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

[Mike Rapoport <rppt@kernel.org>]

On Mon, Jul 14, 2025 at 03:19:17PM +0200, Uladzislau Rezki wrote:
> On Mon, Jul 14, 2025 at 01:39:20PM +0100, David Laight wrote:
> > On Wed, 9 Jul 2025 11:22:34 -0700
> > Dave Hansen <dave.hansen@intel.com> wrote:
> > 
> > > On 7/9/25 11:15, Jacob Pan wrote:
> > > >>> Is there a use case where a SVA user can access kernel memory in the
> > > >>> first place?    
> > > >> No. It should be fully blocked.
> > > >>  
> > > > Then I don't understand what is the "vulnerability condition" being
> > > > addressed here. We are talking about KVA range here.  
> > > 
> > > SVA users can't access kernel memory, but they can compel walks of
> > > kernel page tables, which the IOMMU caches. The trouble starts if the
> > > kernel happens to free that page table page and the IOMMU is using the
> > > cache after the page is freed.
> > > 
> > > That was covered in the changelog, but I guess it could be made a bit
> > > more succinct.

But does this really mean that every flush_tlb_kernel_range() should flush
the IOMMU page tables as well? AFAIU, set_memory flushes TLB even when bits
in pte change and it seems like an overkill...

> > Is it worth just never freeing the page tables used for vmalloc() memory?
> > After all they are likely to be reallocated again.
> > 
> >
> Do we free? Maybe on some arches? According to my tests(AMD x86-64) i did
> once upon a time, the PTE entries were not freed after vfree(). It could be
> expensive if we did it, due to a global "page_table_lock" lock.
> 
> I see one place though, it is in the vmap_try_huge_pud()
> 
> 	if (pud_present(*pud) && !pud_free_pmd_page(pud, addr))
> 		return 0;
> 
> it is when replace a pud by a huge-page.

There's also a place that replaces a pmd by a smaller huge page, but other
than that vmalloc does not free page tables.

> --
> Uladzislau Rezki

-- 
Sincerely yours,
Mike.