On Fri, Feb 2, 2024 at 5:08 PM Jeff Layton [off-list ref] wrote:

On Fri, 2024-02-02 at 16:31 +0100, Ondrej Mosnacek wrote:

quoted

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 :)

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