Re: [PATCH 6/9] x86: Use rd/wr fs/gs base in arch_prctl

[Andy Lutomirski]

On Mon, Mar 21, 2016 at 9:16 AM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>
> Convert arch_prctl to use the new instructions to
> change fs/gs if available, instead of using MSRs.
>
> This is merely a small performance optimization,
> no new functionality.
>
> With the new instructions the syscall is really obsolete,
> as everything can be set directly in ring 3. But the syscall
> is widely used by existing software, so we still support it.
>
> The syscall still enforces that the addresses are not
> in kernel space, even though that is not needed more.
> This is mainly so that the programs written for new CPUs
> do not suddenly fail on old CPUs.
>
> v2: Make kprobes safe
> v3: Rename things.
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/process_64.c | 48 ++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 40 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 53fa839..5f40517 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -530,20 +530,38 @@ void set_personality_ia32(bool x32)
>  }
>  EXPORT_SYMBOL_GPL(set_personality_ia32);
>
> +static noinline __kprobes void reload_user_gs(unsigned long addr)
> +{
> +       local_irq_disable();
> +       swapgs();
> +       loadsegment(gs, 0);
> +       wrgsbase(addr);
> +       swapgs();
> +       local_irq_enable();
> +}

The actual operation this does is to set the selector to zero and the
base to the specified value.  Can you give it a name that makes it
clear (e.g. zero_user_gs_and_set_base)?

I'm also wondering whether it would make sense to move the cpu_has
into these helpers rather than putting it in the callers.

--Andy