linux-kernel - Re: [PATCH v5 5/9] iommu/arm-smmu-v3: Refactor write_ctx

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Message-ID: <CAKHBV25wD2sU-pVWsNBrJRdhDvmt41hWST7sr91qrVbmtkSZdg@mail.gmail.com>
Date:   Tue, 15 Aug 2023 20:36:55 +0800
From:   Michael Shavit <mshavit@...gle.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Will Deacon <will@...nel.org>, iommu@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        robin.murphy@....com, nicolinc@...dia.com, jean-philippe@...aro.org
Subject: Re: [PATCH v5 5/9] iommu/arm-smmu-v3: Refactor write_ctx_desc

On Tue, Aug 15, 2023 at 8:30 PM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Tue, Aug 15, 2023 at 08:03:40PM +0800, Michael Shavit wrote:
> > On Tue, Aug 15, 2023 at 7:38 PM Jason Gunthorpe <jgg@...dia.com> wrote:
> > >
> > > On Tue, Aug 15, 2023 at 01:20:04PM +0800, Michael Shavit wrote:
> > > > On Thu, Aug 10, 2023 at 11:39 PM Jason Gunthorpe <jgg@...dia.com> wrote:
> > > > >
> > > > > Actually, I don't think this even works as nothing on the PASID path
> > > > > adds to the list that arm_smmu_write_ctx_desc_devices() iterates over ??
> > > > >
> > > > > Then the remaining two calls:
> > > > >
> > > > > arm_smmu_share_asid(struct mm_struct *mm, u16 asid)
> > > > >         arm_smmu_write_ctx_desc_devices(smmu_domain, 0, cd);
> > > > >
> > > > >         This is OK only if the sketchy assumption that the CD
> > > > >         we extracted for a conflicting ASID is not asigned to a PASID.
> > > > >
> > > > > static void arm_smmu_mm_release(struct mmu_notifier *mn, struct mm_struct *mm)
> > > > >         arm_smmu_write_ctx_desc_devices(smmu_domain, mm->pasid, &quiet_cd);
> > > > >
> > > > >         This doesn't work because we didn't add the master to the list
> > > > >         during __arm_smmu_sva_bind and this path is expressly working
> > > > >         on the PASID binds, not the RID binds.
> > > >
> > > > Actually it is working on the RID attached domain (as returned by
> > > > iommu_get_domain_for_dev() at sva_bind time) not the SVA domain
> > > > here...
> > >
> > > That can't be right, the purpose of that call and arm_smmu_mm_release is to
> > > disable the PASID that is about the UAF the mm's page table.
> > >
> > > Jason
> >
> > For the sake of this message, let's call "primary domain" whatever RID
> > domain was attached to a master at the time set_dev_pasid() was called
> > on that master. That RID domain is locked in while SVA is enabled and
> > cannot be detached.
> >
> > The arm-smmu-v3-sva.c implementation creates a mapping between an SVA
> > domain and this primary domain (through the sva domain's mm). In
> > arm_smmu_mm_release, the primary domain is looked up and
> > arm_smmu_write_ctx_desc() is called on all masters that this domain is
> > attached to.
>
> My question is still the same - how does arm_smmu_mm_release update the
> Contex descriptor table entry for the *PASID*
>
> The RID on PASID 0 hasn't change and doesn't need updating.
>
> Jason

arm_smmu_mm_release looks-up the CD table(s) to write using the
primary domain's device list, and finds the index into those CD
table(s) to write to using mm->pasid.