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SubjectRe: [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@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

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