From: Bharata B Rao <bharata@linux.ibm.com>
To: Ram Pai <linuxram@us.ibm.com>
Cc: ldufour@linux.ibm.com, cclaudio@linux.ibm.com,
kvm-ppc@vger.kernel.org, sathnaga@linux.vnet.ibm.com,
aneesh.kumar@linux.ibm.com, sukadev@linux.vnet.ibm.com,
linuxppc-dev@lists.ozlabs.org, bauerman@linux.ibm.com,
david@gibson.dropbear.id.au
Subject: Re: [PATCH] KVM: PPC: Book3S HV: fix a oops in kvmppc_uvmem_page_free()
Date: Fri, 31 Jul 2020 15:32:40 +0530 [thread overview]
Message-ID: <20200731100240.GC20199@in.ibm.com> (raw)
In-Reply-To: <20200731083700.GB5787@oc0525413822.ibm.com>
On Fri, Jul 31, 2020 at 01:37:00AM -0700, Ram Pai wrote:
> On Fri, Jul 31, 2020 at 09:59:40AM +0530, Bharata B Rao wrote:
> > On Thu, Jul 30, 2020 at 04:25:26PM -0700, Ram Pai wrote:
> > In our case, device pages that are in use are always associated with a valid
> > pvt member. See kvmppc_uvmem_get_page() which returns failure if it
> > runs out of device pfns and that will result in proper failure of
> > page-in calls.
>
> looked at the code, and yes that code path looks correct. So my
> reasoning behind the root cause of this bug is incorrect. However the
> bug is surfacing and there must be a reason.
>
> >
> > For the case where we run out of device pfns, migrate_vma_finalize() will
> > restore the original PTE and will not replace the PTE with device private PTE.
> >
> > Also kvmppc_uvmem_page_free() (=dev_pagemap_ops.page_free()) is never
> > called for non-device-private pages.
>
> Yes. it should not be called. But as seen above in the stack trace, it is called.
>
> What would cause the HMM to call ->page_free() on a page that is not
> associated with that device's pfn?
I believe it is being called for a device private page, you can verify
it when you hit it next time?
>
> >
> > This could be a use-after-free case possibly arising out of the new state
> > changes in HV. If so, this fix will only mask the bug and not address the
> > original problem.
>
> I can verify by rerunning the tests, without the new state changes. But
> I do not see how those changes can cause this fault?
>
> This could also be caused by a duplicate ->page_free() call due to some
> bug in the migrate_page path? Could there be a race between
> migrate_page() and a page_fault ?
>
>
> Regardless, kvmppc_uvmem_page_free() needs to be fixed. It should not
> access contents of pvt, without verifing pvt is valid.
We don't expect pvt to be NULL here. Checking for NULL and returning
isn't the right fix, I think.
Regards,
Bharata.
prev parent reply other threads:[~2020-07-31 10:04 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-30 23:25 [PATCH] KVM: PPC: Book3S HV: fix a oops in kvmppc_uvmem_page_free() Ram Pai
2020-07-31 4:29 ` Bharata B Rao
2020-07-31 8:37 ` Ram Pai
2020-07-31 10:02 ` Bharata B Rao [this message]
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