x86/nmi: Fix NMI uaccess race against CR3 switching

A NMI can hit in the middle of context switching or in the middle of
switch_mm_irqs_off().  In either case, CR3 might not match current->mm,
which could cause copy_from_user_nmi() and friends to read the wrong
memory.

Fix it by adding a new nmi_uaccess_okay() helper and checking it in
copy_from_user_nmi() and in __copy_from_user_nmi()'s callers.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Rik van Riel <riel@surriel.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jann Horn <jannh@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/dd956eba16646fd0b15c3c0741269dfd84452dac.1535557289.git.luto@kernel.org

arch/x86/lib/usercopy.c

@@ -7,6 +7,8 @@
 #include <linux/uaccess.h>
 #include <linux/export.h>
 
+#include <asm/tlbflush.h>
+
 /*
  * We rely on the nested NMI work to allow atomic faults from the NMI path; the
  * nested NMI paths are careful to preserve CR2.
@@ -19,6 +21,9 @@ copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
 	if (__range_not_ok(from, n, TASK_SIZE))
 		return n;
 
+	if (!nmi_uaccess_okay())
+		return n;
+
 	/*
 	 * Even though this function is typically called from NMI/IRQ context
 	 * disable pagefaults so that its behaviour is consistent even when