f_modown and LSM inconsistency (was [PATCH v2 1/4] Landlock: Add signal control)

[Jann Horn]

On Fri, Aug 9, 2024 at 3:18 PM Mickaël Salaün <mic at digikod.net> wrote:
> Talking about f_modown() and security_file_set_fowner(), it looks like
> there are some issues:
>
> On Fri, Aug 09, 2024 at 02:44:06PM +0200, Jann Horn wrote:
> > On Fri, Aug 9, 2024 at 12:59 PM Mickaël Salaün <mic at digikod.net> wrote:
>
> [...]
>
> > > BTW, I don't understand why neither SELinux nor Smack use (explicit)
> > > atomic operations nor lock.
> >
> > Yeah, I think they're sloppy and kinda wrong - but it sorta works in
> > practice mostly because they don't have to do any refcounting around
> > this?
> >
> > > And it looks weird that
> > > security_file_set_fowner() isn't called by f_modown() with the same
> > > locking to avoid races.
> >
> > True. I imagine maybe the thought behind this design could have been
> > that LSMs should have their own locking, and that calling an LSM hook
> > with IRQs off is a little weird? But the way the LSMs actually use the
> > hook now, it might make sense to call the LSM with the lock held and
> > IRQs off...
> >
>
> Would it be OK (for VFS, SELinux, and Smack maintainers) to move the
> security_file_set_fowner() call into f_modown(), especially where
> UID/EUID are populated.  That would only call security_file_set_fowner()
> when the fown is actually set, which I think could also fix a bug for
> SELinux and Smack.
>
> Could we replace the uid and euid fields with a pointer to the current
> credentials?  This would enables LSMs to not copy the same kind of
> credential informations and save some memory, simplify credential
> management, and improve consistency.

To clarify: These two paragraphs are supposed to be two alternative
options, right? One option is to call security_file_set_fowner() with
the lock held, the other option is to completely rip out the
security_file_set_fowner() hook and instead let the VFS provide LSMs
with the creds they need for the file_send_sigiotask hook?