[PATCH] vfs,shmem,kernfs: fix listxattr to include security.* xattrs

[Stephen Smalley]

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>

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

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 at gmail.com>
---
 fs/kernfs/inode.c |  8 +++++++-
 fs/xattr.c        | 13 +++++++++++++
 mm/shmem.c        |  8 +++++++-
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index b83054da68b3..8fd69e48d32d 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -140,12 +140,18 @@ ssize_t kernfs_iop_listxattr(struct dentry *dentry, char *buf, size_t size)
 {
 	struct kernfs_node *kn = kernfs_dentry_node(dentry);
 	struct kernfs_iattrs *attrs;
+	ssize_t sz;
 
 	attrs = kernfs_iattrs(kn);
 	if (!attrs)
 		return -ENOMEM;
 
-	return simple_xattr_list(d_inode(dentry), &attrs->xattrs, buf, size);
+	sz = simple_xattr_list(d_inode(dentry), &attrs->xattrs, buf, size);
+	if (sz >= 0 && sz <= size)
+		sz += security_inode_listsecurity(d_inode(dentry),
+						buf ? buf + sz : NULL,
+						size - sz);
+	return sz;
 }
 
 static inline void set_default_inode_attr(struct inode *inode, umode_t mode)
diff --git a/fs/xattr.c b/fs/xattr.c
index 02bee149ad96..68ac91d0dbc3 100644
--- a/fs/xattr.c
+++ b/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
@@ -1468,6 +1477,10 @@ 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 separately */
+		if (xattr_is_maclabel(xattr->name))
+			continue;
+
 		err = xattr_list_one(&buffer, &remaining_size, xattr->name);
 		if (err)
 			break;
diff --git a/mm/shmem.c b/mm/shmem.c
index 99327c30507c..69f664668a3a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -4372,7 +4372,13 @@ static const struct xattr_handler * const shmem_xattr_handlers[] = {
 static ssize_t shmem_listxattr(struct dentry *dentry, char *buffer, size_t size)
 {
 	struct shmem_inode_info *info = SHMEM_I(d_inode(dentry));
-	return simple_xattr_list(d_inode(dentry), &info->xattrs, buffer, size);
+	ssize_t sz = simple_xattr_list(d_inode(dentry), &info->xattrs, buffer,
+				size);
+	if (sz >= 0 && sz <= size)
+		sz += security_inode_listsecurity(d_inode(dentry),
+						buffer ? buffer + sz : NULL,
+						size - sz);
+	return sz;
 }
 #endif /* CONFIG_TMPFS_XATTR */
 
-- 
2.49.0