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fs: move SB_I_USERNS_VISIBLE to FS_USERNS_MOUNT_RESTRICTED

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Whether a filesystem's mounts need to undergo a visibility check in user
namespaces is a static property of the filesystem type, not a runtime
property of each superblock instance. Both proc and sysfs always set
SB_I_USERNS_VISIBLE on their superblocks unconditionally (sysfs does so
on first creation, and subsequent mounts reuse the same superblock).

Move this flag from sb->s_iflags (SB_I_USERNS_VISIBLE) to
file_system_type->fs_flags (FS_USERNS_MOUNT_RESTRICTED) so the intent
is expressed at the filesystem type level where it belongs.

All check sites are updated to test sb->s_type->fs_flags instead of
sb->s_iflags. The SB_I_NOEXEC and SB_I_NODEV flags remain on the
superblock as they are runtime properties set during fill_super.

Link: https://patch.msgid.link/72887c5b6204dc3adf5a53104f0be6bd8bc4f6cd.1777278334.git.legion@kernel.org
Reviewed-by: Aleksa Sarai <aleksa@amutable.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2026-04-27 10:26:03 +02:00
parent 672697c37c
commit dc4edae7f4
6 changed files with 8 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
fs/namespace.c
View File

@ -1099,7 +1099,7 @@ static void mnt_add_to_ns(struct mnt_namespace *ns, struct mount *mnt)
rb_link_node(&mnt->mnt_node, parent, link);
rb_insert_color(&mnt->mnt_node, &ns->mounts);
if ((mnt->mnt.mnt_sb->s_iflags & SB_I_USERNS_VISIBLE) &&
if ((mnt->mnt.mnt_sb->s_type->fs_flags & FS_USERNS_MOUNT_RESTRICTED) &&
mnt->mnt.mnt_root == mnt->mnt.mnt_sb->s_root)
hlist_add_head(&mnt->mnt_ns_visible, &ns->mnt_visible_mounts);
@ -6408,10 +6408,10 @@ static bool mount_too_revealing(const struct super_block *sb, int *new_mnt_flags
return false;
/* Can this filesystem be too revealing? */
s_iflags = sb->s_iflags;
if (!(s_iflags & SB_I_USERNS_VISIBLE))
if (!(sb->s_type->fs_flags & FS_USERNS_MOUNT_RESTRICTED))
return false;
s_iflags = sb->s_iflags;
if ((s_iflags & required_iflags) != required_iflags) {
WARN_ONCE(1, "Expected s_iflags to contain 0x%lx\n",
required_iflags);

4
fs/proc/root.c
View File

@ -257,7 +257,7 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
proc_apply_options(fs_info, fc, current_user_ns());
/* User space would break if executables or devices appear on proc */
s->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC | SB_I_NODEV;
s->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
s->s_flags |= SB_NODIRATIME | SB_NOSUID | SB_NOEXEC;
s->s_blocksize = 1024;
s->s_blocksize_bits = 10;
@ -359,7 +359,7 @@ static struct file_system_type proc_fs_type = {
.init_fs_context = proc_init_fs_context,
.parameters = proc_fs_parameters,
.kill_sb = proc_kill_sb,
.fs_flags = FS_USERNS_MOUNT | FS_DISALLOW_NOTIFY_PERM,
.fs_flags = FS_USERNS_MOUNT | FS_USERNS_MOUNT_RESTRICTED | FS_DISALLOW_NOTIFY_PERM,
};
void __init proc_root_init(void)

4
fs/sysfs/mount.c
View File

@ -32,8 +32,6 @@ static int sysfs_get_tree(struct fs_context *fc)
if (ret)
return ret;
if (kfc->new_sb_created)
fc->root->d_sb->s_iflags |= SB_I_USERNS_VISIBLE;
return 0;
}
@ -93,7 +91,7 @@ static struct file_system_type sysfs_fs_type = {
.name = "sysfs",
.init_fs_context = sysfs_init_fs_context,
.kill_sb = sysfs_kill_sb,
.fs_flags = FS_USERNS_MOUNT,
.fs_flags = FS_USERNS_MOUNT | FS_USERNS_MOUNT_RESTRICTED,
};
int __init sysfs_init(void)

1
include/linux/fs.h
View File

@ -2281,6 +2281,7 @@ struct file_system_type {
#define FS_MGTIME 64 /* FS uses multigrain timestamps */
#define FS_LBS 128 /* FS supports LBS */
#define FS_POWER_FREEZE 256 /* Always freeze on suspend/hibernate */
#define FS_USERNS_MOUNT_RESTRICTED 512 /* Restrict mount in userns if not already visible */
#define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move() during rename() internally. */
int (*init_fs_context)(struct fs_context *);
const struct fs_parameter_spec *parameters;

1
include/linux/fs/super_types.h
View File

@ -326,7 +326,6 @@ struct super_block {
#define SB_I_STABLE_WRITES 0x00000008 /* don't modify blks until WB is done */
/* sb->s_iflags to limit user namespace mounts */
#define SB_I_USERNS_VISIBLE 0x00000010 /* fstype already mounted */
#define SB_I_IMA_UNVERIFIABLE_SIGNATURE 0x00000020
#define SB_I_UNTRUSTED_MOUNTER 0x00000040
#define SB_I_EVM_HMAC_UNSUPPORTED 0x00000080

2
kernel/acct.c
View File

@ -249,7 +249,7 @@ static int acct_on(const char __user *name)
return -EINVAL;
/* Exclude procfs and sysfs. */
if (file_inode(file)->i_sb->s_iflags & SB_I_USERNS_VISIBLE)
if (file_inode(file)->i_sb->s_type->fs_flags & FS_USERNS_MOUNT_RESTRICTED)
return -EINVAL;
if (!(file->f_mode & FMODE_CAN_WRITE))