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    SubjectRe: [PATCH 6/7] x86/traps: Fix up invalid PASID
    Date
    Fenghua Yu <fenghua.yu@intel.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@linutronix.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

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