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Date:   Tue, 22 Feb 2022 10:17:02 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, ccross@google.com,
        sumit.semwal@linaro.org, dave.hansen@intel.com,
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Subject: Re: [PATCH 2/3] mm: prevent vm_area_struct::anon_name refcount
 saturation
Message-ID: <YhSqDvSTN7nbz4C9@dhcp22.suse.cz>
References: <20220222054025.3412898-1-surenb@google.com>
 <20220222054025.3412898-2-surenb@google.com>
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On Mon 21-02-22 21:40:24, Suren Baghdasaryan wrote:
> A deep process chain with many vmas could grow really high.

This would really benefit from some numbers. With default
sysctl_max_map_count (64k) and default pid_max (32k) the INT_MAX could
be theoretically reached but I find it impractical because not all vmas
can be anonymous same as all available pids can be consumed for a
theoretical attack (if my counting is proper).
On the other hand any non-default configuration with any of the values
increased could hit this theoretically.

> kref
> refcounting interface used in anon_vma_name structure will detect
> a counter overflow when it reaches REFCOUNT_SATURATED value but will
> only generate a warning about broken refcounting.
> To ensure anon_vma_name refcount does not overflow, stop anon_vma_name
> sharing when the refcount reaches INT_MAX, which still leaves INT_MAX/2
> values before the counter reaches REFCOUNT_SATURATED. This should provide
> enough headroom for raising the refcounts temporarily.
> 
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  include/linux/mm_inline.h | 18 ++++++++++++++----
>  mm/madvise.c              |  3 +--
>  2 files changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index 70b619442d56..b189e2638843 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -156,15 +156,25 @@ static inline void anon_vma_name_get(struct anon_vma_name *anon_name)
>  
>  extern void anon_vma_name_put(struct anon_vma_name *anon_name);
>  
> +static inline
> +struct anon_vma_name *anon_vma_name_reuse(struct anon_vma_name *anon_name)
> +{
> +	/* Prevent anon_name refcount saturation early on */
> +	if (kref_read(&anon_name->kref) < INT_MAX) {

REFCOUNT_MAX seems to be defined by the kref framework.

Other than that looks good to me.

> +		anon_vma_name_get(anon_name);
> +		return anon_name;
> +
> +	}
> +	return anon_vma_name_alloc(anon_name->name);
> +}
> +
>  static inline void dup_vma_anon_name(struct vm_area_struct *orig_vma,
>  				     struct vm_area_struct *new_vma)
>  {
>  	struct anon_vma_name *anon_name = vma_anon_name(orig_vma);
>  
> -	if (anon_name) {
> -		anon_vma_name_get(anon_name);
> -		new_vma->anon_name = anon_name;
> -	}
> +	if (anon_name)
> +		new_vma->anon_name = anon_vma_name_reuse(anon_name);
>  }
>  
>  static inline void free_vma_anon_name(struct vm_area_struct *vma)
> diff --git a/mm/madvise.c b/mm/madvise.c
> index f81d62d8ce9b..a395884aeecb 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -122,8 +122,7 @@ static int replace_vma_anon_name(struct vm_area_struct *vma,
>  	if (anon_vma_name_eq(orig_name, anon_name))
>  		return 0;
>  
> -	anon_vma_name_get(anon_name);
> -	vma->anon_name = anon_name;
> +	vma->anon_name = anon_vma_name_reuse(anon_name);
>  	anon_vma_name_put(orig_name);
>  
>  	return 0;
> -- 
> 2.35.1.473.g83b2b277ed-goog

-- 
Michal Hocko
SUSE Labs