* [PATCH] x86/pti: unpoison pgd for trusted boot
@ 2018-01-10 21:11 Dave Hansen
0 siblings, 0 replies; 3+ messages in thread
From: Dave Hansen @ 2018-01-10 21:11 UTC (permalink / raw)
To: linux-kernel
Cc: x86, Dave Hansen, ning.sun, tglx, mingo, hpa, tboot-devel,
aarcange, jcm, dwmw, pbonzini, gnomes, torvalds, andi, gregkh,
tim.c.chen, law, nickc, luto, peterz
I believe this should replace 262b6b30087 in -tip.
The patch in -tip potentially misses the pgd clearing if pud_alloc()
sets a PGD. It would also be nice to have that comment back.
Note that the -tip commit 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.
--
From: Dave Hansen <dave.hansen@linux.intel.com>
This is another case similar to what EFI does: create a new set of
page tables, map some code at a low address, and jump to it. PTI
mistakes this low address for userspace and mistakenly marks it
non-executable in an effort to make it unusable for userspace. Undo
the poison to allow execution.
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ning Sun <ning.sun@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: tboot-devel@lists.sourceforge.net
Cc: linux-kernel@vger.kernel.org
Cc: Andrea Arcangeli <aarcange@redhat.com>
CC: Jon Masters <jcm@redhat.com>
Cc: "Woodhouse, David" <dwmw@amazon.co.uk>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
CC: "Tim Chen" <tim.c.chen@linux.intel.com>
Cc: Jeff Law <law@redhat.com>
Cc: Nick Clifton <nickc@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
---
b/arch/x86/kernel/tboot.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff -puN arch/x86/kernel/tboot.c~pti-tboot-fix arch/x86/kernel/tboot.c
--- a/arch/x86/kernel/tboot.c~pti-tboot-fix 2018-01-09 17:12:49.776734656 -0800
+++ b/arch/x86/kernel/tboot.c 2018-01-09 17:12:49.784734656 -0800
@@ -138,6 +138,17 @@ static int map_tboot_page(unsigned long
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;
}
_
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] x86/pti: unpoison pgd for trusted boot
2018-01-10 22:49 Dave Hansen
@ 2018-01-10 22:53 ` Andrea Arcangeli
0 siblings, 0 replies; 3+ messages in thread
From: Andrea Arcangeli @ 2018-01-10 22:53 UTC (permalink / raw)
To: Dave Hansen
Cc: linux-kernel, x86, ning.sun, tglx, mingo, hpa, tboot-devel, jcm,
dwmw, pbonzini, gnomes, torvalds, andi, gregkh, tim.c.chen, law,
nickc, luto, peterz
On Wed, Jan 10, 2018 at 02:49:39PM -0800, Dave Hansen wrote:
>
> Updated to make this on top of x86/pti.
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
^ permalink raw reply [flat|nested] 3+ messages in thread
* [PATCH] x86/pti: unpoison pgd for trusted boot
@ 2018-01-10 22:49 Dave Hansen
2018-01-10 22:53 ` Andrea Arcangeli
0 siblings, 1 reply; 3+ messages in thread
From: Dave Hansen @ 2018-01-10 22:49 UTC (permalink / raw)
To: linux-kernel
Cc: x86, Dave Hansen, ning.sun, tglx, mingo, hpa, tboot-devel,
aarcange, jcm, dwmw, pbonzini, gnomes, torvalds, andi, gregkh,
tim.c.chen, law, nickc, luto, peterz
Updated to make this on top of x86/pti.
--
From: Dave Hansen <dave.hansen@linux.intel.com>
The code in -tip potentially misses the pgd clearing if pud_alloc()
sets a PGD. It would also be nice to have that comment back.
Note that the -tip commit 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.
This just defers the NX clearing to a point after it is known that
all top-level allocations have occurred. Add a comment to clarify
why.
Fixes: 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled")
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ning Sun <ning.sun@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: tboot-devel@lists.sourceforge.net
Cc: linux-kernel@vger.kernel.org
Cc: Andrea Arcangeli <aarcange@redhat.com>
CC: Jon Masters <jcm@redhat.com>
Cc: "Woodhouse, David" <dwmw@amazon.co.uk>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
CC: "Tim Chen" <tim.c.chen@linux.intel.com>
Cc: Jeff Law <law@redhat.com>
Cc: Nick Clifton <nickc@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
---
b/arch/x86/kernel/tboot.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff -puN arch/x86/kernel/tboot.c~pti-tboot-fix arch/x86/kernel/tboot.c
--- a/arch/x86/kernel/tboot.c~pti-tboot-fix 2018-01-10 14:24:46.454544324 -0800
+++ b/arch/x86/kernel/tboot.c 2018-01-10 14:25:53.354544157 -0800
@@ -127,7 +127,6 @@ static int map_tboot_page(unsigned long
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
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;
}
_
^ permalink raw reply [flat|nested] 3+ messages in thread
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2018-01-10 21:11 [PATCH] x86/pti: unpoison pgd for trusted boot Dave Hansen
2018-01-10 22:49 Dave Hansen
2018-01-10 22:53 ` Andrea Arcangeli
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