Re: [PATCH 5/5] mm/mprotect: use huge_ptep_get() for hugetlb

[Muchun Song <muchun.song@linux.dev>]

> On Jun 26, 2026, at 12:08, Dev Jain <dev.jain@arm.com> wrote:
> 
> 
> 
> On 26/06/26 9:10 am, Muchun Song wrote:
>> 
>> 
>> On 2026/6/25 19:29, 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.
>>> But 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>
>>> ---
>>>   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
>> 
>> This is very strange, because we defined a stub as NULL for a helper
> 
> I was following pattern elsewhere, search for ".hugetlb_entry" in the
> codebase and you will find others doing the same.

Okay, I understand why you want to do it that way, but I would still
recommend not following that format.

Thanks.

> 
> 
>> function. How about  the following diff?
>> 
>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>> index 9cbf932b028c..4d8c1551fbce 100644
>> --- a/mm/mprotect.c
>> +++ b/mm/mprotect.c
>> @@ -716,7 +716,9 @@ static int prot_none_test(unsigned long addr, unsigned long next,
>> 
>>  static const struct mm_walk_ops prot_none_walk_ops = {
>>         .pte_entry              = prot_none_pte_entry,
>> +#ifdef CONFIG_HUGETLB_PAGE
>>         .hugetlb_entry          = prot_none_hugetlb_entry,
>> +#endif
>>         .test_walk              = prot_none_test,
>>         .walk_lock              = PGWALK_WRLOCK,
>>  };
>> 
>> Thanks,
>> Muchun
>> 
>>> +#endif
>>>     static int prot_none_test(unsigned long addr, unsigned long next,
>>>                 struct mm_walk *walk)