x86, entry: Switch stacks on a paranoid entry from userspace

This causes all non-NMI, non-double-fault kernel entries from
userspace to run on the normal kernel stack.  Double-fault is
exempt to minimize confusion if we double-fault directly from
userspace due to a bad kernel stack.

This is, suprisingly, simpler and shorter than the current code.  It
removes the IMO rather frightening paranoid_userspace path, and it
make sync_regs much simpler.

There is no risk of stack overflow due to this change -- the kernel
stack that we switch to is empty.

This will also enable us to create non-atomic sections within
machine checks from userspace, which will simplify memory failure
handling.  It will also allow the upcoming fsgsbase code to be
simplified, because it doesn't need to worry about usergs when
scheduling in paranoid_exit, as that code no longer exists.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Tony Luck <tony.luck@intel.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>

1 files changed, 5 insertions, 18 deletions

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 88900e288021..28f3e5ffc55d 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -466,27 +466,14 @@ NOKPROBE_SYMBOL(do_int3);
 
 #ifdef CONFIG_X86_64
 /*
- * Help handler running on IST stack to switch back to user stack
- * for scheduling or signal handling. The actual stack switch is done in
- * entry.S
+ * Help handler running on IST stack to switch off the IST stack if the
+ * interrupted code was in user mode. The actual stack switch is done in
+ * entry_64.S
  */
 asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs)
 {
-	struct pt_regs *regs = eregs;
-	/* Did already sync */
-	if (eregs == (struct pt_regs *)eregs->sp)
-		;
-	/* Exception from user space */
-	else if (user_mode(eregs))
-		regs = task_pt_regs(current);
-	/*
-	 * Exception from kernel and interrupts are enabled. Move to
-	 * kernel process stack.
-	 */
-	else if (eregs->flags & X86_EFLAGS_IF)
-		regs = (struct pt_regs *)(eregs->sp -= sizeof(struct pt_regs));
-	if (eregs != regs)
-		*regs = *eregs;
+	struct pt_regs *regs = task_pt_regs(current);
+	*regs = *eregs;
 	return regs;
 }
 NOKPROBE_SYMBOL(sync_regs);