x86/espfix: Document use of _PAGE_GLOBAL

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@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/20180406205507.BCF4D4F0@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/arch/x86/kernel/espfix_64.c b/arch/x86/kernel/espfix_64.c
index e5ec3cafa72e..aebd0d5bc086 100644
--- a/arch/x86/kernel/espfix_64.c
+++ b/arch/x86/kernel/espfix_64.c
@@ -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);