Commit-ID: 8a931d1e24bacf01f00a35d43bfe7917256c5c49
Gitweb: https://git.kernel.org/tip/8a931d1e24bacf01f00a35d43bfe7917256c5c49
Author: Dave Hansen <dave.han...@linux.intel.com>
AuthorDate: Wed, 10 Jan 2018 14:49:39 -0800
Committer: Thomas Gleixner <t...@linutronix.de>
CommitDate: Thu, 11 Jan 2018 00:30:25 +0100
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: 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled")
Signed-off-by: Dave Hansen <dave.han...@linux.intel.com>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
Reviewed-by: Andrea Arcangeli <aarca...@redhat.com>
Cc: Jon Masters <j...@redhat.com>
Cc: "Tim Chen" <tim.c.c...@linux.intel.com>
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
---
arch/x86/kernel/tboot.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index 75869a4..a2486f4 100644
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -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;
}
