Eric Biggers <ebiggers3@xxxxxxxxx> wrote:
Okay, I've folded in all your changes to my mount-context branch, apart from
patch 07 (the legacy wrapper double free fix) which I think needed doing
differently.
> Also, 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.
How about the attached? I need to separate it out anyway if I want to
implement a mount_setattr() syscall.
David
---
commit f24ee400978ad41e48e679f1d422277fc353521c
Author: David Howells <dhowells@xxxxxxxxxx>
Date: Mon Jul 9 16:24:27 2018 +0100
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 now gets parsed with the new fs_context
stuff prior to doing a remount - and this causes the syscall under some
circumstances.
diff --git a/fs/namespace.c b/fs/namespace.c
index 58c1ace07bb6..39e9744beab9 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -275,13 +275,13 @@ static struct mount *alloc_vfsmnt(const char *name)
* 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;
+ return true;
if (sb_rdonly(mnt->mnt_sb))
- return 1;
- return 0;
+ return true;
+ return false;
}
EXPORT_SYMBOL_GPL(__mnt_is_readonly);
@@ -596,11 +596,12 @@ 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)
@@ -2307,21 +2308,85 @@ SYSCALL_DEFINE3(open_tree, int, dfd, const char *, filename, unsigned, flags)
return error;
}
-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)
+ 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 vfsmount *mnt, unsigned int mnt_flags)
+{
+ bool readonly_request = true;
+
+ if (mnt_flags & MNT_READONLY)
readonly_request = 1;
if (readonly_request == __mnt_is_readonly(mnt))
return 0;
- if (readonly_request)
- error = mnt_make_readonly(real_mount(mnt));
- else
- __mnt_unmake_readonly(real_mount(mnt));
- return error;
+ if (!readonly_request)
+ return mnt_make_readonly(real_mount(mnt));
+
+ return __mnt_unmake_readonly(real_mount(mnt));
+}
+
+/*
+ * 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 != path->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(path->mnt, mnt_flags);
+ if (ret == 0) {
+ 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();
+ }
+ up_write(&sb->s_umount);
+ return ret;
}
/*
@@ -2358,32 +2423,8 @@ 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)) {
- 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))) {
+ if (!can_change_locked_flags(mnt, mnt_flags))
return -EPERM;
- }
if (type->init_fs_context) {
fc = vfs_sb_reconfig(path, sb_flags);
@@ -2410,9 +2451,7 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
}
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))
+ if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
err = -EPERM;
else
err = do_remount_sb(sb, sb_flags, data, data_size, 0, fc);
@@ -2961,7 +3000,9 @@ 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, data_size);
else if (flags & MS_BIND)
diff --git a/include/linux/mount.h b/include/linux/mount.h
index ee5af77afc06..41b6b080ffd0 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -82,7 +82,7 @@ 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;
