Re: [PATCH] arm64/mm: restrict transparent contpte mappings to valid PTEs

[Lance Yang]

On 2026/10/14 18:03, Dev Jain wrote:
> On 14/10/26 12:01 pm, Zhao Gongyi wrote:
> > On 10/13/26 11:39 PM, Lance Yang wrote:
> > > > +	/*
> > > > +	 * Keep the producer-side invariant documented in pgtable.h true even
> > > > +	 * if a direct caller passes an invalid descriptor with PTE_CONT set.
> > > > +	 */
> > > > +	if (!pte_valid(pte))
> > > > +		return __set_ptes(mm, addr, ptep, pte_mknoncont(pte), nr);
> > >
> > > set_ptes() is the only in-tree caller, and it already clears PTE_CONT
> > > before we get here ... no? Why not fold !pte_valid(pte) into the
> > > !mm_is_user(mm) check below and return __set_ptes() directly?
> > >
> > > 	if (!mm_is_user(mm) || !pte_valid(pte))
> > > 		return __set_ptes(mm, addr, ptep, pte, nr);
> > >
> > > If a direct caller does pass a !pte_valid PTE with PTE_CONT set, I'd
> > > rather add a VM_WARN_ON() to catch that. Silently clearing the bit would
> > > hide a caller bug :)
> >
> > Hi Lance,
> >
> > Thanks for taking a look.
> >
> > Yes, you are right. The public set_ptes() wrapper already strips PTE_CONT
> > before calling contpte_set_ptes(), so the pte_mknoncont() in the invalid
> > path is unnecessary for the current in-tree path.
> >
> > The issue I was trying to avoid is not that the incoming pte already has
> > PTE_CONT set, but that contpte_set_ptes() rebuilds the pte from pfn/prot
> > and then adds PTE_CONT again based on alignment. For an invalid descriptor,
> > that transparent contiguous encoding should be skipped entirely.
> >
> > So your suggested shape makes sense to me. I will change this to return
> > through __set_ptes() directly for invalid descriptors, and add a warning
> > for any unexpected direct caller that passes an invalid PTE with PTE_CONT
> > set, e.g.:
> >
> > 	if (!mm_is_user(mm) || !pte_valid(pte)) {
> > 		VM_WARN_ON_ONCE(!pte_valid(pte) && pte_cont(pte));
>
> set_ptes does pte_mknoncont(), so the warning will *never* fire I think?

YES. Just a sanity check to catch a future direct caller passing
a !pte_valid PTE with PTE_CONT set, rather than silently clearing
the bit :)

Cheers, Lance

> > 		return __set_ptes(mm, addr, ptep, pte, nr);
> > 	}
> >
> > That keeps the fix focused on not creating new invalid contpte entries,
> > while still catching a bad direct caller instead of silently sanitizing it.
> >
> > Thanks,
> > Zhao