[PATCH v2] x86/ptrace: Fix possible spectre-v1 in ptrace_get_debugreg()

[Dianzhang Chen <dianzhangchen0@gmail.com>]

The index to access the threads ptrace_bps is controlled by userspace
via syscall: sys_ptrace(), hence, hence leading to a potential
exploitation of the Spectre variant 1 vulnerability.
The n can be controlled from:
    ptrace -> arch_ptrace -> ptrace_get_debugreg.

Fix this by sanitizing n before using it to index thread->ptrace_bps.

Signed-off-by: Dianzhang Chen <dianzhangchen0@gmail.com>
---
 arch/x86/kernel/ptrace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index a166c96..cbac646 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/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;
-- 
2.7.4