Re: [PATCH v6 1/7] mseal, system mappings: kernel config and header change

Liam R. Howlett
2025-02-24T20:13:39Z

* Jeff Xu <jeffxu@chromium.org> [250224 14:42]:
> On Mon, Feb 24, 2025 at 11:25 AM Kees Cook <kees@kernel.org> wrote:
> >
> > On Mon, Feb 24, 2025 at 11:10:22AM -0800, Jeff Xu wrote:
> > > On Mon, Feb 24, 2025 at 11:03 AM Liam R. Howlett
> > > <Liam.Howlett@oracle.com> wrote:
> > > >
> > > > * Jeff Xu <jeffxu@chromium.org> [250224 13:44]:
> > > > > On Mon, Feb 24, 2025 at 10:21 AM Dave Hansen <dave.hansen@intel.com> wrote:
> > > > > >
> > > > > > On 2/24/25 09:45, jeffxu@chromium.org wrote:
> > > > > > > +/*
> > > > > > > + * mseal of userspace process's system mappings.
> > > > > > > + */
> > > > > > > +#ifdef CONFIG_MSEAL_SYSTEM_MAPPINGS
> > > > > > > +#define MSEAL_SYSTEM_MAPPINGS_VM_FLAG VM_SEALED
> > > > > > > +#else
> > > > > > > +#define MSEAL_SYSTEM_MAPPINGS_VM_FLAG VM_NONE
> > > > > > > +#endif
> > > > > >
> > > > > > This ends up looking pretty wonky in practice:
> > > > > >
> > > > > > > + vm_flags = VM_READ|VM_MAYREAD|VM_IO|VM_DONTDUMP|VM_PFNMAP;
> > > > > > > + vm_flags |= MSEAL_SYSTEM_MAPPINGS_VM_FLAG;
> > > > > >
> > > > > > because MSEAL_SYSTEM_MAPPINGS_VM_FLAG is so much different from the
> > > > > > other ones.
> > > > > >
> > > > > > Would it really hurt to have
> > > > > >
> > > > > > #ifdef CONFIG_64BIT
> > > > > > /* VM is sealed, in vm_flags */
> > > > > > #define VM_SEALED _BITUL(63)
> > > > > > +#else
> > > > > > +#define VM_SEALED VM_NONE
> > > > > > #endif
> > > > > >
> > > > > > ?
> > > > > >
> > > > > VM_SEALED isn't defined in 32-bit systems, and mseal.c isn't part of
> > > > > the build. This is intentional. Any 32-bit code trying to use the
> > > > > sealing function or the VM_SEALED flag will immediately fail
> > > > > compilation. This makes it easier to identify incorrect usage.
> > > > >
> > > >
> > > > The reason that two #defines are needed is because you can have mseal
> > > > enabled while not sealing system mappings, so for this to be clean we
> > > > need two defines.
> > > >
> > > > However MSEAL_SYSTEM_MAPPINGS_VM_FLAG, is _way_ too long, in my opinion.
> > > > Keeping with "VM_SEALED" I'd suggest "VM_SYSTEM_SEALED".
> > > >
> > > How about MSEAL_SYSTME_MAPPINGS as Kees suggested ?
> > >
> > > The VM_SYSTEM_SEALED doesn't have the MSEAL key or the MAPPING, so it
> > > might take longer for the new reader to understand what it is.
> >
> > I think to address Dave's concern, it should start with "VM_". We use
> > "SEAL" already with VM_SEALED, so the "M" in "MSEAL" may be redundant,
> > and to clarify the system mapping, how avout VM_SEAL_SYSMAP ? That
> > capture's, hopefully, Dave, Liam, and your thoughts about the naming?
> >
> Liam had a comment in the previous version, asking everything related
> with mseal() to have the MSEAL keyword, that is why KCONFIG change has
> MSEAL.
>
> How about VM_MSEAL_SYSMAP ?

I don't recall if it was this set or previous ones, but seal is becoming
overloaded and having mseal would have been good for this one.

VM_MSEAL does gain more greping, but since we have VM_SEALED,
VM_SEAL_SYSMAP is going to show up on VM_SEAL grep, but not VM_SEALED
greps. Maybe VM_SEALED_SYSMAP would be better for searching.

Thanks,
Liam