IMA: add support to restrict the hash algorithms used for file appraisal

The kernel accepts any hash algorithm as a value for the security.ima
xattr. Users may wish to restrict the accepted algorithms to only
support strong cryptographic ones.

Provide the plumbing to restrict the permitted set of hash algorithms
used for verifying file hashes and signatures stored in security.ima
xattr.

Signed-off-by: THOBY Simon <Simon.THOBY@viveris.fr>
Reviewed-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

1 files changed, 16 insertions, 2 deletions

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index fd5d46e511f1..1536e6f5eb22 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -35,6 +35,7 @@
 #define IMA_FSNAME	0x0200
 #define IMA_KEYRINGS	0x0400
 #define IMA_LABEL	0x0800
+#define IMA_VALIDATE_ALGOS	0x1000
 
 #define UNKNOWN		0
 #define MEASURE		0x0001	/* same as IMA_MEASURE */
@@ -79,6 +80,7 @@ struct ima_rule_entry {
 	bool (*uid_op)(kuid_t, kuid_t);    /* Handlers for operators       */
 	bool (*fowner_op)(kuid_t, kuid_t); /* uid_eq(), uid_gt(), uid_lt() */
 	int pcr;
+	unsigned int allowed_algos; /* bitfield of allowed hash algorithms */
 	struct {
 		void *rule;	/* LSM file metadata specific */
 		char *args_p;	/* audit value */
@@ -91,6 +93,14 @@ struct ima_rule_entry {
 };
 
 /*
+ * sanity check in case the kernels gains more hash algorithms that can
+ * fit in an unsigned int
+ */
+static_assert(
+	8 * sizeof(unsigned int) >= HASH_ALGO__LAST,
+	"The bitfield allowed_algos in ima_rule_entry is too small to contain all the supported hash algorithms, consider using a bigger type");
+
+/*
  * Without LSM specific knowledge, the default policy can only be
  * written in terms of .action, .func, .mask, .fsmagic, .uid, and .fowner
  */
@@ -646,6 +656,7 @@ static int get_subaction(struct ima_rule_entry *rule, enum ima_hooks func)
  * @pcr: set the pcr to extend
  * @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
  *
  * Measure decision based on func/mask/fsmagic and LSM(subj/obj/type)
  * conditions.
@@ -658,7 +669,7 @@ int ima_match_policy(struct user_namespace *mnt_userns, 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)
+		     const char *func_data, unsigned int *allowed_algos)
 {
 	struct ima_rule_entry *entry;
 	int action = 0, actmask = flags | (flags << 1);
@@ -684,8 +695,11 @@ int ima_match_policy(struct user_namespace *mnt_userns, struct inode *inode,
 			action &= ~IMA_HASH;
 			if (ima_fail_unverifiable_sigs)
 				action |= IMA_FAIL_UNVERIFIABLE_SIGS;
-		}
 
+			if (allowed_algos &&
+			    entry->flags & IMA_VALIDATE_ALGOS)
+				*allowed_algos = entry->allowed_algos;
+		}
 
 		if (entry->action & IMA_DO_MASK)
 			actmask &= ~(entry->action | entry->action << 1);