linux-kernel - Re: [patch V4 part 3 01/29] x86/traps: Mark fixup_bad

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Date:   Mon, 11 May 2020 21:28:16 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        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>
Subject: Re: [patch V4 part 3 01/29] x86/traps: Mark fixup_bad_iret()
 noinstr

On Tue, 05 May 2020 15:43:55 +0200
Thomas Gleixner <tglx@...utronix.de> wrote:

> This is called from deep entry ASM in a situation where instrumentation
> will cause more harm than providing useful information.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org>

Thank you,

> ---
>  arch/x86/kernel/traps.c |   16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -621,7 +621,7 @@ struct bad_iret_stack {
>  	struct pt_regs regs;
>  };
>  
> -asmlinkage __visible notrace
> +asmlinkage __visible noinstr
>  struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
>  {
>  	/*
> @@ -632,19 +632,21 @@ struct bad_iret_stack *fixup_bad_iret(st
>  	 * just below the IRET frame) and we want to pretend that the
>  	 * exception came from the IRET target.
>  	 */
> -	struct bad_iret_stack *new_stack =
> -		(struct bad_iret_stack *)this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;
> +	struct bad_iret_stack tmp, *new_stack =
> +		(struct bad_iret_stack *)__this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;
>  
> -	/* Copy the IRET target to the new stack. */
> -	memmove(&new_stack->regs.ip, (void *)s->regs.sp, 5*8);
> +	/* Copy the IRET target to the temporary storage. */
> +	memcpy(&tmp.regs.ip, (void *)s->regs.sp, 5*8);
>  
>  	/* Copy the remainder of the stack from the current stack. */
> -	memmove(new_stack, s, offsetof(struct bad_iret_stack, regs.ip));
> +	memcpy(&tmp, s, offsetof(struct bad_iret_stack, regs.ip));
> +
> +	/* Update the entry stack */
> +	memcpy(new_stack, &tmp, sizeof(tmp));
>  
>  	BUG_ON(!user_mode(&new_stack->regs));
>  	return new_stack;
>  }
> -NOKPROBE_SYMBOL(fixup_bad_iret);
>  #endif
>  
>  static bool is_sysenter_singlestep(struct pt_regs *regs)
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>