[PATCH 04/11] x86/espfix: document use of _PAGE_GLOBAL

Dave Hansen
2018-03-23T17:46:58Z

From: Dave Hansen <dave.hansen@linux.intel.com>

The "normal" kernel page table creation mechanisms using
PAGE_KERNEL_* page protections will never set _PAGE_GLOBAL with PTI.
The few places in the kernel that always want _PAGE_GLOBAL must
avoid using PAGE_KERNEL_*.

Document that we want it here and its use is not accidental.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: x86@kernel.org
Cc: Nadav Amit <namit@vmware.com>
---

b/arch/x86/kernel/espfix_64.c | 4 ++++
1 file changed, 4 insertions(+)

diff -puN arch/x86/kernel/espfix_64.c~espfix-use-kern-defaults-not-supported arch/x86/kernel/espfix_64.c
--- a/arch/x86/kernel/espfix_64.c~espfix-use-kern-defaults-not-supported 2018-03-21 16:31:57.951192318 -0700
+++ b/arch/x86/kernel/espfix_64.c 2018-03-21 16:31:57.954192318 -0700
@@ -195,6 +195,10 @@ void init_espfix_ap(int cpu)

pte_p = pte_offset_kernel(&pmd, addr);
stack_page = page_address(alloc_pages_node(node, GFP_KERNEL, 0));
+ /*
+ * __PAGE_KERNEL_* includes _PAGE_GLOBAL, which we want since
+ * this is mapped to userspace.
+ */
pte = __pte(__pa(stack_page) | ((__PAGE_KERNEL_RO | _PAGE_ENC) & ptemask));
for (n = 0; n < ESPFIX_PTE_CLONES; n++)
set_pte(&pte_p[n*PTE_STRIDE], pte);
_