linux-kernel - Re: [PATCH] arm64: Check pxd_leaf() instead of !pxd

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: <61da67fe-ba7a-4b9c-acb2-f1488f00a804@redhat.com>
Date: Thu, 15 May 2025 10:49:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, catalin.marinas@....com, will@...nel.org
Cc: ryan.roberts@....com, anshuman.khandual@....com, mark.rutland@....com,
 yang@...amperecomputing.com, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, stable@...r.kernel.org
Subject: Re: [PATCH] arm64: Check pxd_leaf() instead of !pxd_table() while
 tearing down page tables

On 15.05.25 10:40, Dev Jain wrote:
> 
> 
> On 15/05/25 2:06 pm, David Hildenbrand wrote:
>> On 15.05.25 10:22, Dev Jain wrote:
>>>
>>>
>>> On 15/05/25 1:43 pm, David Hildenbrand wrote:
>>>> On 15.05.25 08:34, Dev Jain wrote:
>>>>> Commit 9c006972c3fe removes the pxd_present() checks because the caller
>>>>> checks pxd_present(). But, in case of vmap_try_huge_pud(), the caller
>>>>> only
>>>>> checks pud_present(); pud_free_pmd_page() recurses on each pmd through
>>>>> pmd_free_pte_page(), wherein the pmd may be none.
>>>> The commit states: "The core code already has a check for pXd_none()",
>>>> so I assume that assumption was not true in all cases?
>>>>
>>>> Should that one problematic caller then check for pmd_none() instead?
>>>
>>>    From what I could gather of Will's commit message, my interpretation is
>>> that the concerned callers are vmap_try_huge_pud and vmap_try_huge_pmd.
>>> These individually check for pxd_present():
>>>
>>> if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr))
>>>      return 0;
>>>
>>> The problem is that vmap_try_huge_pud will also iterate on pte entries.
>>> So if the pud is present, then pud_free_pmd_page -> pmd_free_pte_page
>>> may encounter a none pmd and trigger a WARN.
>>
>> Yeah, pud_free_pmd_page()->pmd_free_pte_page() looks shaky.
>>
>> I assume we should either have an explicit pmd_none() check in
>> pud_free_pmd_page() before calling pmd_free_pte_page(), or one in
>> pmd_free_pte_page().
>>
>> With your patch, we'd be calling pte_free_kernel() on a NULL pointer,
>> which sounds wrong -- unless I am missing something important.
>>
>>>
>>>>
>>>> If you were able to trigger this WARN, it's always a good idea to
>>>> include the splat in the commit.
>>>
>>> I wasn't able to, it is just an observation from code inspection.
>>
>> That better be included in the patch description :)
> 
> I did, actually. My bad for not putting in spaces, I notice now that the
> description looks horrible to the eye :)

Ahh, there it is. Sorry :) Yeah, some empty lines won't hurt!

-- 
Cheers,

David / dhildenb