x86/ptrace: Fix possible spectre-v1 in ptrace_get_debugreg()

The index to access the threads ptrace_bps is controlled by userspace via
syscall: sys_ptrace(), hence leading to a potential exploitation of the
Spectre variant 1 vulnerability.

The index can be controlled from:
    ptrace -> arch_ptrace -> ptrace_get_debugreg.

Fix this by sanitizing the user supplied index before using it access
thread->ptrace_bps.
Signed-off-by: Dianzhang Chen <dianzhangchen0@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: bp@alien8.de
Cc: hpa@zytor.com
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1561476617-3759-1-git-send-email-dianzhangchen0@gmail.com

arch/x86/kernel/ptrace.c

@@ -25,6 +25,7 @@
 #include <linux/rcupdate.h>
 #include <linux/export.h>
 #include <linux/context_tracking.h>
+#include <linux/nospec.h>
 
 #include <linux/uaccess.h>
 #include <asm/pgtable.h>
@@ -643,9 +644,11 @@ static unsigned long ptrace_get_debugreg(struct task_struct *tsk, int n)
 {
 	struct thread_struct *thread = &tsk->thread;
 	unsigned long val = 0;
+	int index = n;
 
 	if (n < HBP_NUM) {
-		struct perf_event *bp = thread->ptrace_bps[n];
+		index = array_index_nospec(index, HBP_NUM);
+		struct perf_event *bp = thread->ptrace_bps[index];
 
 		if (bp)
 			val = bp->hw.info.address;