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Date: Tue, 11 Nov 2014 12:08:04 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Andi Kleen <andi@...stfloor.org>, x86@...nel.org
CC: linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 8/8] x86: Use rd/wr fs/gs base in arch_prctl
On 11/10/2014 03:55 PM, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
>
> 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.
>
> With the new instructions available it prefers to use
> them in the context switch, instead of using the old
> "use GDT segment rewrite" trick.
>
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---
> arch/x86/kernel/process_64.c | 45 ++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 37 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index df554e2..010fe15 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -483,15 +483,23 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
> int ret = 0;
> int doit = task == current;
> int cpu;
> + int fast_seg = boot_cpu_has(X86_FEATURE_FSGSBASE);
>
> switch (code) {
> case ARCH_SET_GS:
> + /*
> + * With fast_seg we don't need that check anymore,
> + * but keep it so that programs do not suddenly
> + * start failing when run on older CPUs.
> + * If you really want to set a address in kernel space
> + * use WRGSBASE directly.
> + */
> if (addr >= TASK_SIZE_OF(task))
> return -EPERM;
> cpu = get_cpu();
> /* handle small bases via the GDT because that's faster to
> switch. */
> - if (addr <= 0xffffffff) {
> + if (addr <= 0xffffffff && !fast_seg) {
> set_32bit_tls(task, GS_TLS, addr);
> if (doit) {
> load_TLS(&task->thread, cpu);
> @@ -503,8 +511,17 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
> task->thread.gsindex = 0;
> task->thread.gs = addr;
> if (doit) {
> - load_gs_index(0);
> - ret = wrmsrl_safe(MSR_KERNEL_GS_BASE, addr);
> + if (fast_seg) {
> + local_irq_disable();
> + swapgs();
> + loadsegment(gs, 0);
> + wrgsbase(addr);
> + swapgs();
> + local_irq_enable();
Does this (and the other copies of this) need kprobe protection?
--Andy
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