arm64: fix possible spectre-v1 write in ptrace_hbp_set_event()

It's possible for userspace to control idx. Sanitize idx when using it
as an array index, to inhibit the potential spectre-v1 write gadget.

Found by smatch.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/ptrace.c

@@ -277,19 +277,22 @@ static int ptrace_hbp_set_event(unsigned int note_type,
 
 	switch (note_type) {
 	case NT_ARM_HW_BREAK:
-		if (idx < ARM_MAX_BRP) {
+		if (idx >= ARM_MAX_BRP)
+			goto out;
+		idx = array_index_nospec(idx, ARM_MAX_BRP);
 		tsk->thread.debug.hbp_break[idx] = bp;
 		err = 0;
-		}
 		break;
 	case NT_ARM_HW_WATCH:
-		if (idx < ARM_MAX_WRP) {
+		if (idx >= ARM_MAX_WRP)
+			goto out;
+		idx = array_index_nospec(idx, ARM_MAX_WRP);
 		tsk->thread.debug.hbp_watch[idx] = bp;
 		err = 0;
-		}
 		break;
 	}
 
+out:
 	return err;
 }