[RFC][PATCH 3/8] ima: Add digest_cache policy keyword
Roberto Sassu
roberto.sassu at huaweicloud.com
Fri Mar 8 09:05:09 UTC 2024
On Thu, 2024-03-07 at 14:43 -0500, Mimi Zohar wrote:
> On Wed, 2024-02-14 at 15:35 +0100, Roberto Sassu wrote:
> > From: Roberto Sassu <roberto.sassu at huawei.com>
> >
> > Add the 'digest_cache=' policy keyword, to enable the usage of digest
> > caches for specific IMA actions and purposes.
> >
> > At the moment, it accepts only 'content' as value, as digest caches can be
> > only used only for measurement and appraisal of file content. In the
> > future, it might be possible to use them for file metadata too.
>
> At least from this patch, it is unclear why 'digest_cache' requires an option.
> The usage - measure, appraise - is based on 'action'. From an IMA perspective,
> does the file content make a difference? And if it did, then file 'data' would
> parallel file 'metadata'.
I wanted to express the fact that digest caches, if available, can only
be used to appraise file data, if there is no metadata (similarly to
module-style appended signatures).
That would prevent for example the scenario where appraisal of file
data is successful without having verified current metadata, and EVM
attaches to the file a valid HMAC on file close, based on the current
xattr value (trust at first use).
An IMA rule with 'digest_cache=metadata' would take a different code
path. It would make IMA send to evm_verifyxattr() the calculated file
digest (since there is no security.ima), and let EVM calculate and
search the digest of file metadata in the digest cache.
I didn't go that far yet, but this is more or less what I would like to
do, also based on my old implementation of the IMA Digest Lists
extension (which supports file metadata lookup).
> Without having to pass around "digest_cache_mask" would simplify this patch.
We need to pass it anyway, to let process_measurement() know that it
can use digest caches. Or we can make 'flags' in ima_iint_cache a u64,
and introduce new flags.
Roberto
> Mimi
>
> > The 'digest_cache=' keyword can be specified for the subset of IMA hooks
> > listed in ima_digest_cache_func_allowed().
> >
> > POLICY_CHECK has been excluded for measurement, because policy changes must
> > be visible in the IMA measurement list. For appraisal, instead, it might be
> > useful to load custom policies in the initial ram disk (no security.ima
> > xattr).
> >
> > Add the digest_cache_mask member to the ima_rule_entry structure, and set
> > the flag IMA_DIGEST_CACHE_MEASURE_CONTENT if 'digest_cache=content' was
> > specified for a measure rule, IMA_DIGEST_CACHE_APPRAISE_CONTENT for an
> > appraise rule.
> >
> > Propagate the mask down to ima_match_policy() and ima_get_action(), so that
> > process_measurement() can make the final decision on whether or not digest
> > caches should be used to measure/appraise the file being evaluated.
> >
> > Since using digest caches changes the meaning of the IMA measurement list,
> > which will include only digest lists and unknown files, enforce specifying
> > 'pcr=' with a non-standard value, when 'digest_cache=content' is specified
> > in a measure rule.
> >
> > This removes the ambiguity on the meaning of the IMA measurement list.
> >
> > Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> > ---
> > Documentation/ABI/testing/ima_policy | 5 +-
> > security/integrity/ima/ima.h | 10 +++-
> > security/integrity/ima/ima_api.c | 6 ++-
> > security/integrity/ima/ima_appraise.c | 2 +-
> > security/integrity/ima/ima_main.c | 8 +--
> > security/integrity/ima/ima_policy.c | 70 ++++++++++++++++++++++++++-
> > 6 files changed, 89 insertions(+), 12 deletions(-)
> >
> > diff --git a/Documentation/ABI/testing/ima_policy
> > b/Documentation/ABI/testing/ima_policy
> > index 22237fec5532..be045fb60530 100644
> > --- a/Documentation/ABI/testing/ima_policy
> > +++ b/Documentation/ABI/testing/ima_policy
> > @@ -29,7 +29,7 @@ Description:
> > [obj_user=] [obj_role=] [obj_type=]]
> > option: [digest_type=] [template=] [permit_directio]
> > [appraise_type=] [appraise_flag=]
> > - [appraise_algos=] [keyrings=]
> > + [appraise_algos=] [keyrings=] [digest_cache=]
> > base:
> > func:=
> > [BPRM_CHECK][MMAP_CHECK][CREDS_CHECK][FILE_CHECK][MODULE_CHECK]
> > [FIRMWARE_CHECK]
> > @@ -77,6 +77,9 @@ Description:
> > For example, "sha256,sha512" to only accept to appraise
> > files where the security.ima xattr was hashed with one
> > of these two algorithms.
> > + digest_cache:= [content]
> > + "content" means that the digest cache is used only
> > + for file content measurement and/or appraisal.
> >
> > default policy:
> > # PROC_SUPER_MAGIC
> > diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> > index c9140a57b591..deee56d99d6f 100644
> > --- a/security/integrity/ima/ima.h
> > +++ b/security/integrity/ima/ima.h
> > @@ -43,6 +43,10 @@ enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8, TPM_PCR10 = 10
> > };
> >
> > #define NR_BANKS(chip) ((chip != NULL) ? chip->nr_allocated_banks : 0)
> >
> > +/* Digest cache usage flags. */
> > +#define IMA_DIGEST_CACHE_MEASURE_CONTENT 0x0000000000000001
> > +#define IMA_DIGEST_CACHE_APPRAISE_CONTENT 0x0000000000000002
> > +
> > /* current content of the policy */
> > extern int ima_policy_flag;
> >
> > @@ -367,7 +371,8 @@ int ima_get_action(struct mnt_idmap *idmap, struct inode
> > *inode,
> > const struct cred *cred, u32 secid, int mask,
> > enum ima_hooks func, int *pcr,
> > struct ima_template_desc **template_desc,
> > - const char *func_data, unsigned int *allowed_algos);
> > + const char *func_data, unsigned int *allowed_algos,
> > + u64 *digest_cache_mask);
> > int ima_must_measure(struct inode *inode, int mask, enum ima_hooks func);
> > int ima_collect_measurement(struct ima_iint_cache *iint, struct file *file,
> > void *buf, loff_t size, enum hash_algo algo,
> > @@ -398,7 +403,8 @@ int ima_match_policy(struct mnt_idmap *idmap, struct inode
> > *inode,
> > const struct cred *cred, u32 secid, enum ima_hooks func,
> > int mask, int flags, int *pcr,
> > struct ima_template_desc **template_desc,
> > - const char *func_data, unsigned int *allowed_algos);
> > + const char *func_data, unsigned int *allowed_algos,
> > + u64 *digest_cache_mask);
> > void ima_init_policy(void);
> > void ima_update_policy(void);
> > void ima_update_policy_flags(void);
> > diff --git a/security/integrity/ima/ima_api.c
> > b/security/integrity/ima/ima_api.c
> > index b37d043d5748..87e286ace43c 100644
> > --- a/security/integrity/ima/ima_api.c
> > +++ b/security/integrity/ima/ima_api.c
> > @@ -173,6 +173,7 @@ void ima_add_violation(struct file *file, const unsigned
> > char *filename,
> > * @template_desc: pointer filled in if matched measure policy sets template=
> > * @func_data: func specific data, may be NULL
> > * @allowed_algos: allowlist of hash algorithms for the IMA xattr
> > + * @digest_cache_mask: Actions and purposes for which digest cache is allowed
> > *
> > * The policy is defined in terms of keypairs:
> > * subj=, obj=, type=, func=, mask=, fsmagic=
> > @@ -190,7 +191,8 @@ int ima_get_action(struct mnt_idmap *idmap, struct inode
> > *inode,
> > const struct cred *cred, u32 secid, int mask,
> > enum ima_hooks func, int *pcr,
> > struct ima_template_desc **template_desc,
> > - const char *func_data, unsigned int *allowed_algos)
> > + const char *func_data, unsigned int *allowed_algos,
> > + u64 *digest_cache_mask)
> > {
> > int flags = IMA_MEASURE | IMA_AUDIT | IMA_APPRAISE | IMA_HASH;
> >
> > @@ -198,7 +200,7 @@ int ima_get_action(struct mnt_idmap *idmap, struct inode
> > *inode,
> >
> > return ima_match_policy(idmap, inode, cred, secid, func, mask,
> > flags, pcr, template_desc, func_data,
> > - allowed_algos);
> > + allowed_algos, digest_cache_mask);
> > }
> >
> > static bool ima_get_verity_digest(struct ima_iint_cache *iint,
> > diff --git a/security/integrity/ima/ima_appraise.c
> > b/security/integrity/ima/ima_appraise.c
> > index 3497741caea9..27ccc9a2c09f 100644
> > --- a/security/integrity/ima/ima_appraise.c
> > +++ b/security/integrity/ima/ima_appraise.c
> > @@ -81,7 +81,7 @@ int ima_must_appraise(struct mnt_idmap *idmap, struct inode
> > *inode,
> > security_current_getsecid_subj(&secid);
> > return ima_match_policy(idmap, inode, current_cred(), secid,
> > func, mask, IMA_APPRAISE | IMA_HASH, NULL,
> > - NULL, NULL, NULL);
> > + NULL, NULL, NULL, NULL);
> > }
> >
> > static int ima_fix_xattr(struct dentry *dentry, struct ima_iint_cache *iint)
> > diff --git a/security/integrity/ima/ima_main.c
> > b/security/integrity/ima/ima_main.c
> > index 18285fc8ac07..e3ca80098c4c 100644
> > --- a/security/integrity/ima/ima_main.c
> > +++ b/security/integrity/ima/ima_main.c
> > @@ -232,7 +232,7 @@ static int process_measurement(struct file *file, const
> > struct cred *cred,
> > */
> > action = ima_get_action(file_mnt_idmap(file), inode, cred, secid,
> > mask, func, &pcr, &template_desc, NULL,
> > - &allowed_algos);
> > + &allowed_algos, NULL);
> > violation_check = ((func == FILE_CHECK || func == MMAP_CHECK ||
> > func == MMAP_CHECK_REQPROT) &&
> > (ima_policy_flag & IMA_MEASURE));
> > @@ -489,11 +489,11 @@ static int ima_file_mprotect(struct vm_area_struct *vma,
> > unsigned long reqprot,
> > inode = file_inode(vma->vm_file);
> > action = ima_get_action(file_mnt_idmap(vma->vm_file), inode,
> > current_cred(), secid, MAY_EXEC, MMAP_CHECK,
> > - &pcr, &template, NULL, NULL);
> > + &pcr, &template, NULL, NULL, NULL);
> > action |= ima_get_action(file_mnt_idmap(vma->vm_file), inode,
> > current_cred(), secid, MAY_EXEC,
> > MMAP_CHECK_REQPROT, &pcr, &template, NULL,
> > - NULL);
> > + NULL, NULL);
> >
> > /* Is the mmap'ed file in policy? */
> > if (!(action & (IMA_MEASURE | IMA_APPRAISE_SUBMASK)))
> > @@ -972,7 +972,7 @@ int process_buffer_measurement(struct mnt_idmap *idmap,
> > security_current_getsecid_subj(&secid);
> > action = ima_get_action(idmap, inode, current_cred(),
> > secid, 0, func, &pcr, &template,
> > - func_data, NULL);
> > + func_data, NULL, NULL);
> > if (!(action & IMA_MEASURE) && !digest)
> > return -ENOENT;
> > }
> > diff --git a/security/integrity/ima/ima_policy.c
> > b/security/integrity/ima/ima_policy.c
> > index 7cfd1860791f..4ac83df8d255 100644
> > --- a/security/integrity/ima/ima_policy.c
> > +++ b/security/integrity/ima/ima_policy.c
> > @@ -122,6 +122,7 @@ struct ima_rule_entry {
> > struct ima_rule_opt_list *keyrings; /* Measure keys added to these
> > keyrings */
> > struct ima_rule_opt_list *label; /* Measure data grouped under this
> > label */
> > struct ima_template_desc *template;
> > + u64 digest_cache_mask; /* Actions and purposes for which digest cache
> > is allowed */
> > };
> >
> > /*
> > @@ -726,6 +727,7 @@ static int get_subaction(struct ima_rule_entry *rule, enum
> > ima_hooks func)
> > * @template_desc: the template that should be used for this rule
> > * @func_data: func specific data, may be NULL
> > * @allowed_algos: allowlist of hash algorithms for the IMA xattr
> > + * @digest_cache_mask: Actions and purposes for which digest cache is allowed
>
> Purpose
> > *
> > * Measure decision based on func/mask/fsmagic and LSM(subj/obj/type)
> > * conditions.
> > @@ -738,7 +740,8 @@ int ima_match_policy(struct mnt_idmap *idmap, struct inode
> > *inode,
> > const struct cred *cred, u32 secid, enum ima_hooks func,
> > int mask, int flags, int *pcr,
> > struct ima_template_desc **template_desc,
> > - const char *func_data, unsigned int *allowed_algos)
> > + const char *func_data, unsigned int *allowed_algos,
> > + u64 *digest_cache_mask)
> > {
> > struct ima_rule_entry *entry;
> > int action = 0, actmask = flags | (flags << 1);
> > @@ -783,6 +786,9 @@ int ima_match_policy(struct mnt_idmap *idmap, struct inode
> > *inode,
> > if (template_desc && entry->template)
> > *template_desc = entry->template;
> >
> > + if (digest_cache_mask)
> > + *digest_cache_mask |= entry->digest_cache_mask;
> > +
> > if (!actmask)
> > break;
> > }
> > @@ -859,6 +865,30 @@ static int ima_appraise_flag(enum ima_hooks func)
> > return 0;
> > }
> >
> > +static bool ima_digest_cache_func_allowed(struct ima_rule_entry *entry)
> > +{
> > + switch (entry->func) {
> > + case NONE:
> > + case FILE_CHECK:
> > + case MMAP_CHECK:
> > + case MMAP_CHECK_REQPROT:
> > + case BPRM_CHECK:
> > + case CREDS_CHECK:
> > + case FIRMWARE_CHECK:
> > + case POLICY_CHECK:
> > + case MODULE_CHECK:
> > + case KEXEC_KERNEL_CHECK:
> > + case KEXEC_INITRAMFS_CHECK:
> > + /* Exception: always add policy updates to measurement list! */
> > + if (entry->action == MEASURE && entry->func == POLICY_CHECK)
> > + return false;
> > +
> > + return true;
> > + default:
> > + return false;
> > + }
> > +}
> > +
> > static void add_rules(struct ima_rule_entry *entries, int count,
> > enum policy_rule_list policy_rule)
> > {
> > @@ -1073,7 +1103,7 @@ enum policy_opt {
> > Opt_digest_type,
> > Opt_appraise_type, Opt_appraise_flag, Opt_appraise_algos,
> > Opt_permit_directio, Opt_pcr, Opt_template, Opt_keyrings,
> > - Opt_label, Opt_err
> > + Opt_label, Opt_digest_cache, Opt_err
> > };
> >
> > static const match_table_t policy_tokens = {
> > @@ -1122,6 +1152,7 @@ static const match_table_t policy_tokens = {
> > {Opt_template, "template=%s"},
> > {Opt_keyrings, "keyrings=%s"},
> > {Opt_label, "label=%s"},
> > + {Opt_digest_cache, "digest_cache=%s"},
> > {Opt_err, NULL}
> > };
> >
> > @@ -1245,6 +1276,18 @@ static bool ima_validate_rule(struct ima_rule_entry
> > *entry)
> > if (entry->action != MEASURE && entry->flags & IMA_PCR)
> > return false;
> >
> > + /* New-style measurements with digest cache cannot be on default PCR. */
> > + if (entry->action == MEASURE &&
> > + (entry->digest_cache_mask & IMA_DIGEST_CACHE_MEASURE_CONTENT)) {
> > + if (!(entry->flags & IMA_PCR) ||
> > + entry->pcr == CONFIG_IMA_MEASURE_PCR_IDX)
> > + return false;
> > + }
> > +
> > + /* Digest caches can be used only for a subset of the IMA hooks. */
> > + if (entry->digest_cache_mask && !ima_digest_cache_func_allowed(entry))
> > + return false;
> > +
> > if (entry->action != APPRAISE &&
> > entry->flags & (IMA_DIGSIG_REQUIRED | IMA_MODSIG_ALLOWED |
> > IMA_CHECK_BLACKLIST | IMA_VALIDATE_ALGOS))
> > @@ -1881,6 +1924,26 @@ static int ima_parse_rule(char *rule, struct
> > ima_rule_entry *entry)
> > &(template_desc->num_fields));
> > entry->template = template_desc;
> > break;
> > + case Opt_digest_cache:
> > + ima_log_string(ab, "digest_cache", args[0].from);
> > +
> > + result = -EINVAL;
> > +
> > + if (!strcmp(args[0].from, "content")) {
> > + switch (entry->action) {
> > + case MEASURE:
> > + entry->digest_cache_mask |=
> > IMA_DIGEST_CACHE_MEASURE_CONTENT;
> > + result = 0;
> > + break;
> > + case APPRAISE:
> > + entry->digest_cache_mask |=
> > IMA_DIGEST_CACHE_APPRAISE_CONTENT;
> > + result = 0;
> > + break;
> > + default:
> > + break;
> > + }
> > + }
> > + break;
> > case Opt_err:
> > ima_log_string(ab, "UNKNOWN", p);
> > result = -EINVAL;
> > @@ -2271,6 +2334,9 @@ int ima_policy_show(struct seq_file *m, void *v)
> > seq_puts(m, "digest_type=verity ");
> > if (entry->flags & IMA_PERMIT_DIRECTIO)
> > seq_puts(m, "permit_directio ");
> > + if ((entry->digest_cache_mask & IMA_DIGEST_CACHE_MEASURE_CONTENT) ||
> > + (entry->digest_cache_mask & IMA_DIGEST_CACHE_APPRAISE_CONTENT))
> > + seq_puts(m, "digest_cache=content ");
> > rcu_read_unlock();
> > seq_puts(m, "\n");
> > return 0;
>
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