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x86/pti: Make unpoison of pgd for trusted boot work for real 

The inital fix for trusted boot and PTI potentially misses the pgd clearing
if pud_alloc() sets a PGD.  It probably works in *practice* because for two
adjacent calls to map_tboot_page() that share a PGD entry, the first will
clear NX, *then* allocate and set the PGD (without NX clear).  The second
call will *not* allocate but will clear the NX bit.

Defer the NX clearing to a point after it is known that all top-level
allocations have occurred.  Add a comment to clarify why.

[ tglx: Massaged changelog ]

Fixes: 262b6b3008 ("x86/tboot: Unbreak tboot with PTI enabled")
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Jon Masters <jcm@redhat.com>
Cc: "Tim Chen" <tim.c.chen@linux.intel.com>
Cc: gnomes@lxorguk.ukuu.org.uk
Cc: peterz@infradead.org
Cc: ning.sun@intel.com
Cc: tboot-devel@lists.sourceforge.net
Cc: andi@firstfloor.org
Cc: luto@kernel.org
Cc: law@redhat.com
Cc: pbonzini@redhat.com
Cc: torvalds@linux-foundation.org
Cc: gregkh@linux-foundation.org
Cc: dwmw@amazon.co.uk
Cc: nickc@redhat.com
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180110224939.2695CD47@viggo.jf.intel.com
This commit is contained in:
Dave Hansen 2018-01-10 14:49:39 -08:00 committed by Thomas Gleixner
parent 612e8e9350
commit 445b69e3b7
1 changed files with 11 additions and 1 deletions
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12
arch/x86/kernel/tboot.c
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@ -127,7 +127,6 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,
p4d = p4d_alloc(&tboot_mm, pgd, vaddr);
if (!p4d)
return -1;
pgd->pgd &= ~_PAGE_NX;
pud = pud_alloc(&tboot_mm, p4d, vaddr);
if (!pud)
return -1;
@ -139,6 +138,17 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,
return -1;
set_pte_at(&tboot_mm, vaddr, pte, pfn_pte(pfn, prot));
pte_unmap(pte);
/*
* PTI poisons low addresses in the kernel page tables in the
* name of making them unusable for userspace. To execute
* code at such a low address, the poison must be cleared.
*
* Note: 'pgd' actually gets set in p4d_alloc() _or_
* pud_alloc() depending on 4/5-level paging.
*/
pgd->pgd &= ~_PAGE_NX;
return 0;
}