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/debugfs/inode.c b/fs/debugfs/inode.c
index f17fcf8..1e30f74 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -187,9 +187,9 @@ static const struct super_operations debugfs_super_operations = {
static struct vfsmount *debugfs_automount(struct path *path)
{
- struct vfsmount *(*f)(void *);
- f = (struct vfsmount *(*)(void *))path->dentry->d_fsdata;
- return f(d_inode(path->dentry)->i_private);
+ debugfs_automount_t f;
+ f = (debugfs_automount_t)path->dentry->d_fsdata;
+ return f(path->dentry, d_inode(path->dentry)->i_private);
}
static const struct dentry_operations debugfs_dops = {
@@ -504,7 +504,7 @@ EXPORT_SYMBOL_GPL(debugfs_create_dir);
*/
struct dentry *debugfs_create_automount(const char *name,
struct dentry *parent,
- struct vfsmount *(*f)(void *),
+ debugfs_automount_t f,
void *data)
{
struct dentry *dentry = start_creating(name, parent);