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Subject: [PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery
To:     linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, Dave Hansen <dave.hansen@linux.intel.com>,
        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 <dave.hansen@linux.intel.com>
Date:   Tue, 03 Apr 2018 18:10:04 -0700
References: <20180404010946.6186729B@viggo.jf.intel.com>
In-Reply-To: <20180404010946.6186729B@viggo.jf.intel.com>
Message-Id: <20180404011004.FE082845@viggo.jf.intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


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

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 <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/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 16:41:16.085605170 -0700
+++ b/arch/x86/kernel/head_64.S	2018-04-02 16:41:16.088605170 -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)
_