linux-kernel - Re: [PATCH v5 4/7] mm: Introduce FPB_RESPECT

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] [day] [month] [year] [list]

Message-ID: <ac0b2fcd-6f90-4802-be46-dc1c20d55d2e@arm.com>
Date: Tue, 22 Jul 2025 12:37:55 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org
Cc: david@...hat.com, willy@...radead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, catalin.marinas@....com, will@...nel.org,
 Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com, vbabka@...e.cz,
 jannh@...gle.com, anshuman.khandual@....com, peterx@...hat.com,
 joey.gouly@....com, ioworker0@...il.com, baohua@...nel.org,
 kevin.brodsky@....com, quic_zhenhuah@...cinc.com,
 christophe.leroy@...roup.eu, yangyicong@...ilicon.com,
 linux-arm-kernel@...ts.infradead.org, hughd@...gle.com,
 yang@...amperecomputing.com, ziy@...dia.com
Subject: Re: [PATCH v5 4/7] mm: Introduce FPB_RESPECT_WRITE for PTE batching
 infrastructure

On 18/07/2025 10:02, Dev Jain wrote:
> Patch 6 optimizes mprotect() by batch clearing the ptes, masking in the new
> protections, and batch setting the ptes. Suppose that the first pte
> of the batch is writable - with the current implementation of
> folio_pte_batch(), it is not guaranteed that the other ptes in the batch
> are already writable too, so we may incorrectly end up setting the
> writable bit on all ptes via modify_prot_commit_ptes().
> 
> Therefore, introduce FPB_RESPECT_WRITE so that all ptes in the batch
> are writable or not.
> 
> Signed-off-by: Dev Jain <dev.jain@....com>

Reviewed-by: Ryan Roberts <ryan.roberts@....com>

> ---
>  mm/internal.h | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 5b0f71e5434b..28d2d5b051df 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -208,17 +208,20 @@ typedef int __bitwise fpb_t;
>  /* Compare PTEs respecting the soft-dirty bit. */
>  #define FPB_RESPECT_SOFT_DIRTY		((__force fpb_t)BIT(1))
>  
> +/* Compare PTEs respecting the writable bit. */
> +#define FPB_RESPECT_WRITE		((__force fpb_t)BIT(2))
> +
>  /*
>   * Merge PTE write bits: if any PTE in the batch is writable, modify the
>   * PTE at @ptentp to be writable.
>   */
> -#define FPB_MERGE_WRITE			((__force fpb_t)BIT(2))
> +#define FPB_MERGE_WRITE			((__force fpb_t)BIT(3))
>  
>  /*
>   * Merge PTE young and dirty bits: if any PTE in the batch is young or dirty,
>   * modify the PTE at @ptentp to be young or dirty, respectively.
>   */
> -#define FPB_MERGE_YOUNG_DIRTY		((__force fpb_t)BIT(3))
> +#define FPB_MERGE_YOUNG_DIRTY		((__force fpb_t)BIT(4))
>  
>  static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>  {
> @@ -226,7 +229,9 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>  		pte = pte_mkclean(pte);
>  	if (likely(!(flags & FPB_RESPECT_SOFT_DIRTY)))
>  		pte = pte_clear_soft_dirty(pte);
> -	return pte_wrprotect(pte_mkold(pte));
> +	if (likely(!(flags & FPB_RESPECT_WRITE)))
> +		pte = pte_wrprotect(pte);
> +	return pte_mkold(pte);
>  }
>  
>  /**