[PATCH 4.4 11/67] x86/pti: Make unpoison of pgd for trusted boot work for real

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

--

From: Dave Hansen 

commit 445b69e3b75e42362a5bdc13c8b8f61599e2228a upstream.

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 ]

[hughd notes: I have not tested tboot, but this looks to me as necessary
and as safe in old-Kaiser backports as it is upstream; I'm not submitting
the commit-to-be-fixed 262b6b30087, since it was undone by 445b69e3b75e,
and makes conflict trouble because of 5-level's p4d versus 4-level's pgd.]

Fixes: 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled")
Signed-off-by: Dave Hansen 
Signed-off-by: Thomas Gleixner 
Reviewed-by: Andrea Arcangeli 
Cc: Jon Masters 
Cc: "Tim Chen" 
Cc: gno...@lxorguk.ukuu.org.uk
Cc: pet...@infradead.org
Cc: ning@intel.com
Cc: tboot-de...@lists.sourceforge.net
Cc: a...@firstfloor.org
Cc: l...@kernel.org
Cc: l...@redhat.com
Cc: pbonz...@redhat.com
Cc: torva...@linux-foundation.org
Cc: gre...@linux-foundation.org
Cc: d...@amazon.co.uk
Cc: ni...@redhat.com
Cc: sta...@vger.kernel.org
Link: https://lkml.kernel.org/r/20180110224939.2695c...@viggo.jf.intel.com
Signed-off-by: Hugh Dickins 
Signed-off-by: Greg Kroah-Hartman 

---
 arch/x86/kernel/tboot.c |   10 ++
 1 file changed, 10 insertions(+)

--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -140,6 +140,16 @@ static int map_tboot_page(unsigned long
return -1;
set_pte_at(_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 pud_alloc().
+*/
+   pgd->pgd &= ~_PAGE_NX;
+
return 0;
 }
 

[PATCH 4.4 11/67] x86/pti: Make unpoison of pgd for trusted boot work for real

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

--

From: Dave Hansen 

commit 445b69e3b75e42362a5bdc13c8b8f61599e2228a upstream.

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 ]

[hughd notes: I have not tested tboot, but this looks to me as necessary
and as safe in old-Kaiser backports as it is upstream; I'm not submitting
the commit-to-be-fixed 262b6b30087, since it was undone by 445b69e3b75e,
and makes conflict trouble because of 5-level's p4d versus 4-level's pgd.]

Fixes: 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled")
Signed-off-by: Dave Hansen 
Signed-off-by: Thomas Gleixner 
Reviewed-by: Andrea Arcangeli 
Cc: Jon Masters 
Cc: "Tim Chen" 
Cc: gno...@lxorguk.ukuu.org.uk
Cc: pet...@infradead.org
Cc: ning@intel.com
Cc: tboot-de...@lists.sourceforge.net
Cc: a...@firstfloor.org
Cc: l...@kernel.org
Cc: l...@redhat.com
Cc: pbonz...@redhat.com
Cc: torva...@linux-foundation.org
Cc: gre...@linux-foundation.org
Cc: d...@amazon.co.uk
Cc: ni...@redhat.com
Cc: sta...@vger.kernel.org
Link: https://lkml.kernel.org/r/20180110224939.2695c...@viggo.jf.intel.com
Signed-off-by: Hugh Dickins 
Signed-off-by: Greg Kroah-Hartman 

---
 arch/x86/kernel/tboot.c |   10 ++
 1 file changed, 10 insertions(+)

--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -140,6 +140,16 @@ static int map_tboot_page(unsigned long
return -1;
set_pte_at(_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 pud_alloc().
+*/
+   pgd->pgd &= ~_PAGE_NX;
+
return 0;
 }