linux-kernel - Re: [PATCH v3] huge_mm.h: disallow is_huge_zero

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: <da6bd1ed-3c35-4f5d-b667-a21a47585fb0@lucifer.local>
Date: Thu, 28 Aug 2025 10:35:06 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Max Kellermann <max.kellermann@...os.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
        baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
        npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
        baohua@...nel.org, shikemeng@...weicloud.com, kasong@...cent.com,
        nphamcs@...il.com, bhe@...hat.com, chrisl@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] huge_mm.h: disallow is_huge_zero_folio(NULL)

Please don't send this in reply to previous versions, just send as a
separate mail :) (yes email for dev sucks and there's a bunch of weird
nuances so totally understandable!)

Hopefully Andrew will pick this up regardless.

On Thu, Aug 28, 2025 at 10:48:20AM +0200, Max Kellermann wrote:
> Calling is_huge_zero_folio(NULL) should not be legal - it makes no
> sense, and a different (theoretical) implementation may dereference
> the pointer.  But currently, lacking any explicit documentation, this
> call is possible.
>
> But if somebody really passes NULL, the function should not return
> true - this isn't the huge zero folio after all!  However, if the
> `huge_zero_folio` hasn't been allocated yet, it's NULL, and
> is_huge_zero_folio(NULL) just happens to return true, which is a lie.
>
> This weird side effect prevented me from reproducing a kernel crash
> that occurred when the elements of a folio_batch were NULL - since
> folios_put_refs() skips huge zero folios, this sometimes causes a
> crash, but sometimes does not.  For debugging, it is better to reveal
> such bugs reliably and not hide them behind random preconditions like
> "has the huge zero folio already been created?"
>
> To improve detection of such bugs, David Hildenbrand suggested adding
> a VM_WARN_ON_ONCE().
>
> Signed-off-by: Max Kellermann <max.kellermann@...os.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
> v1->v2: using VM_WARN_ON_ONCE() instead of checking huge_zero_folio
> v2->v3: use "!" to check NULL; removed the #include

Putting the history in is great though thanks! :)

> ---
>  include/linux/huge_mm.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 7748489fde1b..96ac47603d97 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -479,6 +479,8 @@ extern unsigned long huge_zero_pfn;
>
>  static inline bool is_huge_zero_folio(const struct folio *folio)
>  {
> +	VM_WARN_ON_ONCE(!folio);
> +
>  	return READ_ONCE(huge_zero_folio) == folio;
>  }
>
> --
> 2.47.2
>