[RFC][PATCH 20/24] x86/pti: Execute NMI handler on the kernel stack

[Alexandre Chartre]

After a NMI from userland, the kernel is entered and it switches
the stack to the PTI stack which is mapped both in the kernel and in
the user page-table. When executing the NMI handler, switch to the
kernel stack (which is mapped only in the kernel page-table) so that
no kernel data leak to the userland through the stack.

Signed-off-by: Alexandre Chartre <alexandre.char...@oracle.com>
---
 arch/x86/kernel/nmi.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index 4bc77aaf1303..be0f654c3095 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -506,8 +506,18 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
 
        inc_irq_stat(__nmi_count);
 
-       if (!ignore_nmis)
-               default_do_nmi(regs);
+       if (!ignore_nmis) {
+               if (user_mode(regs)) {
+                       /*
+                        * If we come from userland then we are on the
+                        * trampoline stack, switch to the kernel stack
+                        * to execute the NMI handler.
+                        */
+                       run_idt(default_do_nmi, regs);
+               } else {
+                       default_do_nmi(regs);
+               }
+       }
 
        idtentry_exit_nmi(regs, irq_state);
 
-- 
2.18.4