x86/mm: Comment _PAGE_GLOBAL mystery

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: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kees Cook <keescook@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nadav Amit <namit@vmware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20180406205513.079BB265@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 48385c1..8344dd2 100644 (file)
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -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)