linux-kernel - Re: [PATCH RFC rebase 2/9] powerpc: Use barrier_nospec in copy_from

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Date:   Thu, 15 Mar 2018 14:37:25 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Michal Suchanek <msuchanek@...e.de>
Cc:     ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Neuling <mikey@...ling.org>,
        "Bryant G. Ly" <bryantly@...ux.vnet.ibm.com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Daniel Axtens <dja@...ens.net>,
        Nicholas Piggin <npiggin@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Gibson <david@...son.dropbear.id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Cédric Le Goater <clg@...d.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Joe Perches <joe@...ches.com>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Tobin C. Harding" <me@...in.cc>
Subject: Re: [PATCH RFC rebase 2/9] powerpc: Use barrier_nospec in copy_from_user

On Thu, Mar 15, 2018 at 12:15 PM, Michal Suchanek <msuchanek@...e.de> wrote:
> This is based on x86 patch doing the same.
>
> Signed-off-by: Michal Suchanek <msuchanek@...e.de>
> ---
> --- 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