linux-kernel - Re: [PATCH v3 1/1] iommu/sva: Invalidate KVA range on kernel TLB flush

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Message-ID: <b6122394-0c4e-4082-ae8d-47f4219a0642@intel.com>
Date: Fri, 15 Aug 2025 07:31:39 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, "Tian, Kevin"
 <kevin.tian@...el.com>, Jason Gunthorpe <jgg@...dia.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....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>,
 "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 v3 1/1] iommu/sva: Invalidate KVA range on kernel TLB
 flush

On 8/15/25 02:16, Baolu Lu wrote:
> On 8/8/2025 10:57 AM, Tian, Kevin wrote:
>> pud_free_pmd_page()
>>     ...
>>     for (i = 0; i < PTRS_PER_PMD; i++) {
>>         if (!pmd_none(pmd_sv[i])) {
>>             pte = (pte_t *)pmd_page_vaddr(pmd_sv[i]);
>>             pte_free_kernel(&init_mm, pte);
>>         }
>>     }
>>
>>     free_page((unsigned long)pmd_sv);
>>
>> Otherwise the risk still exists if the pmd page is repurposed before the
>> pte work is scheduled.
> 
> You're right that freeing high-level page table pages also requires an
> IOTLB flush before the pages are freed. But I question the practical
> risk of the race given the extremely small time window.

I hear that Linux is gaining popularity these days. There might even be
dozens of users! Given that large scale of dozens (or even hundreds??)
of users, I would suggest exercising some care. The race might be small
but it only needs to happen once to cause chaos.

Seriously, though... A race is a race. Preemption or interrupts or SMIs
or VMExits or a million other things can cause a "small time window" to
become a big time window.

Even perceived small races need to be fixed.

> If this is a real concern, a potential mitigation would be to clear
> the U/S bits in all page table entries for kernel address space? But
> I am not confident in making that change at this time as I am unsure
> of the side effects it might cause.

That doesn't do any good. I even went as far as double-checking months
ago with the IOMMU hardware folks to confirm the actual implementation.
I'm really surprised this is being brought up again.

>> another observation - pte_free_kernel is not used in remove_pagetable ()
>> and __change_page_attr(). Is it straightforward to put it in those paths
>> or do we need duplicate some deferring logic there?
> 
> The remove_pagetable() function is called in the path where memory is
> hot-removed from the system, right?

No. Not right.

This is in the vmalloc() code: the side of things that _creates_
mappings for new allocations, not tears them down.