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

Andy Lutomirski
2017-11-23T04:08:04Z

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