From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f70.google.com (mail-pl0-f70.google.com [209.85.160.70]) by kanga.kvack.org (Postfix) with ESMTP id E10276B002F for ; Mon, 2 Apr 2018 13:29:55 -0400 (EDT) Received: by mail-pl0-f70.google.com with SMTP id f19-v6so5988630plr.23 for ; Mon, 02 Apr 2018 10:29:55 -0700 (PDT) Received: from mga06.intel.com (mga06.intel.com. [134.134.136.31]) by mx.google.com with ESMTPS id m125si524506pgm.163.2018.04.02.10.29.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 02 Apr 2018 10:29:54 -0700 (PDT) Subject: [PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery From: Dave Hansen Date: Mon, 02 Apr 2018 10:27:10 -0700 References: <20180402172700.65CAE838@viggo.jf.intel.com> In-Reply-To: <20180402172700.65CAE838@viggo.jf.intel.com> Message-Id: <20180402172710.9AEB9D53@viggo.jf.intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Dave Hansen , aarcange@redhat.com, luto@kernel.org, torvalds@linux-foundation.org, keescook@google.com, hughd@google.com, jgross@suse.com, x86@kernel.org, namit@vmware.com From: Dave Hansen I was mystified as to where the _PAGE_GLOBAL in the kernel page tables for kernel text came from. I audited all the places I could find, but I missed one: head_64.S. The page tables that we create in here live for a long time, and they also have _PAGE_GLOBAL set, despite whether the processor supports it or not. It's harmless, and we got *lucky* that the pageattr code accidentally clears it when we wipe it out of __supported_pte_mask and then later try to mark kernel text read-only. Comment some of these properties to make it easier to find and understand in the future. Signed-off-by: Dave Hansen Cc: Andrea Arcangeli Cc: Andy Lutomirski Cc: Linus Torvalds Cc: Kees Cook Cc: Hugh Dickins Cc: Juergen Gross Cc: x86@kernel.org Cc: Nadav Amit --- b/arch/x86/kernel/head_64.S | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff -puN arch/x86/kernel/head_64.S~comment-global-page arch/x86/kernel/head_64.S --- a/arch/x86/kernel/head_64.S~comment-global-page 2018-04-02 10:26:46.113661210 -0700 +++ b/arch/x86/kernel/head_64.S 2018-04-02 10:26:46.116661210 -0700 @@ -399,8 +399,13 @@ NEXT_PAGE(level3_ident_pgt) .quad level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC .fill 511, 8, 0 NEXT_PAGE(level2_ident_pgt) - /* Since I easily can, map the first 1G. + /* + * Since I easily can, map the first 1G. * Don't set NX because code runs from these pages. + * + * Note: This sets _PAGE_GLOBAL despite whether + * the CPU supports it or it is enabled. But, + * the CPU should ignore the bit. */ PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD) #else @@ -431,6 +436,10 @@ NEXT_PAGE(level2_kernel_pgt) * (NOTE: at +512MB starts the module area, see MODULES_VADDR. * If you want to increase this then increase MODULES_VADDR * too.) + * + * This table is eventually used by the kernel during normal + * runtime. Care must be taken to clear out undesired bits + * later, like _PAGE_RW or _PAGE_GLOBAL in some cases. */ PMDS(0, __PAGE_KERNEL_LARGE_EXEC, KERNEL_IMAGE_SIZE/PMD_SIZE) _