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

[Lance Yang <lance.yang@linux.dev>]

On Tue, Oct 13, 2026 at 05:36:00PM +0800, Zhao Gongyi wrote:
[...]
>
>Fixes: 4602e5757bcc ("arm64/mm: wire up PTE_CONT for user mappings")
>Cc: stable@vger.kernel.org
>Signed-off-by: Zhao Gongyi <zhaogongyi@bytedance.com>
>---

Good catch! Thanks!

[...]
>diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>index 0acab179f..851fa2e44 100644
>--- a/arch/arm64/mm/contpte.c
>+++ b/arch/arm64/mm/contpte.c
>@@ -463,6 +463,13 @@ void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
> 	if (!mm_is_user(mm))
> 		return __set_ptes(mm, addr, ptep, pte, nr);
> 
>+	/*
>+	 * 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 :)

Cheers, Lance

>+
> 	end = addr + (nr << PAGE_SHIFT);
> 	pfn = pte_pfn(pte);
> 	prot = pte_pgprot(pte);
>-- 
>2.39.5 (Apple Git-154)
>