Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page tables (core patch)

[Andy Lutomirski <luto@kernel.org>]

On Wed, Nov 22, 2017 at 4:34 PM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
>
> These actions when dealing with a user address *and* the
> PGD has _PAGE_USER set.  That way, in-kernel users of low addresses
> typically used by userspace are not accidentally poisoned.

This seems sane.

> +/*
> + * Take a PGD location (pgdp) and a pgd value that needs
> + * to be set there.  Populates the shadow and returns
> + * the resulting PGD that must be set in the kernel copy
> + * of the page tables.
> + */
> +static inline pgd_t kaiser_set_shadow_pgd(pgd_t *pgdp, pgd_t pgd)
> +{
> +#ifdef CONFIG_KAISER
> +       if (pgd_userspace_access(pgd)) {
> +               if (pgdp_maps_userspace(pgdp)) {
> +                       /*
> +                        * The user/shadow page tables get the full
> +                        * PGD, accessible from userspace:
> +                        */
> +                       kernel_to_shadow_pgdp(pgdp)->pgd = pgd.pgd;
> +                       /*
> +                        * For the copy of the pgd that the kernel
> +                        * uses, make it unusable to userspace.  This
> +                        * ensures if we get out to userspace with the
> +                        * wrong CR3 value, userspace will crash
> +                        * instead of running.
> +                        */
> +                       pgd.pgd |= _PAGE_NX;
> +               }
> +       } else if (pgd_userspace_access(*pgdp)) {
> +               /*
> +                * We are clearing a _PAGE_USER PGD for which we
> +                * presumably populated the shadow.  We must now
> +                * clear the shadow PGD entry.
> +                */
> +               if (pgdp_maps_userspace(pgdp)) {
> +                       kernel_to_shadow_pgdp(pgdp)->pgd = pgd.pgd;
> +               } else {
> +                       /*
> +                        * Attempted to clear a _PAGE_USER PGD which
> +                        * is in the kernel porttion of the address
> +                        * space.  PGDs are pre-populated and we
> +                        * never clear them.
> +                        */
> +                       WARN_ON_ONCE(1);
> +               }
> +       } else {
> +               /*
> +                * _PAGE_USER was not set in either the PGD being set
> +                * or cleared.  All kernel PGDs should be
> +                * pre-populated so this should never happen after
> +                * boot.
> +                */
> +       }
> +#endif
> +       /* return the copy of the PGD we want the kernel to use: */
> +       return pgd;
> +}
> +

The more I read this code, the more I dislike "shadow".  Shadow
pagetables mean something specific in the virtualization world and,
more importantly, the word "shadow" fails to convey *which* table it
is.  Unless I'm extra confused, mm->pgd points to the kernelmode
tables.  So can we replace the word "shadow" with "usermode"?  That
will also make the entry stuff way clearer.  (Or I have it backwards,
in which case "kernelmode" would be the right choice.)  And rename the
argument.

That confusion aside, I'm trying to wrap my head around this.  I think
the description above makes sense, but I'm struggling to grok the code
and how it matches the description.  May I suggest an alternative
implementation?  (Apologies for epic whitespace damage.)

/*
 * Install an entry into the usermode pgd.  pgdp points to the kernelmode
 * entry whose usermode counterpart we're supposed to set.  pgd is the
 * desired entry.  Returns pgd, possibly modified if the actual entry installed
 * into the kernelmode needs different mode bits.
 */
static inline pgd_t kaiser_set_usermode_pgd(pgd_t *pgdp, pgd_t pgd) {
  VM_BUG_ON(pgdp points to a usermode table);

  if (pgdp_maps_userspace(pgdp)) {
    /* Install the pgd as requested into the usermode tables. */
    kernelmode_to_usermode_pgdp(pgdp)->pgd = pgd.pgd;

    if (pgd_val(pgd) & _PAGE_USER) {
      /*
       * This is a normal user pgd -- the kernelmode mapping should have NX
       * set to prevent erroneous usermode execution with the kernel tables.
       */
      return __pgd(pgd_val(pgd) | _PAGE_NX;
    } else {
      /* This is a weird mapping, e.g. EFI.  Map it straight through. */
      return pgd;
    }
  } else {
    /*
     * We can get here due to vmalloc, a vmalloc fault, memory
hot-add, or initial setup
     * of kernelmode page tables.  Regardless of which particular code
path we're in,
     * these mappings should not be automatically propagated to the
usermode tables.
     */
    return pgd;
  }
}

As a side benefit, this shouldn't have magical interactions with the
vsyscall page any more.

Are there cases that this would get wrong?

--Andy