[RFD] A mount api that notices previous mounts

[Casey Schaufler]

On 1/29/2019 1:44 PM, Eric W. Biederman wrote:
> All,
>
> With the existing mount API it is possible to mount a filesystem
> like:
>
> mount -t ext4 /dev/sda1 -o user_xattr /some/path
> mount -t ext4 /dev/sda1 -o nouser_xattr /some/other/path
>
> And have both mount commands succeed and have two mounts of the same
> filesystem.  If the mounter is not attentive or the first mount is added
> earlier it may not be immediately noticed that a mount option that is
> needed for the correct operation or the security of the system is lost.
>
> We have seen this failure mode with both devpts and proc.  So it is not
> theoretical, and it has resulted in CVEs.
>
> In some cases the existing mount API (such as a conflict between ro and
> rw) handles this by returning -EBUSY.  So we may be able to correct this
> in the existing mount API.  But it is always very tricky to to get
> adequate testing for a change like that to avoid regressions, so I am
> proposing we change this in the new mount api.
>
> This has been brought up before and I have been told it is technically
> infeasible to make this work.  To counter that I have sat down and
> implemented it.
>
> The basic idea is:
>  - get a handle to a filesystem
>    (passing enough options to uniquely identify the super block).
>    Also capture enough state in the file handle to let you know if
>    the file system has it's mount options changed between system calls.
>    (essentially this is just the fs code that calls sget)
>
>  - If the super block has not been configured allow setting the file
>    systems options.
>
>  - If the super block has already been configured require reading the
>    file systems mount options before setting/updating the file systems
>    mount options.
>
> To complement that I have functionality that:
>  - Allows reading a file systems current mount options.
>  - Allows reading the mount options that are needed to get a handle to
>    the filesystem.  For most filesystems it is just the block device
>    name.  For nfs is is essentially all mount options.  For btrfs
>    it is the block device name, and the "devices=" mount option for
>    secondary block device names.

Are you taking the LSM specific mount options into account?

>
> Please find below a tree where all of this is implemented and working.
> Not all file systems have been converted but the most of the unconverted
> ones are just a couple minutes work as I have converted all of the file
> system mount helper routines.
>
> Also please find below an example mount program that takes the same set
> of mount options as mount(8) today and mounts filesystems with the
> proposed new mount api.
>  - Without having any filesystem mount knowledge it sucessfully figures
>
> out which system calls different mount options needs to be applied
>    to.
>
> - Without having any filesystem specific knowledge in most cases it
>    can detect if a mount option you specify is already specified to an
>    existing mount or not.  For duplicates it can detect it ignores them.
>    For the other cases it fails the mount as it thinks the mount options
>    are different.
>
>  - Which demonstrates it safe to put the detection and remediation of
>    multiple mount commands resolving to the same filesystem in user
>    space.
>
> I really don't care whose code gets used as long as it works.  I do very
> much care that we don't add a new mount api that has the confusion flaws
> of the existing mount api.
>
> Along the way I have also detected a lot of room for improvement on the
> mount path for filesystems.  Those cleanup patches are in my tree below
> and will be extracting them and sending them along shortly.
>
> Comments?
>
> git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git no-losing-mount-options-proof-of-concept
>
>
>
> Eric
>