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

[Serge Hallyn]

Quoting Amir Goldstein (amir@xxxxxxxxxxx):
> On Tue, Jul 28, 2015 at 11:40 PM, Seth Forshee
> <seth.forshee@xxxxxxxxxxxxx> 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.
> 
> Seth,
> 
> There were 2 main concerns discussed in this thread:
> 1. trusting LSM labels outside the namespace
> 2. trusting the content of the image file/loopdev
> 
> While your approach addresses the first concern, I suspect it may be placing
> an obstacle in a way for resolving the second concern.
> 
> A viable security policy to mitigate the second concern could be:
> - Allow only trusted programs (e.g. mkfs, fsck) to write to 'Loopback' images
> - Allow mount only of 'Loopback' images
> 
> This should allow the system as a whole to trust unprivileged mounts based on
> the trust of the entities that had raw access the the fs layout.

Just to be sure I understand right, you're looking for a way to let
the host admin trust that the kernel's superblock parsers aren't being
fed trash or an exploit?

> Alas, if you choose to propagate the backing dev label to contained files,
> they would all share the designated 'Loopback' label and render the policy above
> useless.
> 
> Any thoughts on how to reconcile this conflict?
> 
> Amir.
> 
> 
> > > >  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.
> >
> > ---
> >
> > diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> > index a143328f75eb..8e631a66b03c 100644
> > --- a/security/smack/smack_lsm.c
> > +++ b/security/smack/smack_lsm.c
> > @@ -662,6 +662,8 @@ static int smack_sb_kern_mount(struct super_block *sb, int flags, void *data)
> >                 skp = smk_of_current();
> >                 sp->smk_root = skp;
> >                 sp->smk_default = skp;
> > +               if (sb_in_userns(sb))
> > +                       transmute = 1;
> >         }
> >         /*
> >          * Initialize the root inode.
> > @@ -1023,6 +1025,12 @@ static int smack_inode_permission(struct inode *inode, int mask)
> >         if (mask == 0)
> >                 return 0;
> >
> > +       if (sb_in_userns(inode->i_sb)) {
> > +               struct superblock_smack *sbsp = inode->i_sb->s_security;
> > +               if (smk_of_inode(inode) != sbsp->smk_root)
> > +                       return -EACCES;
> > +       }
> > +
> >         /* May be droppable after audit */
> >         if (no_block)
> >                 return -ECHILD;
> > @@ -3220,14 +3228,16 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
> >                         if (rc >= 0)
> >                                 transflag = SMK_INODE_TRANSMUTE;
> >                 }
> > -               /*
> > -                * Don't let the exec or mmap label be "*" or "@".
> > -                */
> > -               skp = smk_fetch(XATTR_NAME_SMACKEXEC, inode, dp);
> > -               if (IS_ERR(skp) || skp == &smack_known_star ||
> > -                   skp == &smack_known_web)
> > -                       skp = NULL;
> > -               isp->smk_task = skp;
> > +               if (!sb_in_userns(inode->i_sb)) {
> > +                       /*
> > +                        * Don't let the exec or mmap label be "*" or "@".
> > +                        */
> > +                       skp = smk_fetch(XATTR_NAME_SMACKEXEC, inode, dp);
> > +                       if (IS_ERR(skp) || skp == &smack_known_star ||
> > +                           skp == &smack_known_web)
> > +                               skp = NULL;
> > +                       isp->smk_task = skp;
> > +               }
> >
> >                 skp = smk_fetch(XATTR_NAME_SMACKMMAP, inode, dp);
> >                 if (IS_ERR(skp) || skp == &smack_known_star ||
> > --
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