[PATCH v4 2/5] IMA: block writes of the security.ima xattr with unsupported algorithms

[THOBY Simon <Simon.THOBY@viveris.fr>]

By default, any write to the extended attributes security.ima will be
accepted, even if the xattr value uses a hash algorithm not compiled in
the kernel (which doesn't make sense, because the kernel wouldn't be able
to appraise that file, as it lacks support for validating the hash).

Prevent such writes: only writes using hash algorithms
available in the current kernel are now allowed. Any attempt to
perform these writes will be denied with an audit message.

Note however that CONFIG_IMA depends on CONFIG_CRYPTO_SHA1, which
somewhat hampers the security benefits of this measure (but MD4 and
MD5 can be disabled, which is already a significant improvement).

Signed-off-by: Simon Thoby <simon.thoby@viveris.fr>
---
 security/integrity/ima/ima_appraise.c | 51 ++++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index ef9dcfce45d4..989da2fbf496 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -575,12 +575,55 @@ static void ima_reset_appraise_flags(struct inode *inode, int digsig)
 		clear_bit(IMA_DIGSIG, &iint->atomic_flags);
 }
 
+/**
+ * ima_setxattr_validate_hash_alg() - Block setxattr with invalid digests
+ * @dentry: file being altered
+ * @xattr_value: value supplied by userland for the xattr
+ * @xattr_value_len: length of xattr_value
+ *
+ * Context: called when the user tries to write the security.ima xattr.
+ * The xattr value is mapped to some hash algorithm, and this algorithm
+ * must be built in the kernel for the setxattr to be allowed.
+ *
+ * Emit an audit message when the algorithm is invalid.
+ *
+ * Return: 0 on success, else an error.
+ */
+int ima_setxattr_validate_hash_alg(struct dentry *dentry,
+				   const void *xattr_value,
+				   size_t xattr_value_len)
+{
+	int res = -EACCES;
+	char *path = NULL, *pathbuf = NULL;
+	enum hash_algo dentry_hash;
+
+	dentry_hash = ima_get_hash_algo((struct evm_ima_xattr_data *)xattr_value,
+				     xattr_value_len);
+
+	if (likely(dentry_hash == ima_hash_algo
+	    || crypto_has_alg(hash_algo_name[dentry_hash], 0, 0)))
+		return 0;
+
+	pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
+	/* no memory available ? no file path for you */
+	if (pathbuf)
+		path = dentry_path(dentry, pathbuf, PATH_MAX);
+
+	/* disallow xattr writes with algorithms not built in the kernel */
+	integrity_audit_msg(AUDIT_INTEGRITY_DATA, d_inode(dentry),
+		path, "collect_data", "unavailable-hash-algorithm", res, 0);
+
+	kfree(pathbuf);
+
+	return res;
+}
+
 int ima_inode_setxattr(struct dentry *dentry, const char *xattr_name,
 		       const void *xattr_value, size_t xattr_value_len)
 {
 	const struct evm_ima_xattr_data *xvalue = xattr_value;
 	int digsig = 0;
-	int result;
+	int result, rc;
 
 	result = ima_protect_xattr(dentry, xattr_name, xattr_value,
 				   xattr_value_len);
@@ -592,6 +635,12 @@ int ima_inode_setxattr(struct dentry *dentry, const char *xattr_name,
 		digsig = (xvalue->type == EVM_XATTR_PORTABLE_DIGSIG);
 	}
 	if (result == 1 || evm_revalidate_status(xattr_name)) {
+		/* the user-supplied xattr must use an allowed hash algo */
+		rc = ima_setxattr_validate_hash_alg(dentry, xattr_value,
+							xattr_value_len);
+		if (!rc)
+			return rc;
+
 		ima_reset_appraise_flags(d_backing_inode(dentry), digsig);
 		if (result == 1)
 			result = 0;
-- 
2.31.1