Re: [PATCH v4 12/12] x86/traps: Fix up invalid PASID

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi Fenghua,

On 2020/6/26 4:17, Fenghua Yu wrote:
> A #GP fault is generated when ENQCMD instruction is executed without
> a valid PASID value programmed in the current thread's PASID MSR. The
> #GP fault handler will initialize the MSR if a PASID has been allocated
> for this process.
> 
> Decoding the user instruction is ugly and sets a bad architecture
> precedent. It may not function if the faulting instruction is modified
> after #GP.
> 
> Thomas suggested to provide a reason for the #GP caused by executing ENQCMD
> without a valid PASID value programmed. #GP error codes are 16 bits and all
> 16 bits are taken. Refer to SDM Vol 3, Chapter 16.13 for details. The other
> choice was to reflect the error code in an MSR. ENQCMD can also cause #GP
> when loading from the source operand, so its not fully comprehending all
> the reasons. Rather than special case the ENQCMD, in future Intel may
> choose a different fault mechanism for such cases if recovery is needed on
> #GP.
> 
> 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 will hit case "2".

For changes in Intel VT-d driver,

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>

Best regards,
baolu

> 
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> ---
> v4:
> - Change PASID type to u32 (Christoph)
> 
> v3:
> - Check and set current->has_valid_pasid in fixup() (PeterZ)
> - Add fpu__pasid_write() to update the MSR (PeterZ)
> - Add ioasid_find() sanity check in fixup()
> 
> v2:
> - Update the first paragraph of the commit message (Thomas)
> - Add reasons why don't decode the user instruction and don't use
>    #GP error code (Thomas)
> - Change get_task_mm() to current->mm (Thomas)
> - Add comments on why IRQ is disabled during PASID fixup (Thomas)
> - Add comment in fixup() that the function is called when #GP is from
>    user (so mm is not NULL) (Dave Hansen)
> 
>   arch/x86/include/asm/iommu.h |  1 +
>   arch/x86/kernel/traps.c      | 14 +++++++
>   drivers/iommu/intel/svm.c    | 78 ++++++++++++++++++++++++++++++++++++
>   3 files changed, 93 insertions(+)
> 
> diff --git a/arch/x86/include/asm/iommu.h b/arch/x86/include/asm/iommu.h
> index ed41259fe7ac..e9365a5d6f7d 100644
> --- a/arch/x86/include/asm/iommu.h
> +++ b/arch/x86/include/asm/iommu.h
> @@ -27,5 +27,6 @@ arch_rmrr_sanity_check(struct acpi_dmar_reserved_memory *rmrr)
>   }
>   
>   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 f9727b96961f..2352f3f1f7ed 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -59,6 +59,7 @@
>   #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>
> @@ -514,6 +515,16 @@ 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"
>   
>   DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
> @@ -526,6 +537,9 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
>   
>   	cond_local_irq_enable(regs);
>   
> +	if (user_mode(regs) && fixup_pasid_exception())
> +		goto exit;
> +
>   	if (static_cpu_has(X86_FEATURE_UMIP)) {
>   		if (user_mode(regs) && fixup_umip_exception(regs))
>   			goto exit;
> diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c
> index 4c788880b037..4a84c82a4f8c 100644
> --- a/drivers/iommu/intel/svm.c
> +++ b/drivers/iommu/intel/svm.c
> @@ -1105,3 +1105,81 @@ void __free_pasid(struct mm_struct *mm)
>   	 */
>   	ioasid_free(pasid);
>   }
> +
> +/*
> + * Write the current task's PASID MSR/state. This is called only when PASID
> + * is enabled.
> + */
> +static void fpu__pasid_write(u32 pasid)
> +{
> +	u64 msr_val = pasid | MSR_IA32_PASID_VALID;
> +
> +	fpregs_lock();
> +
> +	/*
> +	 * If the MSR is active and owned by the current task's FPU, it can
> +	 * be directly written.
> +	 *
> +	 * Otherwise, write the fpstate.
> +	 */
> +	if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
> +		wrmsrl(MSR_IA32_PASID, msr_val);
> +	} else {
> +		struct ia32_pasid_state *ppasid_state;
> +
> +		ppasid_state = get_xsave_addr(&current->thread.fpu.state.xsave,
> +					      XFEATURE_PASID);
> +		/*
> +		 * ppasid_state shouldn't be NULL because XFEATURE_PASID
> +		 * is enabled.
> +		 */
> +		WARN_ON_ONCE(!ppasid_state);
> +		ppasid_state->pasid = msr_val;
> +	}
> +	fpregs_unlock();
> +}
> +
> +/*
> + * Apply some heuristics to see if the #GP fault was caused by a thread
> + * that hasn't had the IA32_PASID MSR initialized.  If it looks like that
> + * is the problem, try initializing the IA32_PASID MSR. If the heuristic
> + * guesses incorrectly, take one more #GP fault.
> + */
> +bool __fixup_pasid_exception(void)
> +{
> +	struct intel_svm *svm;
> +	struct mm_struct *mm;
> +	u32 pasid;
> +
> +	/*
> +	 * If the current task already has a valid PASID in the MSR,
> +	 * the #GP must be for some other reason.
> +	 */
> +	if (current->has_valid_pasid)
> +		return false;
> +
> +	/*
> +	 * This function is called only when this #GP was triggered from user
> +	 * space. So the mm cannot be NULL.
> +	 */
> +	mm = current->mm;
> +	pasid = mm->pasid;
> +
> +	/*
> +	 * If the PASID isn't found, cannot help.
> +	 *
> +	 * Don't care if the PASID is bound to the mm here. #PF will handle the
> +	 * case that the MSR is fixed up by an unbound PASID.
> +	 */
> +	svm = ioasid_find(NULL, pasid, NULL);
> +	if (IS_ERR_OR_NULL(svm))
> +		return false;
> +
> +	/* Fix up the MSR by the PASID in the mm. */
> +	fpu__pasid_write(pasid);
> +
> +	/* Now the current task has a valid PASID in the MSR. */
> +	current->has_valid_pasid = 1;
> +
> +	return true;
> +}
> 
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