linux-kernel - Re: [PATCH] iommu/intel: Fix __domain_mapping()'s usage of switch_to_super

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Message-ID: <e119e295535deafd93e283db143e9f17b1148169.camel@infradead.org>
Date: Thu, 28 Aug 2025 08:29:40 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Baolu Lu <baolu.lu@...ux.intel.com>, Eugene Koira <eugkoira@...zon.com>,
  iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc: joro@...tes.org, will@...nel.org, robin.murphy@....com,
 longpeng2@...wei.com,  graf@...zon.de, nsaenz@...zon.com,
 nh-open-source@...zon.com,  stable@...r.kernel.org
Subject: Re: [PATCH] iommu/intel: Fix __domain_mapping()'s usage of
 switch_to_super_page()

On Thu, 2025-08-28 at 14:33 +0800, Baolu Lu wrote:
> On 8/26/25 23:48, David Woodhouse wrote:
> > On Tue, 2025-08-26 at 14:38 +0000, Eugene Koira wrote:
> > > switch_to_super_page() assumes the memory range it's working on is aligned
> > > to the target large page level. Unfortunately, __domain_mapping() doesn't
> > > take this into account when using it, and will pass unaligned ranges
> > > ultimately freeing a PTE range larger than expected.
> > > 
> > > Take for example a mapping with the following iov_pfn range [0x3fe400,
> > > 0x4c0600], which should be backed by the following mappings:
> > 
> > The range is [0x3fe400, 0x4c0600) ?
> > 
> > >     iov_pfn [0x3fe400, 0x3fffff] covered by 2MiB pages
> > >     iov_pfn [0x400000, 0x4bffff] covered by 1GiB pages
> > >     iov_pfn [0x4c0000, 0x4c05ff] covered by 2MiB pages
> > > 
> > > Under this circumstance, __domain_mapping() will pass [0x400000, 0x4c05ff]
> > > to switch_to_super_page() at a 1 GiB granularity, which will in turn
> > > free PTEs all the way to iov_pfn 0x4fffff.
> > > 
> > > Mitigate this by rounding down the iov_pfn range passed to
> > > switch_to_super_page() in __domain_mapping()
> > > to the target large page level.
> > > 
> > > Additionally add range alignment checks to switch_to_super_page.
> > > 
> > > Fixes: 9906b9352a35 ("iommu/vt-d: Avoid duplicate removing in __domain_mapping()")
> > > Signed-off-by: Eugene Koira <eugkoira@...zon.com>
> > > Cc: stable@...r.kernel.org
> > > ---
> > >   drivers/iommu/intel/iommu.c | 7 ++++++-
> > >   1 file changed, 6 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> > > index 9c3ab9d9f69a..dff2d895b8ab 100644
> > > --- a/drivers/iommu/intel/iommu.c
> > > +++ b/drivers/iommu/intel/iommu.c
> > > @@ -1575,6 +1575,10 @@ static void switch_to_super_page(struct dmar_domain *domain,
> > >   	unsigned long lvl_pages = lvl_to_nr_pages(level);
> > >   	struct dma_pte *pte = NULL;
> > >   
> > > +	if (WARN_ON(!IS_ALIGNED(start_pfn, lvl_pages) ||
> > > +		    !IS_ALIGNED(end_pfn + 1, lvl_pages)))
> > > +		return;
> > > +
> > >   	while (start_pfn <= end_pfn) {
> > >   		if (!pte)
> > >   			pte = pfn_to_dma_pte(domain, start_pfn, &level,
> > > @@ -1650,7 +1654,8 @@ __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
> > >   				unsigned long pages_to_remove;
> > >   
> > >   				pteval |= DMA_PTE_LARGE_PAGE;
> > > -				pages_to_remove = min_t(unsigned long, nr_pages,
> > > +				pages_to_remove = min_t(unsigned long,
> > > +							round_down(nr_pages, lvl_pages),
> > >   							nr_pte_to_next_page(pte) * lvl_pages);
> > >   				end_pfn = iov_pfn + pages_to_remove - 1;
> > >   				switch_to_super_page(domain, iov_pfn, end_pfn, largepage_lvl);
> > 
> > I'm mildly entertained by the fact that the *only* comment in this
> > block of code is a lie. Would you care to address that while you're
> > here? Maybe the comment could look something like...
> > 
> > 			/* If the new mapping is eligible for a large page, then
> > 			 * remove all smaller pages that the existing pte at this
> > 			 * level references.
> > 			 * XX: do we even need to bother calling switch_to_super_page()
> > 			 * if this PTE wasn't *present* before?
> > 			 */
> > 
> > I bet it would benefit from one or two other one-line comments to make
> > it clearer what's going on, too...
> > 
> > But in general, I think this looks sane even though this code makes my
> > brain hurt. Could do with a test case, in an ideal world. Maybe we can
> > work on that as part of the generic pagetable support which is coming?
> 
> Agreed. The generic pagetable work will replace this code, so this will
> be removed. Therefore, we need a fix patch that can be backported before
> the generic pagetable for vt-d lands.

Yep. And since Jason has in fact already posted the patches which
*delete* all this code, I don't think I even care about fixing the
comments. Eugene's patch is fine as-is.

Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>

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