Re: [PATCH v6] vfs, security: Fix automount superblock LSM init problem, preventing NFS sb sharing

From: David Howells
Date: Fri Aug 04 2023 - 11:17:15 EST

Paul Moore <paul@xxxxxxxxxxxxxx> wrote:

> I generally dislike core kernel code which makes LSM calls conditional
> on some kernel state maintained outside the LSM. Sometimes it has to
> be done as there is no other good options, but I would like us to try
> and avoid it if possible. The commit description mentioned that this
> was put here to avoid a SELinux complaint, can you provide an example
> of the complain? Does it complain about a double/invalid mount, e.g.
> "SELinux: mount invalid. Same superblock, different security ..."?
> I'd like to understand why the sb_set_mnt_opts() call fails when it
> comes after the fs_context_init() call. I'm particulary curious to
> know if the failure is due to conflicting SELinux state in the
> fs_context, or if it is simply an issue of sb_set_mnt_opts() not
> properly handling existing values. Perhaps I'm being overly naive,
> but I'm hopeful that we can address both of these within the SELinux
> code itself.
> In a worst case situation, we could always implement a flag *inside*
> the SELinux code, similar to what has been done with 'lsm_set' here.

IIRC, the issue is when you make a mount with an explicit context= setting and
make another mount from some way down the export tree that doesn't have an
explicit setting, e.g.:

mount carina:/ /mnt -o context=system_u:object_r:root_t:s0
mount carina:/nfs/scratch /mnt2

and then cause an automount to walk from one to the other:

stat /mnt/nfs/scratch/foo

For reference, my server has:


and if I look in /proc/fs/nfsfs/volumes, I can see the individual superblocks:

v4 c0a80601 801 0:51 0:0 no
v4 c0a80601 801 0:56 3:0 no
v4 c0a80601 801 0:52 1:0 no
v4 c0a80601 801 0:55 3:0 no

As you can see, there are two referring to the same 'volume'.

Without the "fc->lsm_set=true" bit, you get an error something like:

SELinux: mount invalid. Same superblock, different security settings for (dev 0:56, type nfs4)

One important question is how should sharing of a mount with unspecified
context be handled when we try to unify it with a mount that has an explicit