fs/xattr.c: fix simple_xattr_list to always include security.* xattrs

The vfs has long had a fallback to obtain the security.* xattrs from the
LSM when the filesystem does not implement its own listxattr, but
shmem/tmpfs and kernfs later gained their own xattr handlers to support
other xattrs. Unfortunately, as a side effect, tmpfs and kernfs-based
filesystems like sysfs no longer return the synthetic security.* xattr
names via listxattr unless they are explicitly set by userspace or
initially set upon inode creation after policy load. coreutils has
recently switched from unconditionally invoking getxattr for security.*
for ls -Z via libselinux to only doing so if listxattr returns the xattr
name, breaking ls -Z of such inodes.

Before:
$ getfattr -m.* /run/initramfs
<no output>
$ getfattr -m.* /sys/kernel/fscaps
<no output>
$ setfattr -n user.foo /run/initramfs
$ getfattr -m.* /run/initramfs
user.foo

After:
$ getfattr -m.* /run/initramfs
security.selinux
$ getfattr -m.* /sys/kernel/fscaps
security.selinux
$ setfattr -n user.foo /run/initramfs
$ getfattr -m.* /run/initramfs
security.selinux
user.foo

Link: https://lore.kernel.org/selinux/CAFqZXNtF8wDyQajPCdGn=iOawX4y77ph0EcfcqcUUj+T87FKyA@mail.gmail.com/
Link: https://lore.kernel.org/selinux/20250423175728.3185-2-stephen.smalley.work@gmail.com/
Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Link: https://lore.kernel.org/20250424152822.2719-1-stephen.smalley.work@gmail.com
Fixes: b09e0fa ("tmpfs: implement generic xattr support")
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: stephensmalley

fs/xattr.c

@@ -1428,6 +1428,15 @@ static bool xattr_is_trusted(const char *name)
 	return !strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN);
 }
 
+static bool xattr_is_maclabel(const char *name)
+{
+	const char *suffix = name + XATTR_SECURITY_PREFIX_LEN;
+
+	return !strncmp(name, XATTR_SECURITY_PREFIX,
+			XATTR_SECURITY_PREFIX_LEN) &&
+		security_ismaclabel(suffix);
+}
+
 /**
  * simple_xattr_list - list all xattr objects
  * @inode: inode from which to get the xattrs
@@ -1460,6 +1469,17 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
 	if (err)
 		return err;
 
+	err = security_inode_listsecurity(inode, buffer, remaining_size);
+	if (err < 0)
+		return err;
+
+	if (buffer) {
+		if (remaining_size < err)
+			return -ERANGE;
+		buffer += err;
+	}
+	remaining_size -= err;
+
 	read_lock(&xattrs->lock);
 	for (rbp = rb_first(&xattrs->rb_root); rbp; rbp = rb_next(rbp)) {
 		xattr = rb_entry(rbp, struct simple_xattr, rb_node);
@@ -1468,6 +1488,10 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
 		if (!trusted && xattr_is_trusted(xattr->name))
 			continue;
 
+		/* skip MAC labels; these are provided by LSM above */
+		if (xattr_is_maclabel(xattr->name))
+			continue;
+
 		err = xattr_list_one(&buffer, &remaining_size, xattr->name);
 		if (err)
 			break;