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Date:   Sun, 26 Nov 2017 08:10:31 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        richard.fellner@...dent.tugraz.at, moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page
 tables (core patch)

On Wed, Nov 22, 2017 at 8:07 PM, Andy Lutomirski <luto@...nel.org> wrote:
> On Wed, Nov 22, 2017 at 4:34 PM, Dave Hansen
> <dave.hansen@...ux.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?
>

Quick ping: did this get lost?

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