[PATCH 4.19 92/93] x86/ptrace: fix up botched merge of spectrev1 fix

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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 <spender@xxxxxxxxxxxxxx>
Cc: Dianzhang Chen <dianzhangchen0@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: <bp@xxxxxxxxx>
Cc: <hpa@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 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;