Re: [PATCH v2 6/6] mm/mprotect: use huge_ptep_get() for hugetlb

[Dev Jain <dev.jain@arm.com>]

On 02/07/26 10:43 am, Dev Jain wrote:
> prot_none_hugetlb_entry() is the hugetlb callback for the early
> mprotect(PROT_NONE) PFN permission walk on x86.
> 
> The callback passes the decoded PFN to pfn_modify_allowed(). For a
> hugetlb callback, the pte pointer refers to a hugetlb entry. On
> architectures where hugetlb entries need huge_ptep_get(), reading that
> entry with ptep_get() can make the permission check use the wrong PFN.
> 
> Use huge_ptep_get() before decoding the hugetlb PFN.
> 
> Currently there is no path which can trigger a bug: huge_ptep_get() is a
> simple ptep_get() for x86, and the prot_none walk occurs only for x86.
> 
> So no need to backport - use the correct helper anyways.
> 
> Fixes: 42e4089c7890 ("x86/speculation/l1tf: Disallow non privileged high MMIO PROT_NONE mappings")
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---

@Muchun, I did not do the change you suggested: if we do #ifdef around
the hugetlb callback, then for CONFIG_HUGETLB_PAGE=n, prot_none_hugetlb_entry()
is defined but not used, giving compile error.


>  mm/mprotect.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 9cbf932b028cf..23779632d18bf 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -699,14 +699,20 @@ static int prot_none_pte_entry(pte_t *pte, unsigned long addr,
>  		0 : -EACCES;
>  }
>  
> +#ifdef CONFIG_HUGETLB_PAGE
>  static int prot_none_hugetlb_entry(pte_t *pte, unsigned long hmask,
>  				   unsigned long addr, unsigned long next,
>  				   struct mm_walk *walk)
>  {
> -	return pfn_modify_allowed(pte_pfn(ptep_get(pte)),
> +	pte_t entry = huge_ptep_get(walk->mm, addr, pte);
> +
> +	return pfn_modify_allowed(pte_pfn(entry),
>  				  *(pgprot_t *)(walk->private)) ?
>  		0 : -EACCES;
>  }
> +#else
> +#define prot_none_hugetlb_entry	NULL
> +#endif
>  
>  static int prot_none_test(unsigned long addr, unsigned long next,
>  			  struct mm_walk *walk)