mm/tlb: Provide default nmi_uaccess_okay()

x86 has an nmi_uaccess_okay(), but other architectures do not.
Arch-independent code might need to know whether access to user
addresses is ok in an NMI context or in other code whose execution
context is unknown.  Specifically, this function is needed for
bpf_probe_write_user().

Add a default implementation of nmi_uaccess_okay() for architectures
that do not have such a function.

Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: <akpm@linux-foundation.org>
Cc: <ard.biesheuvel@linaro.org>
Cc: <deneen.t.dock@intel.com>
Cc: <kernel-hardening@lists.openwall.com>
Cc: <kristen@linux.intel.com>
Cc: <linux_dti@icloud.com>
Cc: <will.deacon@arm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190426001143.4983-23-namit@vmware.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/tlbflush.h

@@ -274,6 +274,8 @@ static inline bool nmi_uaccess_okay(void)
 	return true;
 }
 
+#define nmi_uaccess_okay nmi_uaccess_okay
+
 /* Initialize cr4 shadow for this CPU. */
 static inline void cr4_init_shadow(void)
 {

include/asm-generic/tlb.h

@@ -20,6 +20,15 @@
 #include <asm/pgalloc.h>
 #include <asm/tlbflush.h>
 
+/*
+ * Blindly accessing user memory from NMI context can be dangerous
+ * if we're in the middle of switching the current user task or switching
+ * the loaded mm.
+ */
+#ifndef nmi_uaccess_okay
+# define nmi_uaccess_okay() true
+#endif
+
 #ifdef CONFIG_MMU
 
 #ifdef CONFIG_HAVE_RCU_TABLE_FREE