Calls to vfs_setlease() from NFSD code cause unnecessary CAP_LEASE security checks

[Ondrej Mosnacek]

On Fri, Feb 2, 2024 at 5:31 PM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>
> On Fri, Feb 2, 2024 at 5:08 PM Jeff Layton <jlayton at kernel.org> wrote:
> >
> > On Fri, 2024-02-02 at 16:31 +0100, Ondrej Mosnacek wrote:
> > > Hello,
> > >
> > > In [1] a user reports seeing SELinux denials from NFSD when it writes
> > > into /proc/fs/nfsd/threads with the following kernel backtrace:
> > >  => trace_event_raw_event_selinux_audited
> > >  => avc_audit_post_callback
> > >  => common_lsm_audit
> > >  => slow_avc_audit
> > >  => cred_has_capability.isra.0
> > >  => security_capable
> > >  => capable
> > >  => generic_setlease
> > >  => destroy_unhashed_deleg
> > >  => __destroy_client
> > >  => nfs4_state_shutdown_net
> > >  => nfsd_shutdown_net
> > >  => nfsd_last_thread
> > >  => nfsd_svc
> > >  => write_threads
> > >  => nfsctl_transaction_write
> > >  => vfs_write
> > >  => ksys_write
> > >  => do_syscall_64
> > >  => entry_SYSCALL_64_after_hwframe
> > >
> > > It seems to me that the security checks in generic_setlease() should
> > > be skipped (at least) when called through this codepath, since the
> > > userspace process merely writes into /proc/fs/nfsd/threads and it's
> > > just the kernel's internal code that releases the lease as a side
> > > effect. For example, for vfs_write() there is kernel_write(), which
> > > provides a no-security-check equivalent. Should there be something
> > > similar for vfs_setlease() that could be utilized for this purpose?
> > >
> > > [1] https://bugzilla.redhat.com/show_bug.cgi?id=2248830
> > >
> >
> > Thanks for the bug report!
> >
> > Am I correct that we only want to do this check when someone from
> > userland tries to set a lease via fcntl? The rest of the callers are all
> > in-kernel callers and I don't think we need to check for any of them. It
> > may be simpler to just push this check into the appropriate callers of
> > generic_setlease instead.
> >
> > Hmm now that I look too...it looks like we aren't checking CAP_LEASE on
> > filesystems that have their own ->setlease operation. I'll have a look
> > at that soon too.
>
> I did briefly check this while analyzing the issue and all of the
> setlease fops implementations seemed to be either simple_nosetlease()
> or some wrappers around generic_setlease(), which should both be OK.
> But it can't hurt to double-check :)

To close the loop here - there is now a fix from Jeff in linux-next:
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=7b8001013d720c232ad9ae7aae0ef0e7c281c6d4

Thank you, Jeff, for taking care of it!

-- 
Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.