x86/tls: Fix possible spectre-v1 in do_get_thread_area()

The index to access the threads tls array 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 -> do_get_thread_area.

Fix this by sanitizing the user supplied index before using it to access
the p->thread.tls_array.
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/1561524630-3642-1-git-send-email-dianzhangchen0@gmail.com

arch/x86/kernel/tls.c

@@ -5,6 +5,7 @@
 #include <linux/user.h>
 #include <linux/regset.h>
 #include <linux/syscalls.h>
+#include <linux/nospec.h>
 
 #include <linux/uaccess.h>
 #include <asm/desc.h>
@@ -220,6 +221,7 @@ int do_get_thread_area(struct task_struct *p, int idx,
 		       struct user_desc __user *u_info)
 {
 	struct user_desc info;
+	int index;
 
 	if (idx == -1 && get_user(idx, &u_info->entry_number))
 		return -EFAULT;
@@ -227,8 +229,11 @@ int do_get_thread_area(struct task_struct *p, int idx,
 	if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 		return -EINVAL;
 
-	fill_user_desc(&info, idx,
-		       &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
+	index = idx - GDT_ENTRY_TLS_MIN;
+	index = array_index_nospec(index,
+			GDT_ENTRY_TLS_MAX - GDT_ENTRY_TLS_MIN + 1);
+
+	fill_user_desc(&info, idx, &p->thread.tls_array[index]);
 
 	if (copy_to_user(u_info, &info, sizeof(info)))
 		return -EFAULT;