linux-kernel - Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1674efca-6d4e-4247-8b1c-b6816360d8bb@redhat.com>
Date: Fri, 17 Oct 2025 10:11:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Wei Yang <richard.weiyang@...il.com>, Dev Jain <dev.jain@....com>
Cc: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 lorenzo.stoakes@...cle.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 baohua@...nel.org, ioworker0@...il.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with
 pte_present()

On 17.10.25 03:27, Wei Yang wrote:
> On Thu, Oct 16, 2025 at 11:47:06AM +0530, Dev Jain wrote:
>>
>> On 16/10/25 9:06 am, Lance Yang wrote:
>>> From: Lance Yang <lance.yang@...ux.dev>
>>>
>>> A non-present entry, like a swap PTE, contains completely different data
>>> (swap type and offset). pte_pfn() doesn't know this, so if we feed it a
>>> non-present entry, it will spit out a junk PFN.
>>>
>>> What if that junk PFN happens to match the zeropage's PFN by sheer
>>> chance? While really unlikely, this would be really bad if it did.
>>>
>>> So, let's fix this potential bug by ensuring all calls to is_zero_pfn()
>>> in khugepaged.c are properly guarded by a pte_present() check.
>>>
>>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>>> ---
>>>    mm/khugepaged.c | 13 ++++++++-----
>>>    1 file changed, 8 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index d635d821f611..0341c3d13e9e 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -516,7 +516,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
>>>    		pte_t pteval = ptep_get(_pte);
>>>    		unsigned long pfn;
>>> -		if (pte_none(pteval))
>>> +		if (!pte_present(pteval))
>>>    			continue;
>>>    		pfn = pte_pfn(pteval);
>>>    		if (is_zero_pfn(pfn))
>>> @@ -690,9 +690,10 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>>>    	     address += nr_ptes * PAGE_SIZE) {
>>>    		nr_ptes = 1;
>>>    		pteval = ptep_get(_pte);
>>> -		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>>> +		if (pte_none(pteval) ||
>>> +		    (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) {
>>>    			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
>>> -			if (is_zero_pfn(pte_pfn(pteval))) {
>>> +			if (!pte_none(pteval)) {
>>
>> Could save a level of indentation by saying
>> if (pte_none(pteval))
>>     continue;
>>
> 
> Vote for this :-)

I suspect there will be a v2, correct?

-- 
Cheers

David / dhildenb