Re: [PATCH v12] x86/split_lock: Enable split lock detection by kernel

[Arvind Sankar]

On Wed, Jan 22, 2020 at 10:55:14AM -0800, Luck, Tony wrote:
> +
> +static enum split_lock_detect_state sld_state = sld_warn;
> +

This sets sld_state to sld_warn even on CPUs that don't support
split-lock detection. split_lock_init will then try to read/write the
MSR to turn it on. Would it be better to initialize it to sld_off and
set it to sld_warn in split_lock_setup instead, which is only called if
the CPU supports the feature?

>  
> +dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code)
> +{
> +	const char str[] = "alignment check";
> +
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> +
> +	if (notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_AC, SIGBUS) == NOTIFY_STOP)
> +		return;
> +
> +	if (!split_lock_detect_enabled())
> +		return;

This misses one comment from Sean [1] that this check should be dropped,
otherwise user-space alignment check via EFLAGS.AC will get ignored when
split lock detection is disabled.

[1] https://lore.kernel.org/lkml/20191122184457.GA31235@xxxxxxxxxxxxxxx/

> +
> +	if (!user_mode(regs))
> +		die("Split lock detected\n", regs, error_code);
> +
> +	cond_local_irq_enable(regs);
> +
> +	if (handle_user_split_lock(regs, error_code))
> +		return;
> +
> +	do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
> +		error_code, BUS_ADRALN, NULL);
> +}
> +

Peter [2] called this a possible DOS vector. If userspace is malicious
rather than buggy, couldn't it simply ignore SIGBUS?

[2] https://lore.kernel.org/lkml/20191121131522.GX5671@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/