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

[Jeff Xu <jeffxu@chromium.org>]

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".
>
> > For example:
> > Consider the case below in src/third_party/kernel/v6.6/fs/proc/task_mmu.c,
>
> third_party?
>
Sorry, I pasted the code path from ChromeOS code base, it is actually
in the kernel itself.

fs/proc/task_mmu.c

> >
> > #ifdef CONFIG_64BIT
> > [ilog2(VM_SEALED)] = "sl",
> > #endif
> >
> > Redefining VM_SEALED  to VM_NONE for 32 bit won't detect the problem
> > in case that  "#ifdef CONFIG_64BIT" line is missing.
>
> I don't think it is reasonable to insist on doing things differently in
> the kernel because you have external tests that would need updating.
> These things can change independently, so I don't think this is a valid
> argument.
>
> If these are upstream tests, and we need these tests to work then they
> can be fixed.
>
As above, this is actually kernel code, not test.

-Jeff

> >
> > Please note, this has been like this since the first version of
> > mseal() RFC patch, and I prefer to keep it this way.
>
> Thanks,
> Liam