[PATCH] x86: fix possible spectre-v1 in do_get_thread_area()

[Dianzhang Chen]

The idx in do_get_thread_area() is controlled by userspace via syscall: 
ptrace(defined in kernel/ptrace.c), hence leading to a potential exploitation 
of the Spectre variant 1 vulnerability.
The idx can be controlled from: ptrace -> arch_ptrace -> do_get_thread_area.

Fix this by sanitizing idx before using it to index p->thread.tls_array.

Signed-off-by: Dianzhang Chen <dianzhangch...@gmail.com>
---
 arch/x86/kernel/tls.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index a5b802a..e3dc05b 100644
--- a/arch/x86/kernel/tls.c
+++ b/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>
@@ -220,15 +221,19 @@ int do_get_thread_area(struct task_struct *p, int idx,
                       struct user_desc __user *u_info)
 {
        struct user_desc info;
+       int index = idx - GDT_ENTRY_TLS_MIN;
 
        if (idx == -1 && get_user(idx, &u_info->entry_number))
                return -EFAULT;
 
-       if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
+       if (index < 0 || index > GDT_ENTRY_TLS_MAX - GDT_ENTRY_TLS_MIN)
                return -EINVAL;
 
+       index = array_index_nospec(index,
+                               GDT_ENTRY_TLS_MAX - GDT_ENTRY_TLS_MIN + 1);
+
        fill_user_desc(&info, idx,
-                      &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
+                      &p->thread.tls_array[index]);
 
        if (copy_to_user(u_info, &info, sizeof(info)))
                return -EFAULT;
-- 
2.7.4