[PATCH 4.14 57/57] x86/ptrace: fix up botched merge of spectrev1 fix

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gre...@linuxfoundation.org>

I incorrectly merged commit 31a2fbb390fe ("x86/ptrace: Fix possible
spectre-v1 in ptrace_get_debugreg()") when backporting it, as was
graciously pointed out at
https://grsecurity.net/teardown_of_a_failed_linux_lts_spectre_fix.php

Resolve the upstream difference with the stable kernel merge to properly
protect things.

Reported-by: Brad Spengler <spen...@grsecurity.net>
Cc: Dianzhang Chen <dianzhangch...@gmail.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: <b...@alien8.de>
Cc: <h...@zytor.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 arch/x86/kernel/ptrace.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -652,11 +652,10 @@ static unsigned long ptrace_get_debugreg
 {
        struct thread_struct *thread = &tsk->thread;
        unsigned long val = 0;
-       int index = n;
 
        if (n < HBP_NUM) {
+               int index = array_index_nospec(n, HBP_NUM);
                struct perf_event *bp = thread->ptrace_bps[index];
-               index = array_index_nospec(index, HBP_NUM);
 
                if (bp)
                        val = bp->hw.info.address;