linux-kernel - [PATCH 04/11] x86/espfix: document use of _PAGE

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Date:   Tue, 03 Apr 2018 18:09:56 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        aarcange@...hat.com, luto@...nel.org,
        torvalds@...ux-foundation.org, keescook@...gle.com,
        hughd@...gle.com, jgross@...e.com, x86@...nel.org, namit@...are.com
Subject: [PATCH 04/11] x86/espfix: document use of _PAGE_GLOBAL


From: Dave Hansen <dave.hansen@...ux.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@...ux.intel.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kees Cook <keescook@...gle.com>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: x86@...nel.org
Cc: Nadav Amit <namit@...are.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	2018-04-02 16:41:14.274605175 -0700
+++ b/arch/x86/kernel/espfix_64.c	2018-04-02 16:41:14.278605175 -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);
_