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

Andrea Arcangeli
2018-01-05T19:55:40Z

On Fri, Jan 05, 2018 at 08:17:17PM +0100, Jiri Kosina wrote:
> On Fri, 5 Jan 2018, Dave Hansen wrote:
>
> > > --- a/arch/x86/platform/efi/efi_64.c
> > > +++ b/arch/x86/platform/efi/efi_64.c
> > > @@ -95,6 +95,12 @@ pgd_t * __init efi_call_phys_prolog(void
> > > save_pgd[pgd] = *pgd_offset_k(pgd * PGDIR_SIZE);
> > > vaddress = (unsigned long)__va(pgd * PGDIR_SIZE);
> > > set_pgd(pgd_offset_k(pgd * PGDIR_SIZE), *pgd_offset_k(vaddress));
> > > + /*
> > > + * pgprot API doesn't clear it for PGD
> > > + *
> > > + * Will be brought back automatically in _epilog()
> > > + */
> > > + pgd_offset_k(pgd * PGDIR_SIZE)->pgd &= ~_PAGE_NX;
> > > }
> > > __flush_tlb_all();

Upstream & downstream looks different, how the above looks completely
different I don't know, but I got it and updating is easy. Great
catch.

> >
> > Wait a sec... Where does the _PAGE_USER come from? Shouldn't we see
> > the &init_mm in there and *not* set _PAGE_USER?
>
> That's because pgd_populate() uses _PAGE_TABLE and not _KERNPG_TABLE for
> reasons that are behind me.

For vsyscalls? I also had to single out warnings out of init_mm.pgd
for the same reasons.

How does the below (untested) look?

>From ab949b80124588c4791568429cf8a234dda16340 Mon Sep 17 00:00:00 2001
From: Jiri Kosina <jikos@kernel.org>
Date: Fri, 5 Jan 2018 20:00:25 +0100
Subject: [RHEL7.5 PATCH 1/1] x86/kaiser/efi: unbreak EFI old_memmap

old_memmap's efi_call_phys_prolog() calls set_pgd() with swapper PGD that
has PAGE_USER set, which makes PTI set NX on it, and therefore EFI can't
execute it's code.

Fix that by forcefully clearing _PAGE_NX from the PGD (this can't be done
by the pgprot API).

_PAGE_NX will be automatically reintroduced in efi_call_phys_epilog(), as
_set_pgd() will again notice that this is _PAGE_USER, and set _PAGE_NX on
it.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
arch/x86/platform/efi/efi_64.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index f951026ea2d2..395079128d98 100644
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -110,6 +110,7 @@ void __init efi_call_phys_prelog(void)
vaddr = (unsigned long)__va(pgd * PGDIR_SIZE);
pgd_efi = pgd_offset_k(addr_pgd);
save_pgd[pgd] = *pgd_efi;
+ pgd_efi->pgd &= ~_PAGE_NX;

pud = pud_alloc(&init_mm, pgd_efi, addr_pgd);
if (!pud) {