linux-kernel - Re: [PATCH 1/2] huge_mm.h: 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: <733CF0F6-6936-4765-8EC5-2430E683C717@nvidia.com>
Date: Tue, 26 Aug 2025 21:19:38 -0400
From: Zi Yan <ziy@...dia.com>
To: Max Kellermann <max.kellermann@...os.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.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 1/2] huge_mm.h: is_huge_zero_folio(NULL) should return
 false

On 26 Aug 2025, at 19:16, 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 legal.
>
> 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.
>
> I believe this is a negligible corner case and I don't want to add any
> overhead for this; but in debugging kernels, it may be helpful to add
> this check, therefore I put it inside an `#ifdef CONFIG_DEBUG_VM`.

IMHO, we should make is_huge_zero_folio() return the correct answer.
Adding hzf == NULL check should not add much overhead. So removing
#ifdef is better.

>
> 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?"
>
> Signed-off-by: Max Kellermann <max.kellermann@...os.com>
> ---
>  include/linux/huge_mm.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 7748489fde1b..e4c617c0b445 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -479,7 +479,12 @@ extern unsigned long huge_zero_pfn;
>
>  static inline bool is_huge_zero_folio(const struct folio *folio)
>  {
> -	return READ_ONCE(huge_zero_folio) == folio;
> +	const struct folio *hzf = READ_ONCE(huge_zero_folio);
> +#ifdef CONFIG_DEBUG_VM
> +	if (hzf == NULL)
> +		return false;
> +#endif
> +	return hzf == folio;
>  }
>
>  static inline bool is_huge_zero_pfn(unsigned long pfn)
> -- 
> 2.47.2


--
Best Regards,
Yan, Zi