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Date:   Wed, 13 Jul 2022 08:45:36 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Maurizio Lombardi <mlombard@redhat.com>
Cc:     alexander.duyck@gmail.com, kuba@kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        chen45464546@163.com
Subject: Re: [PATCH V2] mm: prevent page_frag_alloc() from corrupting the
 memory
Message-Id: <20220713084536.1af461b016a16c58baad7db2@linux-foundation.org>
In-Reply-To: <20220713150143.147537-1-mlombard@redhat.com>
References: <20220713150143.147537-1-mlombard@redhat.com>
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On Wed, 13 Jul 2022 17:01:43 +0200 Maurizio Lombardi <mlombard@redhat.com> wrote:

> A number of drivers call page_frag_alloc() with a
> fragment's size > PAGE_SIZE.
> In low memory conditions, __page_frag_cache_refill() may fail the order 3
> cache allocation and fall back to order 0;
> In this case, the cache will be smaller than the fragment, causing
> memory corruptions.
> 
> Prevent this from happening by checking if the newly allocated cache
> is large enough for the fragment; if not, the allocation will fail
> and page_frag_alloc() will return NULL.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5617,6 +5617,8 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
>  		/* reset page count bias and offset to start of new frag */
>  		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
>  		offset = size - fragsz;
> +		if (unlikely(offset < 0))
> +			return NULL;
>  	}
>  
>  	nc->pagecnt_bias--;

I think we should have a comment here explaining (at least) why we'd
bale after a successful allocation and explaining why we don't call
free_the_page().