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vfs: Separate changing mount flags full remount 

Separate just the changing of mount flags (MS_REMOUNT|MS_BIND) from full
remount because the mount data will get parsed with the new fs_context
stuff prior to doing a remount - and this causes the syscall to fail under
some circumstances.

To quote Eric's explanation:

  [...] mount(..., MS_REMOUNT|MS_BIND, ...) now validates the mount options
  string, which breaks systemd unit files with ProtectControlGroups=yes
  (e.g.  systemd-networkd.service) when systemd does the following to
  change a cgroup (v1) mount to read-only:

    mount(NULL, "/run/systemd/unit-root/sys/fs/cgroup/systemd", NULL,
	  MS_RDONLY|MS_NOSUID|MS_NODEV|MS_NOEXEC|MS_REMOUNT|MS_BIND, NULL)

  ... when the kernel has CONFIG_CGROUPS=y but no cgroup subsystems
  enabled, since in that case the error "cgroup1: Need name or subsystem
  set" is hit when the mount options string is empty.

  Probably it doesn't make sense to validate the mount options string at
  all in the MS_REMOUNT|MS_BIND case, though maybe you had something else
  in mind.

This is also worthwhile doing because we will need to add a mount_setattr()
syscall to take over the remount-bind function.

Reported-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: David Howells <dhowells@redhat.com>
This commit is contained in:
David Howells 2018-11-01 23:07:25 +00:00
parent e262e32d6b
commit 43f5e655ef
2 changed files with 93 additions and 55 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

146
fs/namespace.c
View File

@ -246,13 +246,9 @@ out_free_cache:
* mnt_want/drop_write() will _keep_ the filesystem
* r/w.
*/
int __mnt_is_readonly(struct vfsmount *mnt)
bool __mnt_is_readonly(struct vfsmount *mnt)
{
if (mnt->mnt_flags & MNT_READONLY)
return 1;
if (sb_rdonly(mnt->mnt_sb))
return 1;
return 0;
return (mnt->mnt_flags & MNT_READONLY) || sb_rdonly(mnt->mnt_sb);
}
EXPORT_SYMBOL_GPL(__mnt_is_readonly);
@ -508,11 +504,12 @@ static int mnt_make_readonly(struct mount *mnt)
return ret;
}
static void __mnt_unmake_readonly(struct mount *mnt)
static int __mnt_unmake_readonly(struct mount *mnt)
{
lock_mount_hash();
mnt->mnt.mnt_flags &= ~MNT_READONLY;
unlock_mount_hash();
return 0;
}
int sb_prepare_remount_readonly(struct super_block *sb)
@ -2204,21 +2201,91 @@ out:
return err;
}
static int change_mount_flags(struct vfsmount *mnt, int ms_flags)
/*
* Don't allow locked mount flags to be cleared.
*
* No locks need to be held here while testing the various MNT_LOCK
* flags because those flags can never be cleared once they are set.
*/
static bool can_change_locked_flags(struct mount *mnt, unsigned int mnt_flags)
{
int error = 0;
int readonly_request = 0;
unsigned int fl = mnt->mnt.mnt_flags;
if (ms_flags & MS_RDONLY)
readonly_request = 1;
if (readonly_request == __mnt_is_readonly(mnt))
if ((fl & MNT_LOCK_READONLY) &&
!(mnt_flags & MNT_READONLY))
return false;
if ((fl & MNT_LOCK_NODEV) &&
!(mnt_flags & MNT_NODEV))
return false;
if ((fl & MNT_LOCK_NOSUID) &&
!(mnt_flags & MNT_NOSUID))
return false;
if ((fl & MNT_LOCK_NOEXEC) &&
!(mnt_flags & MNT_NOEXEC))
return false;
if ((fl & MNT_LOCK_ATIME) &&
((fl & MNT_ATIME_MASK) != (mnt_flags & MNT_ATIME_MASK)))
return false;
return true;
}
static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
{
bool readonly_request = (mnt_flags & MNT_READONLY);
if (readonly_request == __mnt_is_readonly(&mnt->mnt))
return 0;
if (readonly_request)
error = mnt_make_readonly(real_mount(mnt));
else
__mnt_unmake_readonly(real_mount(mnt));
return error;
return mnt_make_readonly(mnt);
return __mnt_unmake_readonly(mnt);
}
/*
* Update the user-settable attributes on a mount. The caller must hold
* sb->s_umount for writing.
*/
static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
{
lock_mount_hash();
mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
mnt->mnt.mnt_flags = mnt_flags;
touch_mnt_namespace(mnt->mnt_ns);
unlock_mount_hash();
}
/*
* Handle reconfiguration of the mountpoint only without alteration of the
* superblock it refers to. This is triggered by specifying MS_REMOUNT|MS_BIND
* to mount(2).
*/
static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
{
struct super_block *sb = path->mnt->mnt_sb;
struct mount *mnt = real_mount(path->mnt);
int ret;
if (!check_mnt(mnt))
return -EINVAL;
if (path->dentry != mnt->mnt.mnt_root)
return -EINVAL;
if (!can_change_locked_flags(mnt, mnt_flags))
return -EPERM;
down_write(&sb->s_umount);
ret = change_mount_ro_state(mnt, mnt_flags);
if (ret == 0)
set_mount_attributes(mnt, mnt_flags);
up_write(&sb->s_umount);
return ret;
}
/*
@ -2239,50 +2306,19 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
if (path->dentry != path->mnt->mnt_root)
return -EINVAL;
/* Don't allow changing of locked mnt flags.
*
* No locks need to be held here while testing the various
* MNT_LOCK flags because those flags can never be cleared
* once they are set.
*/
if ((mnt->mnt.mnt_flags & MNT_LOCK_READONLY) &&
!(mnt_flags & MNT_READONLY)) {
if (!can_change_locked_flags(mnt, mnt_flags))
return -EPERM;
}
if ((mnt->mnt.mnt_flags & MNT_LOCK_NODEV) &&
!(mnt_flags & MNT_NODEV)) {
return -EPERM;
}
if ((mnt->mnt.mnt_flags & MNT_LOCK_NOSUID) &&
!(mnt_flags & MNT_NOSUID)) {
return -EPERM;
}
if ((mnt->mnt.mnt_flags & MNT_LOCK_NOEXEC) &&
!(mnt_flags & MNT_NOEXEC)) {
return -EPERM;
}
if ((mnt->mnt.mnt_flags & MNT_LOCK_ATIME) &&
((mnt->mnt.mnt_flags & MNT_ATIME_MASK) != (mnt_flags & MNT_ATIME_MASK))) {
return -EPERM;
}
err = security_sb_remount(sb, data);
if (err)
return err;
down_write(&sb->s_umount);
if (ms_flags & MS_BIND)
err = change_mount_flags(path->mnt, ms_flags);
else if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
err = -EPERM;
else
err = -EPERM;
if (ns_capable(sb->s_user_ns, CAP_SYS_ADMIN)) {
err = do_remount_sb(sb, sb_flags, data, 0);
if (!err) {
lock_mount_hash();
mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
mnt->mnt.mnt_flags = mnt_flags;
touch_mnt_namespace(mnt->mnt_ns);
unlock_mount_hash();
if (!err)
set_mount_attributes(mnt, mnt_flags);
}
up_write(&sb->s_umount);
return err;
@ -2777,7 +2813,9 @@ long do_mount(const char *dev_name, const char __user *dir_name,
SB_LAZYTIME |
SB_I_VERSION);
if (flags & MS_REMOUNT)
if ((flags & (MS_REMOUNT | MS_BIND)) == (MS_REMOUNT | MS_BIND))
retval = do_reconfigure_mnt(&path, mnt_flags);
else if (flags & MS_REMOUNT)
retval = do_remount(&path, flags, sb_flags, mnt_flags,
data_page);
else if (flags & MS_BIND)

2
include/linux/mount.h
View File

@ -81,7 +81,7 @@ extern void mnt_drop_write_file(struct file *file);
extern void mntput(struct vfsmount *mnt);
extern struct vfsmount *mntget(struct vfsmount *mnt);
extern struct vfsmount *mnt_clone_internal(const struct path *path);
extern int __mnt_is_readonly(struct vfsmount *mnt);
extern bool __mnt_is_readonly(struct vfsmount *mnt);
extern bool mnt_may_suid(struct vfsmount *mnt);
struct path;