Re: [PATCH v2] x86/tls: Fix possible spectre-v1 in do_get_thread_area()

[Dianzhang Chen]

On Wed, Jun 26, 2019 at 12:38 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Wed, 26 Jun 2019, Dianzhang Chen wrote:
>
> > 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 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.
>
> Just that I can't find a place which sanitizes the value....
>
> > +#include <linux/nospec.h>
>
> and nothing which uses anything from this header file.
>
> >  #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 +229,9 @@ 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;
> > +
> > +     fill_user_desc(&info, idx, &p->thread.tls_array[index]);
>
> So this is just a cosmetic change and the compiler will create probably
> exactly the same binary.
>
> Thanks,
>
>         tglx
>

sorry for being careless, my bad.
And thanks for suggestion, i'll fix that in next version.