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

[Jeff Xu <jeffxu@chromium.org>]

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.

For example:
Consider the case below in src/third_party/kernel/v6.6/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.

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

Thanks
-Jeff