linux-kernel - Re: [PATCH RFC] x86/bus_lock: Enable bus lock detection

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Date:   Wed, 29 Jul 2020 10:49:47 +0200
From:   peterz@...radead.org
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     "Thomas Gleixner" <tglx@...utronix.de>,
        "Borislav Petkov" <bp@...en8.de>, "Ingo Molnar" <mingo@...hat.com>,
        "Tony Luck" <tony.luck@...el.com>, "H Peter Anvin" <hpa@...or.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        "Ravi V Shankar" <ravi.v.shankar@...el.com>,
        "Xiaoyao Li " <xiaoyao.li@...el.com>, "x86" <x86@...nel.org>,
        "linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] x86/bus_lock: Enable bus lock detection

On Fri, Jul 17, 2020 at 02:35:00PM -0700, Fenghua Yu wrote:

> #DB for bus lock detect fixes all issues in #AC for split lock detect:
> 1) It's architectural ... just need to look at one CPUID bit to know it
>    exists
> 2) The IA32_DEBUGCTL MSR, which reports bus lock in #DB, is per-thread.
>    So each process or guest can have different behavior.

And it generates a whole new problem due to #DB being an IST, and

> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index b038695f36c5..58725567da39 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -812,6 +812,16 @@ static void handle_debug(struct pt_regs *regs, unsigned long dr6, bool user)
>  	if (!user && !dr6)
>  		return;
>  
> +	/* Handle bus lock. */
> +	if (!(dr6 & DR_BUS_LOCK)) {
> +		cond_local_irq_enable(regs);
> +		if (user)
> +			handle_user_bus_lock(regs);
> +		else
> +			handle_kernel_bus_lock(regs);
> +		goto out;
> +	}
> +
>  	/*
>  	 * If dr6 has no reason to give us about the origin of this trap,
>  	 * then it's very likely the result of an icebp/int01 trap.

we very much rely on #DB never recursing, which we carefully crafted by
disallowing hardare breakpoints on noinstr code and clearing DR7 early.

But now it can... please keep the pieces.