Re: [PATCH RFC rebase 2/9] powerpc: Use barrier_nospec in copy_from_user

[Linus Torvalds]

On Thu, Mar 15, 2018 at 12:15 PM, Michal Suchanek <msuchanek@xxxxxxx> wrote:
> This is based on x86 patch doing the same.
>
> Signed-off-by: Michal Suchanek <msuchanek@xxxxxxx>
> ---
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -258,8 +259,10 @@ do {                                                               \
>         long __gu_err = -EFAULT;                                        \
>         unsigned long  __gu_val = 0;                                    \
>         const __typeof__(*(ptr)) __user *__gu_addr = (ptr);             \
> +       int can_access = access_ok(VERIFY_READ, __gu_addr, (size));     \
>         might_fault();                                                  \
> -       if (access_ok(VERIFY_READ, __gu_addr, (size)))                  \
> +       barrier_nospec();                                               \
> +       if (can_access)                                                 \
>                 __get_user_size(__gu_val, __gu_addr, (size), __gu_err); \
>         (x) = (__force __typeof__(*(ptr)))__gu_val;                             \
>         __gu_err;                                                       \

Is the above really correct? The barrier is *before* the conditional
branch that might be mis-predicted.

I don't know how the ppc barrier works, but that sounds completely bogus.

               Linus