x86/ptrace: fix up botched merge of spectrev1 fix

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@grsecurity.net>
Cc: Dianzhang Chen <dianzhangchen0@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <bp@alien8.de>
Cc: <hpa@zytor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index aeba778..516ec75 100644 (file)
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -652,11 +652,10 @@ 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) {
+               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;