Re: [RFC PATCH 0/3] permit write-sealed memfd read-only shared mappings

[Lorenzo Stoakes]

On Mon, Apr 24, 2023 at 02:19:36PM +0200, Jan Kara wrote:
> On Fri 21-04-23 22:23:12, Lorenzo Stoakes wrote:
> > On Fri, Apr 21, 2023 at 11:01:26AM +0200, Jan Kara wrote:
> > > Hi!
> > >
> > > On Mon 03-04-23 23:28:29, Lorenzo Stoakes wrote:
> > > > This patch series is in two parts:-
> > > >
> > > > 1. Currently there are a number of places in the kernel where we assume
> > > >    VM_SHARED implies that a mapping is writable. Let's be slightly less
> > > >    strict and relax this restriction in the case that VM_MAYWRITE is not
> > > >    set.
> > > >
> > > >    This should have no noticeable impact as the lack of VM_MAYWRITE implies
> > > >    that the mapping can not be made writable via mprotect() or any other
> > > >    means.
> > > >
> > > > 2. Align the behaviour of F_SEAL_WRITE and F_SEAL_FUTURE_WRITE on mmap().
> > > >    The latter already clears the VM_MAYWRITE flag for a sealed read-only
> > > >    mapping, we simply extend this to F_SEAL_WRITE too.
> > > >
> > > >    For this to have effect, we must also invoke call_mmap() before
> > > >    mapping_map_writable().
> > > >
> > > > As this is quite a fundamental change on the assumptions around VM_SHARED
> > > > and since this causes a visible change to userland (in permitting read-only
> > > > shared mappings on F_SEAL_WRITE mappings), I am putting forward as an RFC
> > > > to see if there is anything terribly wrong with it.
> > >
> > > So what I miss in this series is what the motivation is. Is it that you need
> > > to map F_SEAL_WRITE read-only? Why?
> > >
> >
> > This originated from the discussion in [1], which refers to the bug
> > reported in [2]. Essentially the user is write-sealing a memfd then trying
> > to mmap it read-only, but receives an -EPERM error.
> >
> > F_SEAL_FUTURE_WRITE _does_ explicitly permit this but F_SEAL_WRITE does not.
> >
> > The fcntl() man page states:
> >
> >     Furthermore, trying to create new shared, writable memory-mappings via
> >     mmap(2) will also fail with EPERM.
> >
> > So the kernel does not behave as the documentation states.
> >
> > I took the user-supplied repro and slightly modified it, enclosed
> > below. After this patch series, this code works correctly.
> >
> > I think there's definitely a case for the VM_MAYWRITE part of this patch
> > series even if the memfd bits are not considered useful, as we do seem to
> > make the implicit assumption that MAP_SHARED == writable even if
> > !VM_MAYWRITE which seems odd.
>
> Thanks for the explanation! Could you please include this information in
> the cover letter (perhaps in a form of a short note and reference to the
> mailing list) for future reference? Thanks!
>
> 								Honza
>

Sure, apologies for not being clear about that :)

I may respin this as a non-RFC (with updated description of course) as its
received very little attention as an RFC and I don't think it's so
insane/huge a concept as to warrant remaining one.

> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR