Re: [PATCH v2 14/39] commoncap: handle idmapped mounts

[Christian Brauner <christian.brauner@ubuntu.com>]

On Sun, Nov 22, 2020 at 04:18:55PM -0500, Paul Moore wrote:
> On Sun, Nov 15, 2020 at 5:39 AM Christian Brauner
> <christian.brauner@ubuntu.com> wrote:
> > When interacting with user namespace and non-user namespace aware
> > filesystem capabilities the vfs will perform various security checks to
> > determine whether or not the filesystem capabilities can be used by the
> > caller (e.g. during exec), or even whether they need to be removed. The
> > main infrastructure for this resides in the capability codepaths but they
> > are called through the LSM security infrastructure even though they are not
> > technically an LSM or optional. This extends the existing security hooks
> > security_inode_removexattr(), security_inode_killpriv(),
> > security_inode_getsecurity() to pass down the mount's user namespace and
> > makes them aware of idmapped mounts.
> > In order to actually get filesystem capabilities from disk the capability
> > infrastructure exposes the get_vfs_caps_from_disk() helper. For user
> > namespace aware filesystem capabilities a root uid is stored alongside the
> > capabilities.
> > In order to determine whether the caller can make use of the filesystem
> > capability or whether it needs to be ignored it is translated according to
> > the superblock's user namespace. If it can be translated to uid 0 according
> > to that id mapping the caller can use the filesystem capabilities stored on
> > disk. If we are accessing the inode that holds the filesystem capabilities
> > through an idmapped mount we need to map the root uid according to the
> > mount's user namespace.
> > Afterwards the checks are identical to non-idmapped mounts. Reading
> > filesystem caps from disk enforces that the root uid associated with the
> > filesystem capability must have a mapping in the superblock's user
> > namespace and that the caller is either in the same user namespace or is a
> > descendant of the superblock's user namespace. For filesystems that are
> > mountable inside user namespace the container can just mount the filesystem
> > and won't usually need to idmap it. If it does create an idmapped mount it
> > can mark it with a user namespace it has created and which is therefore a
> > descendant of the s_user_ns. For filesystems that are not mountable inside
> > user namespaces the descendant rule is trivially true because the s_user_ns
> > will be the initial user namespace.
> >
> > If the initial user namespace is passed all operations are a nop so
> > non-idmapped mounts will not see a change in behavior and will also not see
> > any performance impact.
> >
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: David Howells <dhowells@redhat.com>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: linux-fsdevel@vger.kernel.org
> > Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> 
> ...
> 
> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > index 8dba8f0983b5..ddb9213a3e81 100644
> > --- a/kernel/auditsc.c
> > +++ b/kernel/auditsc.c
> > @@ -1944,7 +1944,7 @@ static inline int audit_copy_fcaps(struct audit_names *name,
> >         if (!dentry)
> >                 return 0;
> >
> > -       rc = get_vfs_caps_from_disk(dentry, &caps);
> > +       rc = get_vfs_caps_from_disk(&init_user_ns, dentry, &caps);
> >         if (rc)
> >                 return rc;
> >
> > @@ -2495,7 +2495,8 @@ int __audit_log_bprm_fcaps(struct linux_binprm *bprm,
> >         ax->d.next = context->aux;
> >         context->aux = (void *)ax;
> >
> > -       get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
> > +       get_vfs_caps_from_disk(mnt_user_ns(bprm->file->f_path.mnt),
> > +                              bprm->file->f_path.dentry, &vcaps);
> 
> As audit currently records information in the context of the
> initial/host namespace I'm guessing we don't want the mnt_user_ns()
> call above; it seems like &init_user_ns would be the right choice
> (similar to audit_copy_fcaps()), yes?

Ok, sounds good. It also makes the patchset simpler.
Note that I'm currently not on the audit mailing list so this is likely
not going to show up there.

(Fwiw, I responded to you in your other mail too.)

Christian