linux-kernel - Re: [PATCH v3 1/9] mm: add overflow and underflow checks for page->

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Message-ID: <72cae3c0-e06e-4fe5-24d5-a2c94d99780f@suse.cz>
Date:   Thu, 27 Jan 2022 19:30:59 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Pasha Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-m68k@...ts.linux-m68k.org, anshuman.khandual@....com,
        willy@...radead.org, akpm@...ux-foundation.org,
        william.kucharski@...cle.com, mike.kravetz@...cle.com,
        geert@...ux-m68k.org, schmitzmic@...il.com, rostedt@...dmis.org,
        mingo@...hat.com, hannes@...xchg.org, guro@...com,
        songmuchun@...edance.com, weixugc@...gle.com, gthelen@...gle.com,
        rientjes@...gle.com, pjt@...gle.com, hughd@...gle.com
Subject: Re: [PATCH v3 1/9] mm: add overflow and underflow checks for
 page->_refcount

On 1/26/22 19:34, Pasha Tatashin wrote:
> The problems with page->_refcount are hard to debug, because usually
> when they are detected, the damage has occurred a long time ago. Yet,
> the problems with invalid page refcount may be catastrophic and lead to
> memory corruptions.
> 
> Reduce the scope of when the _refcount problems manifest themselves by
> adding checks for underflows and overflows into functions that modify
> _refcount.
> 
> Use atomic_fetch_* functions to get the old values of the _refcount,
> and use it to check for overflow/underflow.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
>  include/linux/page_ref.h | 59 +++++++++++++++++++++++++++++-----------
>  1 file changed, 43 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index 2e677e6ad09f..fe4864f7f69c 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -117,7 +117,10 @@ static inline void init_page_count(struct page *page)
>  
>  static inline void page_ref_add(struct page *page, int nr)
>  {
> -	atomic_add(nr, &page->_refcount);
> +	int old_val = atomic_fetch_add(nr, &page->_refcount);
> +	int new_val = old_val + nr;
> +
> +	VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);

This seems somewhat weird, as it will trigger not just on overflow, but also
if nr is negative. Which I think is valid usage, even though the function
has 'add' in name, because 'nr' is signed?