linux-kernel - Re: [patch V4 part 1 08/36] x86/doublefault: Remove memmove() call

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Date:   Wed, 6 May 2020 15:47:03 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [patch V4 part 1 08/36] x86/doublefault: Remove memmove() call


On 5/5/20 3:16 PM, Thomas Gleixner wrote:
> Use of memmove() in #DF is problematic considered tracing and other
> instrumentation.
> 
> Remove the memmove() call and simply write out what needs doing; this
> even clarifies the code, win-win! The code copies from the espfix64
> stack to the normal task stack, there is no possible way for that to
> overlap.
> 
> Survives selftests/x86, specifically sigreturn_64.
> 
> Suggested-by: Borislav Petkov <bp@...en8.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Acked-by: Andy Lutomirski <luto@...nel.org>
> Link: https://lkml.kernel.org/r/20200220121727.GB507@zn.tnic
> ---
>   arch/x86/kernel/traps.c |    7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)

Reviewed-by: Alexandre Chartre <alexandre.chartre@...cle.com>

alex.

> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -278,6 +278,7 @@ dotraplinkage void do_double_fault(struc
>   		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
> @@ -285,7 +286,11 @@ dotraplinkage void do_double_fault(struc
>   		 * in gpregs->ss through gpregs->ip.
>   		 *
>   		 */
> -		memmove(&gpregs->ip, (void *)regs->sp, 5*8);
> +		gpregs->ip	= p[0];
> +		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 */
>   
>   		/*
>