Re: [PATCH 0/7] Initial support for user namespace owned mounts

Eric W. Biederman
2015-07-30T17:40:39Z

Seth Forshee <seth.forshee@canonical.com> writes:

> On Thu, Jul 30, 2015 at 12:05:27PM -0500, Eric W. Biederman wrote:
>> Casey Schaufler <casey@schaufler-ca.com> writes:
>>
>> > On 7/28/2015 1:40 PM, Seth Forshee wrote:
>> >> On Wed, Jul 22, 2015 at 05:05:17PM -0700, Casey Schaufler wrote:
>> >>>> This is what I currently think you want for user ns mounts:
>> >>>>
>> >>>> 1. smk_root and smk_default are assigned the label of the backing
>> >>>> device.
>> >>>> 2. s_root is assigned the transmute property.
>> >>>> 3. For existing files:
>> >>>> a. Files with the same label as the backing device are accessible.
>> >>>> b. Files with any other label are not accessible.
>> >>> That's right. Accept correct data, reject anything that's not right.
>> >>>
>> >>>> If this is right, there are a couple lingering questions in my mind.
>> >>>>
>> >>>> First, what happens with files created in directories with the same
>> >>>> label as the backing device but without the transmute property set? The
>> >>>> inode for the new file will initially be labeled with smk_of_current(),
>> >>>> but then during d_instantiate it will get smk_default and thus end up
>> >>>> with the label we want. So that seems okay.
>> >>> Yes.
>> >>>
>> >>>> The second is whether files with the SMACK64EXEC attribute is still a
>> >>>> problem. It seems it is, for files with the same label as the backing
>> >>>> store at least. I think we can simply skip the code that reads out this
>> >>>> xattr and sets smk_task for user ns mounts, or else skip assigning the
>> >>>> label to the new task in bprm_set_creds. The latter seems more
>> >>>> consistent with the approach you've suggested for dealing with labels
>> >>>> from disk.
>> >>> Yes, I think that skipping the smk_fetch(XATTR_NAME_SMACKEXEC, ...) in
>> >>> smack_d_instantiate for unprivileged mounts would do the trick.
>> >>>
>> >>>> So I guess all of that seems okay, though perhaps a bit restrictive
>> >>>> given that the user who mounted the filesystem already has full access
>> >>>> to the backing store.
>> >>> In truth, there is no reason to expect that the "user" who did the
>> >>> mount will ever have a Smack label that differs from the label of
>> >>> the backing store. If what we've got here seems restrictive, it's
>> >>> because you've got access from someone other than the "user".
>> >>>
>> >>>> Please let me know whether or not this matches up with what you are
>> >>>> thinking, then I can procede with the implementation.
>> >>> My current mindset is that, if you're going to allow unprivileged
>> >>> mounts of user defined backing stores, this is as safe as we can
>> >>> make it.
>> >> All right, I've got a patch which I think does this, and I've managed to
>> >> do some testing to confirm that it behaves like I expect. How does this
>> >> look?
>> >>
>> >> What's missing is getting the label from the block device inode; as
>> >> Stephen discovered the inode that I thought we could get the label from
>> >> turned out to be the wrong one. Afaict we would need a new hook in order
>> >> to do that, so for now I'm using the label of the proccess calling
>> >> mount.
>> >
>> > That will be OK if the mount processing checks for write access to
>> > the backing store. I haven't looked to see if it does. If it doesn't
>> > the problems should be pretty obvious.
>>
>>
>> do_new_mount
>> vfs_kern_mount
>> mount_fs
>> ...
>> mount_bdev
>> blkdev_get_by_path(...,FMODE_READ| FMODE_WRITE | FMODE_EXCL,...)
>> lookup_bdev
>> kern_path
>> filename_lookup
>> path_lookupat
>> lookup_last
>> walk_component
>> blkdev_get(...,mode,...)
>> __blkdev_get(...,mode,...)
>> devcgroup_inode_permission(bdev->bd_inode, perm)
>>
>> *scratches my head*
>>
>> It looks like we don't actually check the permissions on the block
>> device. Tomoyo has a hack for it. nfsd does something. There is
>> devcgroup silliness.
>>
>> But overall it looks like we depend on capable(CAP_SYS_ADMIN).
>>
>> Seth I do believe we have found another area of the vfs we will need to
>> short up before allowing unprivileged mounts of block device based
>> filesystems.
>>
>> It looks like there are enough hacks someone with a clue coming through
>> and making the code make more sense seems like a good idea anyway.
>
> Yep, I just came to the same conclusion myself, and I also verified the
> behavior emperically. That's definitely a problem. I'll get to work on
> fixing that.

At a quick glance it looks like lookup_bdev, and most of it's callers
need to be modified to do potentially do the additional permission
checking.

I expect we could move the devcgroup checks into whatever new checks we
wind up adding.

Fun, fun fun.

Eric