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

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Date:   Sun, 26 Apr 2020 17:25:06 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>, 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>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        iommu@...ts.linux-foundation.org, Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH 6/7] x86/traps: Fix up invalid PASID

Fenghua Yu <fenghua.yu@...el.com> writes:
> A #GP fault is generated when ENQCMD instruction is executed without
> a valid PASID value programmed in.

Programmed in what?

> The #GP fault handler will initialize the current thread's PASID MSR.
>
> The following heuristic is used to avoid decoding the user instructions
> to determine the precise reason for the #GP fault:
> 1) If the mm for the process has not been allocated a PASID, this #GP
>    cannot be fixed.
> 2) If the PASID MSR is already initialized, then the #GP was for some
>    other reason
> 3) Try initializing the PASID MSR and returning. If the #GP was from
>    an ENQCMD this will fix it. If not, the #GP fault will be repeated
>    and we will hit case "2".
>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>

Just for the record I also suggested to have a proper errorcode in the
#GP for ENQCMD and I surely did not suggest to avoid decoding the user
instructions.

>  void __free_pasid(struct mm_struct *mm);
> +bool __fixup_pasid_exception(void);
>  
>  #endif /* _ASM_X86_IOMMU_H */
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 6ef00eb6fbb9..369b5ba94635 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -56,6 +56,7 @@
>  #include <asm/umip.h>
>  #include <asm/insn.h>
>  #include <asm/insn-eval.h>
> +#include <asm/iommu.h>
>  
>  #ifdef CONFIG_X86_64
>  #include <asm/x86_init.h>
> @@ -488,6 +489,16 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
>  	return GP_CANONICAL;
>  }
>  
> +static bool fixup_pasid_exception(void)
> +{
> +	if (!IS_ENABLED(CONFIG_INTEL_IOMMU_SVM))
> +		return false;
> +	if (!static_cpu_has(X86_FEATURE_ENQCMD))
> +		return false;
> +
> +	return __fixup_pasid_exception();
> +}
> +
>  #define GPFSTR "general protection fault"
>  
>  dotraplinkage void do_general_protection(struct pt_regs *regs, long error_code)
> @@ -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.

> +		return;
> +	}
> +
>  	cond_local_irq_enable(regs);
>  
>  	if (static_cpu_has(X86_FEATURE_UMIP)) {
> diff --git a/drivers/iommu/intel-svm.c b/drivers/iommu/intel-svm.c
> index da718a49e91e..5ed39a022adb 100644
> --- a/drivers/iommu/intel-svm.c
> +++ b/drivers/iommu/intel-svm.c
> @@ -759,3 +759,40 @@ void __free_pasid(struct mm_struct *mm)
>  	 */
>  	ioasid_free(pasid);
>  }
> +
> +/*
> + * Fix up the PASID MSR if possible.
> + *
> + * But if the #GP was due to another reason, a second #GP might be triggered
> + * to force proper behavior.
> + */
> +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 ?

> +	/* This #GP was triggered from user mode. So mm cannot be NULL. */
> +	pasid = mm->context.pasid;
> +	/* Ensure this process has been bound to a PASID. */
> +	if (!pasid) {
> +		ret = false;
> +		goto out;
> +	}
> +
> +	/* Check to see if the PASID MSR has already been set for this task. */
> +	rdmsrl(MSR_IA32_PASID, pasid_msr);
> +	if (pasid_msr & MSR_IA32_PASID_VALID) {
> +		ret = false;
> +		goto out;
> +	}
> +
> +	/* Fix up the MSR. */
> +	wrmsrl(MSR_IA32_PASID, pasid | MSR_IA32_PASID_VALID);
> +out:
> +	mmput(mm);

Thanks,

        tglx