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

[Jan Kara]

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

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