• Eric W. Biederman's avatar
    fs: Better permission checking for submounts · 93faccbb
    Eric W. Biederman authored
    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: 069d5ac9 ("autofs:  Fix automounts by using current_real_cred()->uid")
    Fixes: aeaa4a79 ("fs: Call d_automount with the filesystems creds")
    Reviewed-by: default avatarTrond Myklebust <trond.myklebust@primarydata.com>
    Reviewed-by: default avatarSeth Forshee <seth.forshee@canonical.com>
    Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
    93faccbb
mount.h 3.3 KB