fs: Better permission checking for submounts
To support unprivileged users mounting filesystems two permission
checks have to be performed: a test to see if the user allowed to
create a mount in the mount namespace, and a test to see if
the user is allowed to access the specified filesystem.
The automount case is special in that mounting the original filesystem
grants permission to mount the sub-filesystems, to any user who
happens to stumble across the their mountpoint and satisfies the
ordinary filesystem permission checks.
Attempting to handle the automount case by using override_creds
almost works. It preserves the idea that permission to mount
the original filesystem is permission to mount the sub-filesystem.
Unfortunately using override_creds messes up the filesystems
ordinary permission checks.
Solve this by being explicit that a mount is a submount by introducing
vfs_submount, and using it where appropriate.
vfs_submount uses a new mount internal mount flags MS_SUBMOUNT, to let
sget and friends know that a mount is a submount so they can take appropriate
action.
sget and sget_userns are modified to not perform any permission checks
on submounts.
follow_automount is modified to stop using override_creds as that
has proven problemantic.
do_mount is modified to always remove the new MS_SUBMOUNT flag so
that we know userspace will never by able to specify it.
autofs4 is modified to stop using current_real_cred that was put in
there to handle the previous version of submount permission checking.
cifs is modified to pass the mountpoint all of the way down to vfs_submount.
debugfs is modified to pass the mountpoint all of the way down to
trace_automount by adding a new parameter. To make this change easier
a new typedef debugfs_automount_t is introduced to capture the type of
the debugfs automount function.
Cc: stable@vger.kernel.org
Fixes: 069d5ac9ae0d ("autofs: Fix automounts by using current_real_cred()->uid")
Fixes: aeaa4a79ff6a ("fs: Call d_automount with the filesystems creds")
Reviewed-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reviewed-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/fs/namespace.c b/fs/namespace.c
index 487ba30..089a6b2 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -989,6 +989,21 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
}
EXPORT_SYMBOL_GPL(vfs_kern_mount);
+struct vfsmount *
+vfs_submount(const struct dentry *mountpoint, struct file_system_type *type,
+ const char *name, void *data)
+{
+ /* Until it is worked out how to pass the user namespace
+ * through from the parent mount to the submount don't support
+ * unprivileged mounts with submounts.
+ */
+ if (mountpoint->d_sb->s_user_ns != &init_user_ns)
+ return ERR_PTR(-EPERM);
+
+ return vfs_kern_mount(type, MS_SUBMOUNT, name, data);
+}
+EXPORT_SYMBOL_GPL(vfs_submount);
+
static struct mount *clone_mnt(struct mount *old, struct dentry *root,
int flag)
{
@@ -2794,7 +2809,7 @@ long do_mount(const char *dev_name, const char __user *dir_name,
flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
- MS_STRICTATIME | MS_NOREMOTELOCK);
+ MS_STRICTATIME | MS_NOREMOTELOCK | MS_SUBMOUNT);
if (flags & MS_REMOUNT)
retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
