linux-kernel - RE: [PATCH v2 1/1] iommu/sva: Invalidate KVA range on kernel TLB flush

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Message-ID: <BN9PR11MB527627087764E82D508AEDED8C48A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Thu, 10 Jul 2025 02:55:35 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, "Hansen, Dave"
	<dave.hansen@...el.com>, Joerg Roedel <joro@...tes.org>, Will Deacon
	<will@...nel.org>, Robin Murphy <robin.murphy@....com>, Jason Gunthorpe
	<jgg@...dia.com>, Jann Horn <jannh@...gle.com>, Vasant Hegde
	<vasant.hegde@....com>, Alistair Popple <apopple@...dia.com>, Peter Zijlstra
	<peterz@...radead.org>, Uladzislau Rezki <urezki@...il.com>, "Jean-Philippe
 Brucker" <jean-philippe@...aro.org>, Andy Lutomirski <luto@...nel.org>, "Lai,
 Yi1" <yi1.lai@...el.com>
CC: "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, "security@...nel.org"
	<security@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "stable@...r.kernel.org"
	<stable@...r.kernel.org>
Subject: RE: [PATCH v2 1/1] iommu/sva: Invalidate KVA range on kernel TLB
 flush

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Thursday, July 10, 2025 10:15 AM
> 
> On 7/9/25 23:29, Dave Hansen wrote:
> > On 7/8/25 23:28, Lu Baolu wrote:
> >> Modern IOMMUs often cache page table entries to optimize walk
> performance,
> >> even for intermediate page table levels. If kernel page table mappings are
> >> changed (e.g., by vfree()), but the IOMMU's internal caches retain stale
> >> entries, Use-After-Free (UAF) vulnerability condition arises. If these
> >> freed page table pages are reallocated for a different purpose, potentially
> >> by an attacker, the IOMMU could misinterpret the new data as valid page
> >> table entries. This allows the IOMMU to walk into attacker-controlled
> >> memory, leading to arbitrary physical memory DMA access or privilege
> >> escalation.
> >
> > The approach here is certainly conservative and simple. It's also not
> > going to cause big problems on systems without fancy IOMMUs.
> >
> > But I am a _bit_ worried that it's _too_ conservative. The changelog
> > talks about page table page freeing, but the actual code:
> >
> >> @@ -1540,6 +1541,7 @@ void flush_tlb_kernel_range(unsigned long start,
> unsigned long end)
> >>   		kernel_tlb_flush_range(info);
> >>
> >>   	put_flush_tlb_info();
> >> +	iommu_sva_invalidate_kva_range(start, end);
> >>   }
> >
> > is in a very generic TLB flushing spot that's used for a lot more than
> > just freeing page tables.
> >
> > If the problem is truly limited to freeing page tables, it needs to be
> > commented appropriately.
> 
> Yeah, good comments. It should not be limited to freeing page tables;
> freeing page tables is just a real case that we can see in the vmalloc/
> vfree paths. Theoretically, whenever a kernel page table update is done
> and the CPU TLB needs to be flushed, the secondary TLB (i.e., the caches
> on the IOMMU) should be flushed accordingly. It's assumed that this
> happens in flush_tlb_kernel_range().
> 

this conservative approach sounds safer - even if we overlooked any
threat beyond relying on page table free, doing invalidation in this
function is sufficient to mitigate. 

but as Dave suggested let's add a comment in above to clarify the
motivation of doing so.