zpl_xattr: stop heap-allocating prefixed xattr names

The six __zpl_xattr_{user,trusted,security}_{get,set} entry points
built their prefixed name via kmem_asprintf("%s%s", prefix, name)
and freed it with kmem_strfree on the way out.

The Linux xattr API caps the full prefix+name length at
XATTR_NAME_MAX (255), the same bound fs/xattr.c's syscall handlers
rely on with their stack-resident struct xattr_name, and so do
the same in our xattr handlers.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <rob.norris@truenas.com>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Andrew Walker <andrew.walker@truenas.com>
Closes #18570

module/os/linux/zfs/zpl_xattr.c

@@ -701,6 +701,24 @@ zpl_xattr_set(struct inode *ip, const char *name, const void *value,
  * ZFS allows extended user attributes to be disabled administratively
  * by setting the 'xattr=off' property on the dataset.
  */
+
+/*
+ * Concatenate prefix + name into a NUL-terminated stack buffer.
+ * Linux fs/xattr.c (import_xattr_name) caps the full xattr name at
+ * XATTR_NAME_MAX before any handler runs, so XATTR_NAME_MAX + 1
+ * bytes always fit.
+ */
+static inline void
+zpl_xattr_join_name(char *buf, size_t buflen, const char *prefix,
+    size_t prefix_len, const char *name, size_t name_len)
+{
+	ASSERT3U(prefix_len + name_len + 1, <=, buflen);
+
+	memcpy(buf, prefix, prefix_len);
+	memcpy(buf + prefix_len, name, name_len);
+	buf[prefix_len + name_len] = '\0';
+}
+
 static int
 __zpl_xattr_user_list(struct inode *ip, char *list, size_t list_size,
     const char *name, size_t name_len)
@@ -726,9 +744,13 @@ __zpl_xattr_user_get(struct inode *ip, const char *name,
 	 * try again without the namespace prefix for compatibility with
 	 * other platforms.
 	 */
-	char *xattr_name = kmem_asprintf("%s%s", XATTR_USER_PREFIX, name);
+	char xattr_name[XATTR_NAME_MAX + 1];
+
+	zpl_xattr_join_name(xattr_name, sizeof (xattr_name),
+	    XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN,
+	    name, strlen(name));
+
 	error = zpl_xattr_get(ip, xattr_name, value, size);
-	kmem_strfree(xattr_name);
 	if (error == -ENODATA)
 		error = zpl_xattr_get(ip, name, value, size);
 
@@ -758,8 +780,13 @@ __zpl_xattr_user_set(zidmap_t *user_ns,
 	 *   XATTR_CREATE: fail if xattr already exists
 	 *   XATTR_REPLACE: fail if xattr does not exist
 	 */
-	char *prefixed_name = kmem_asprintf("%s%s", XATTR_USER_PREFIX, name);
+	char prefixed_name[XATTR_NAME_MAX + 1];
 	const char *clear_name, *set_name;
+
+	zpl_xattr_join_name(prefixed_name, sizeof (prefixed_name),
+	    XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN,
+	    name, strlen(name));
+
 	if (zfs_xattr_compat) {
 		clear_name = prefixed_name;
 		set_name = name;
@@ -776,7 +803,7 @@ __zpl_xattr_user_set(zidmap_t *user_ns,
 	 * because it already exists.  Stop here.
 	 */
 	if (error == -EEXIST)
-		goto out;
+		return (error);
 	/*
 	 * If XATTR_REPLACE was specified and we succeeded to clear
 	 * an xattr, we don't need to replace anything when setting
@@ -788,10 +815,7 @@ __zpl_xattr_user_set(zidmap_t *user_ns,
 	/*
 	 * Set the new value with the configured name format.
 	 */
-	error = zpl_xattr_set(ip, set_name, value, size, flags);
+	return (zpl_xattr_set(ip, set_name, value, size, flags));
-out:
-	kmem_strfree(prefixed_name);
-	return (error);
 }
 ZPL_XATTR_SET_WRAPPER(zpl_xattr_user_set);
 
@@ -824,17 +848,16 @@ static int
 __zpl_xattr_trusted_get(struct inode *ip, const char *name,
     void *value, size_t size)
 {
-	char *xattr_name;
+	char xattr_name[XATTR_NAME_MAX + 1];
-	int error;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return (-EACCES);
-	/* xattr_resolve_name will do this for us if this is defined */
-	xattr_name = kmem_asprintf("%s%s", XATTR_TRUSTED_PREFIX, name);
-	error = zpl_xattr_get(ip, xattr_name, value, size);
-	kmem_strfree(xattr_name);
 
-	return (error);
+	zpl_xattr_join_name(xattr_name, sizeof (xattr_name),
+	    XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN,
+	    name, strlen(name));
+
+	return (zpl_xattr_get(ip, xattr_name, value, size));
 }
 ZPL_XATTR_GET_WRAPPER(zpl_xattr_trusted_get);
 
@@ -844,17 +867,16 @@ __zpl_xattr_trusted_set(zidmap_t *user_ns,
     const void *value, size_t size, int flags)
 {
 	(void) user_ns;
-	char *xattr_name;
+	char xattr_name[XATTR_NAME_MAX + 1];
-	int error;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return (-EACCES);
-	/* xattr_resolve_name will do this for us if this is defined */
-	xattr_name = kmem_asprintf("%s%s", XATTR_TRUSTED_PREFIX, name);
-	error = zpl_xattr_set(ip, xattr_name, value, size, flags);
-	kmem_strfree(xattr_name);
 
-	return (error);
+	zpl_xattr_join_name(xattr_name, sizeof (xattr_name),
+	    XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN,
+	    name, strlen(name));
+
+	return (zpl_xattr_set(ip, xattr_name, value, size, flags));
 }
 ZPL_XATTR_SET_WRAPPER(zpl_xattr_trusted_set);
 
@@ -889,14 +911,13 @@ static int
 __zpl_xattr_security_get(struct inode *ip, const char *name,
     void *value, size_t size)
 {
-	char *xattr_name;
+	char xattr_name[XATTR_NAME_MAX + 1];
-	int error;
-	/* xattr_resolve_name will do this for us if this is defined */
-	xattr_name = kmem_asprintf("%s%s", XATTR_SECURITY_PREFIX, name);
-	error = zpl_xattr_get(ip, xattr_name, value, size);
-	kmem_strfree(xattr_name);
 
-	return (error);
+	zpl_xattr_join_name(xattr_name, sizeof (xattr_name),
+	    XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN,
+	    name, strlen(name));
+
+	return (zpl_xattr_get(ip, xattr_name, value, size));
 }
 ZPL_XATTR_GET_WRAPPER(zpl_xattr_security_get);
 
@@ -906,14 +927,13 @@ __zpl_xattr_security_set(zidmap_t *user_ns,
     const void *value, size_t size, int flags)
 {
 	(void) user_ns;
-	char *xattr_name;
+	char xattr_name[XATTR_NAME_MAX + 1];
-	int error;
-	/* xattr_resolve_name will do this for us if this is defined */
-	xattr_name = kmem_asprintf("%s%s", XATTR_SECURITY_PREFIX, name);
-	error = zpl_xattr_set(ip, xattr_name, value, size, flags);
-	kmem_strfree(xattr_name);
 
-	return (error);
+	zpl_xattr_join_name(xattr_name, sizeof (xattr_name),
+	    XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN,
+	    name, strlen(name));
+
+	return (zpl_xattr_set(ip, xattr_name, value, size, flags));
 }
 ZPL_XATTR_SET_WRAPPER(zpl_xattr_security_set);