linux-kernel - Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

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Date:   Thu, 20 Feb 2020 13:37:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, mingo@...nel.org,
        joel@...lfernandes.org, gregkh@...uxfoundation.org,
        gustavo@...eddedor.com, tglx@...utronix.de, paulmck@...nel.org,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, luto@...nel.org, tony.luck@...el.com,
        frederic@...nel.org, dan.carpenter@...cle.com, mhiramat@...nel.org
Subject: Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

On Thu, Feb 20, 2020 at 01:17:27PM +0100, Borislav Petkov wrote:
> On Wed, Feb 19, 2020 at 04:57:15PM +0100, Peter Zijlstra wrote:
> > -		memmove(&gpregs->ip, (void *)regs->sp, 5*8);
> > +		for (i = 0; i < count; i++) {
> > +			int idx = (dst <= src) ? i : count - i;
> > +			dst[idx] = src[idx];
> > +		}
> 
> Or, you can actually unroll it. This way it even documents clearly what
> it does:
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index fe38015ed50a..2b790a574ba5 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -298,6 +298,7 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code, unsign
>  		regs->ip == (unsigned long)native_irq_return_iret)
>  	{
>  		struct pt_regs *gpregs = (struct pt_regs *)this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;
> +		unsigned long *p = (unsigned long *)regs->sp;
>  
>  		/*
>  		 * regs->sp points to the failing IRET frame on the
> @@ -305,7 +306,11 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code, unsign
>  		 * in gpregs->ss through gpregs->ip.
>  		 *
>  		 */
> -		memmove(&gpregs->ip, (void *)regs->sp, 5*8);
> +		gpregs->ip	= *p;
> +		gpregs->cs	= *(p + 1);
> +		gpregs->flags	= *(p + 2);
> +		gpregs->sp	= *(p + 3);
> +		gpregs->ss	= *(p + 4);
>  		gpregs->orig_ax = 0;  /* Missing (lost) #GP error code */
>  
>  		/*

While I love that; is that actually correct? This is an unroll of
memcpy() not memmove(). IFF the ranges overlap, the above is buggered.

Was the original memmove() really needed?