| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <BN9PR11MB52757A48BE716BECB20FB240CA922@BN9PR11MB5275.namprd11.prod.outlook.com>
Date: Mon, 2 Sep 2024 05:44:12 +0000
From: "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>
To: Jacob Pan <jacob.jun.pan@...ux.intel.com>, "Tian, Kevin"
<kevin.tian@...el.com>
CC: "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, LKML
<linux-kernel@...r.kernel.org>, Lu Baolu <baolu.lu@...ux.intel.com>, "Liu, Yi
L" <yi.l.liu@...el.com>, "Zhang, Tina" <tina.zhang@...el.com>
Subject: RE: [PATCH] iommu/vt-d: Fix potential soft lockup due to reclaim
Sorry, just catching up with emails.
I noticed another thing.
In qi_submit_sync() input parameters, when the count is 0, the expectation should be that desc should be NULL right?
In that case, the below code will cause a problem.
type = desc->qw0 & GENMASK_ULL(3, 0);
The above line requires caller (when calling with count = 0) to pass a fake non-NULL desc pointer. Should we fix this as well? Alternatively, we can fix it whenever we create the use case of caller calling with count=0.
Thanks,
Sanjay
-----Original Message-----
From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
Sent: Thursday, July 25, 2024 7:42 PM
To: Tian, Kevin <kevin.tian@...el.com>
Cc: iommu@...ts.linux.dev; LKML <linux-kernel@...r.kernel.org>; Lu Baolu <baolu.lu@...ux.intel.com>; Liu, Yi L <yi.l.liu@...el.com>; Zhang, Tina <tina.zhang@...el.com>; Kumar, Sanjay K <sanjay.k.kumar@...el.com>; jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH] iommu/vt-d: Fix potential soft lockup due to reclaim
On Fri, 26 Jul 2024 00:18:13 +0000, "Tian, Kevin" <kevin.tian@...el.com>
wrote:
> > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > Sent: Friday, July 26, 2024 5:11 AM
> > > > > > @@ -1463,8 +1462,14 @@ int qi_submit_sync(struct intel_iommu
> > > > *iommu,
> > > > > > struct qi_desc *desc,
> > > > > > raw_spin_lock(&qi->q_lock);
> > > > > > }
> > > > > >
> > > > > > - for (i = 0; i < count; i++)
> > > > > > - qi->desc_status[(index + i) % QI_LENGTH] =
> > > > > > QI_DONE;
> > > > > > + /*
> > > > > > + * The reclaim code can free descriptors from multiple
> > > > > > submissions
> > > > > > + * starting from the tail of the queue. When count ==
> > > > > > 0, the
> > > > > > + * status of the standalone wait descriptor at the
> > > > > > tail of the queue
> > > > > > + * must be set to QI_TO_BE_FREED to allow the reclaim
> > > > > > code to proceed.
> > > > > > + */
> > > > > > + for (i = 0; i <= count; i++)
> > > > > > + qi->desc_status[(index + i) % QI_LENGTH] =
> > > > > > QI_TO_BE_FREED;
> > > > >
> > > > > We don't really need a new flag. Just set them to QI_FREE and
> > > > > then reclaim QI_FREE slots until hitting qi->head in
> > > > > reclaim_free_desc().
> > > > We do need to have a separate state for descriptors pending to
> > > > be freed. Otherwise, reclaim code will advance pass the intended range.
> > > >
> > >
> > > The commit msg said that QI_DONE is currently used for conflicting
> > > purpose.
> > >
> > > Using QI_FREE keeps it only for signaling that a wait desc is
> > > completed.
> > >
> > > The key is that reclaim() should not change a desc's state before
> > > it's consumed by the owner. Instead we always let the owner to
> > > change the state and reclaim() only does scan and adjust the
> > > tracking fields then such race condition disappears.
> > >
> > > In this example T2's slots are changed to QI_FREE by T2 after it
> > > completes all the checks. Only at this point those slots can be
> > > reclaimed.
> >
> > The problem is that without the TO_BE_FREED state, the reclaim code
> > would have no way of knowing which ones are to be reclaimed and
> > which ones are currently free. Therefore, it cannot track free_cnt.
> >
> > The current reclaim code is not aware of owners nor how many to reclaim.
> >
> > If I make the following changes and run, free_cnt will keep going up
> > and system cannot boot. Perhaps you meant another way?
> >
> > --- a/drivers/iommu/intel/dmar.c
> > +++ b/drivers/iommu/intel/dmar.c
> > @@ -1204,8 +1204,7 @@ static void free_iommu(struct intel_iommu
> > *iommu)
> > */
> > static inline void reclaim_free_desc(struct q_inval *qi) {
> > - while (qi->desc_status[qi->free_tail] == QI_TO_BE_FREED) {
> > - qi->desc_status[qi->free_tail] = QI_FREE;
> > + while (qi->desc_status[qi->free_tail] == QI_FREE) {
> > qi->free_tail = (qi->free_tail + 1) % QI_LENGTH;
> > qi->free_cnt++;
>
> Here miss a check to prevent reclaiming unused slots:
>
> if (qi->free_tail == qi->free_head)
> break;
>
> In the example flow reclaim_free_desc() in T1 will only reclaim slots
> used by T1 as slots of T2 are either QI_IN_USE or QI_DONE. T2 slots
> will be reclaimed when T2 calls reclaim_free_desc() after setting them
> to QI_FREE, and reclaim will stop at qi->free_head.
>
> If for some reason T2 completes earlier than T1. reclaim_free_desc()
> in T2 does nothing as the first slot qi->free_tail belongs to T1 still
> IN_USE. T2's slots will then wait until reclaim is triggered by T1 later.
>
This makes sense. Unlike the original code, we now only allow freeing the submitter's own descriptors.
> > }
> > @@ -1466,10 +1465,10 @@ int qi_submit_sync(struct intel_iommu
> > *iommu, struct qi_desc *desc,
> > * The reclaim code can free descriptors from multiple
> > submissions
> > * starting from the tail of the queue. When count == 0, the
> > * status of the standalone wait descriptor at the tail of
> > the queue
> > - * must be set to QI_TO_BE_FREED to allow the reclaim code to
> > proceed.
> > + * must be set to QI_FREE to allow the reclaim code to proceed.
> > */
> > for (i = 0; i <= count; i++)
> > - qi->desc_status[(index + i) % QI_LENGTH] =
> > QI_TO_BE_FREED;
> > + qi->desc_status[(index + i) % QI_LENGTH] = QI_FREE;
> >
> > reclaim_free_desc(qi);
> > raw_spin_unlock_irqrestore(&qi->q_lock, flags); diff --git
> > a/drivers/iommu/intel/iommu.h b/drivers/iommu/intel/iommu.h index
> > 1ab39f9145f2..eaf015b4353b 100644
> > --- a/drivers/iommu/intel/iommu.h
> > +++ b/drivers/iommu/intel/iommu.h
> > @@ -382,8 +382,7 @@ enum {
> > QI_FREE,
> > QI_IN_USE,
> > QI_DONE,
> > - QI_ABORT,
> > - QI_TO_BE_FREED
> > + QI_ABORT
> > };
> >
> > Thanks,
> >
> > Jacob
>
Thanks,
Jacob
Powered by blists - more mailing lists