[PATCH 07/11] x86/mm: comment _PAGE_GLOBAL mystery

From: Dave Hansen
Date: Tue Apr 03 2018 - 21:14:01 EST

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Nadav Amit <namit@xxxxxxxxxx>

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
- /* 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.
@@ -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.