Re: [RFC PATCH 4/4] s390/mm: implement __ptent-aware pte_t type

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] In `set_pte()` when `STRICT_MM_TYPECHECKS` is defined, the `_PAGE_UNUSED` bit is incorrectly left asserted on present PTEs because a stale snapshot of the PTE is written to the page table instead of the updated value.
--

--- Patch [4]: [RFC PATCH 4/4] s390/mm: implement __ptent-aware pte_t type ---
commit 7300d6f6725c9361fd35e232d50cadaf34b8a823
Author: Alexander Gordeev <agordeev@linux.ibm.com>

s390/mm: implement __ptent-aware pte_t type

This commit introduces a new pteraw_t type to enforce strict mm type checks
for PTEs on s390, modifying how set_pte() and other helpers handle page
table entries.

[ ... ]

> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index e882663a58e77..473e6f58ff9e8 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h

[ ... ]

> @@ -978,6 +978,49 @@ static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
>  	WRITE_ONCE(*pmdp, pmd);
>  }
>  
> +#ifdef STRICT_MM_TYPECHECKS
> +static inline void set_pte(pte_t *ptep, pte_t pte)
> +{
> +	pteraw_t *pteraw_ptep = (__force pteraw_t *)ptep;
> +	pteraw_t pteraw = __pteraw(pte_val(pte));
> +
> +	if (pte_present(pte))
> +		pte = clear_pte_bit(pte, __pgprot(_PAGE_UNUSED));
> +	WRITE_ONCE(*pteraw_ptep, pteraw);

[Severity: High]
Does this WRITE_ONCE() write the unmodified original pte instead of the
cleared one?

Since pteraw is captured from the initial pte argument before clear_pte_bit()
is called, it looks like the _PAGE_UNUSED modification to pte is completely
ignored when writing to memory.

Should pteraw be updated with the modified pte before calling WRITE_ONCE()?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1784292223.git.agordeev@linux.ibm.com?part=4