Re: [PATCH 11/39] selinux: reject writable opens of policy file, drop mmap shared/write check

[Stephen Smalley]

On Fri, Sep 11, 2026 at 6:16 AM Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx> wrote:
>
> On Thu, Sep 10, 2026 at 02:11:18PM -0400, Stephen Smalley wrote:
> > On Tue, Sep 8, 2026 at 4:07 PM Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx> wrote:
> > >
> > > The policy file has no write method and is exposed read-only (S_IRUGO in
> > > selinux_files[]), yet sel_open_policy() performs no open mode check, so a
> > > CAP_DAC_OVERRIDE caller can open it O_RDWR. Reject FMODE_WRITE at open, as
> > > kernfs does.
> > >
> > > The file can then never be mapped with FMODE_WRITE, so do_mmap() always
> > > clears VM_MAYWRITE and VM_SHARED for MAP_SHARED mappings and the VM_SHARED
> > > check in sel_mmap_policy() cannot be reached. Remove it.
> > >
> > > This also stops sel_mmap_policy() clearing VM_MAYWRITE on a mapping that is
> > > neither a PFN map nor a mixed map, ahead of the core enforcing that only
> > > such mappings may do so.
> > >
> > > Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> >
> > We should likely do something similar for the other sel_open_*() and
> > sel_mmap_*() functions too.
> > In particular, truncation of the status file can bring down systemd
> > since it is mapped by libselinux.
>
> Yeah I think there's definitely more that can be done there.
>
> Though I think obviously that kind of stuff is better as a follow up as this
> patch is focused on eliminating a case where VMA_MAYWRITE_BIT is cleared
> incorrectly.

Agreed, posted one for the status handlers at:
https://lore.kernel.org/selinux/20260911145840.19039-2-stephen.smalley.work@xxxxxxxxx/