 ktype; + + if (!desc) + goto invalid; + + desc_len = sizeof(desc); + + /* + * clavis_acl format: + * xx:yyyy... + * + * xx - Single byte of the key type + * : - Ascii colon + * yyyy.. - Even number of hexadecimal characters representing the keyid + */ + + /* The min clavis acl is 7 characters. */ + if (desc_len < 7) + goto invalid; + + /* Check the first byte is a valid key type. */ + if (sscanf(desc, "%2hx", &ktype) != 1) + goto invalid; + + if (ktype >= VERIFYING_CLAVIS_SIGNATURE) + goto invalid; + + /* Check that there is a colon following the key type */ + if (desc[2] != ':') + goto invalid; + + /* Move past the colon. */ + desc += 3; + + for (i = 0; *desc && i < CLAVIS_ASCII_KID_MAX; desc++, i++) { + /* Check if lowercase hex number */ + if (!isxdigit(*desc) || isupper(*desc)) + goto invalid; + } + + /* Check if the has is greater than CLAVIS_ASCII_KID_MAX. */ + if (*desc) + goto invalid; + + /* Check for even number of hex characters. */ + if (i == 0 || i & 1) + goto invalid; + + return 0; + +invalid: + pr_err("Unparsable clavis key id: %s\n", desc); + return -EINVAL; +} + +static struct key_type clavis_key_acl = { + .name = "clavis_key_acl", + .preparse = key_acl_preparse, + .free_preparse = key_acl_free_preparse, + .instantiate = key_acl_instantiate, + .update = key_acl_update, + .revoke = key_acl_revoke, + .destroy = key_acl_destroy, + .vet_description = key_acl_vet_description, + .read = user_read, +}; + static int restrict_link_for_clavis(struct key *dest_keyring, const struct key_type *type, const union key_payload *payload, struct key *restrict_key) { @@ -28,6 +195,9 @@ static int restrict_link_for_clavis(struct key *dest_keyring, const struct key_t return 0; } + if (type == &clavis_key_acl) + return 0; + return -EOPNOTSUPP; } @@ -93,6 +263,9 @@ static int __init clavis_keyring_init(void) { struct key_restriction *restriction; + if (register_key_type(&clavis_key_acl) < 0) + panic("Can't allocate clavis key type\n"); + restriction = clavis_restriction_alloc(restrict_link_for_clavis); if (!restriction) panic("Can't allocate clavis keyring restriction\n");
Introduce a new key type for keyring access control. The new key type is called clavis_key_acl. The clavis_key_acl contains the subject key identifier along with the allowed usage type for the key. The format is as follows: XX:YYYYYYYYYYY XX - Single byte of the key type VERIFYING_MODULE_SIGNATURE 00 VERIFYING_FIRMWARE_SIGNATURE 01 VERIFYING_KEXEC_PE_SIGNATURE 02 VERIFYING_KEY_SIGNATURE 03 VERIFYING_KEY_SELF_SIGNATURE 04 VERIFYING_UNSPECIFIED_SIGNATURE 05 : - ASCII colon YY - Even number of hexadecimal characters representing the key id This key type will be used in the clavis keyring for access control. To be added to the clavis keyring, the clavis_key_acl must be S/MIME signed by the sole asymmetric key contained within it. Below is an example of how this could be used. Within the example, the key (b360d113c848ace3f1e6a80060b43d1206f0487d) is already in the machine keyring. The intended usage for this key is to validate a signed kernel for kexec: echo "02:b360d113c848ace3f1e6a80060b43d1206f0487d" > kernel-acl.txt The next step is to sign it: openssl smime -sign -signer clavis-lsm.x509 -inkey clavis-lsm.priv -in \ kernel-acl.txt -out kernel-acl.pkcs7 -binary -outform DER \ -nodetach -noattr The final step is how to add the acl to the .clavis keyring: keyctl padd clavis_key_acl "" %:.clavis < kernel-acl.pkcs7 Afterwards the new clavis_key_acl can be seen in the .clavis keyring: keyctl show %:.clavis Keyring keyring: .clavis \_ asymmetric: Clavis LSM key: 4a00ab9f35c9dc3aed7c225d22bafcbd9285e1e8 \_ clavis_key_acl: 02:b360d113c848ace3f1e6a80060b43d1206f0487d Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com> --- security/clavis/clavis.h | 1 + security/clavis/clavis_keyring.c | 173 +++++++++++++++++++++++++++++++ 2 files changed, 174 insertions(+)