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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 15 Mar 2018 14:37:25 -0700
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Subject: Re: [PATCH RFC rebase 2/9] powerpc: Use barrier_nospec in copy_from_user
To: Michal Suchanek <msuchanek@suse.de>
Cc: ppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Kate Stewart <kstewart@linuxfoundation.org>,
	Madhavan Srinivasan <maddy@linux.vnet.ibm.com>, Paul Mackerras <paulus@samba.org>,
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	Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>,
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	Philippe Ombredanne <pombredanne@nexb.com>, Joe Perches <joe@perches.com>,
	"Oliver O'Halloran" <oohall@gmail.com>, Andrew Morton <akpm@linux-foundation.org>,
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On Thu, Mar 15, 2018 at 12:15 PM, Michal Suchanek <msuchanek@suse.de> wrote:
> This is based on x86 patch doing the same.
>
> Signed-off-by: Michal Suchanek <msuchanek@suse.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