linux-kernel - Re: [PATCH 6/7] x86/traps: Fix up invalid PASID

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Date:   Tue, 28 Apr 2020 02:13:11 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Sohil Mehta <sohil.mehta@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 6/7] x86/traps: Fix up invalid PASID

Fenghua Yu <fenghua.yu@...el.com> writes:
> On Sun, Apr 26, 2020 at 05:25:06PM +0200, Thomas Gleixner wrote:
>> > @@ -499,6 +510,12 @@ dotraplinkage void do_general_protection(struct pt_regs *regs, long error_code)
>> >  	int ret;
>> >  
>> >  	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
>> > +
>> > +	if (user_mode(regs) && fixup_pasid_exception()) {
>> > +		cond_local_irq_enable(regs);
>> 
>> The point of this conditional irq enable _AFTER_ calling into the fixup
>> function is? Also what's the reason for keeping interrupts disabled
>> while calling into that function? Comments exist for a reason.
>
> irq needs to be disabled because the fixup function requires to disable
> preempt in order to update the PASID MSR on the faulting CPU.

No, that's just wrong. It's not about the update itself.

> Will add comments here.

Factual ones and not some fairy tales please.

>> > +bool __fixup_pasid_exception(void)
>> > +{
>> > +	struct mm_struct *mm;
>> > +	bool ret = true;
>> > +	u64 pasid_msr;
>> > +	int pasid;
>> > +
>> > +	mm = get_task_mm(current);
>> 
>> Why do you need a reference to current->mm ?
>
> The PASID for the address space is per mm and is stored in mm.
> To get the PASID, we need to get the mm and the
> pasid=mm->context.pasid.

It's obvious that you need to access current-mm in order to check
current->mm->context.pasid. Let me rephrase the question:

   Why do you need to take a reference on current->mm ?

Thanks,

        tglx