Message-ID: <a93988da-80fb-dd32-4717-a6a0bae9e4ee@huawei.com>
Date:   Fri, 14 Jan 2022 13:00:27 +0800
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Subject: Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page
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To:     David Hildenbrand <david@redhat.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        <wangzhigang17@huawei.com>, Matthew Wilcox <willy@infradead.org>,
        "Linus Torvalds" <torvalds@linux-foundation.org>
References: <20220113140318.11117-1-zhangliang5@huawei.com>
 <YeA5oP/iaxtVPHb3@casper.infradead.org>
 <CAHk-=wjB0i-B=U-DhpAajQx3f6bp1X==neLOrg0jwq29mgz=3g@mail.gmail.com>
 <172ccfbb-7e24-db21-7d84-8c8d8c3805fd@redhat.com>
From:   "zhangliang (AG)" <zhangliang5@huawei.com>
In-Reply-To: <172ccfbb-7e24-db21-7d84-8c8d8c3805fd@redhat.com>
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On 2022/1/14 0:48, David Hildenbrand wrote:
> On 13.01.22 17:37, Linus Torvalds wrote:
>> On Thu, Jan 13, 2022 at 6:39 AM Matthew Wilcox <willy@infradead.org> wrote:
>>>
>>> Let's bring Linus in on this, but I think this reintroduces all of the
>>> mapcount problems that we've been discussing recently.
>>>
>>> How about this as an alternative?
>>
>> No, at that point reuse_swap_page() is the better thing to do.
>>
>> Don't play games with page_count() (or even worse games with
>> swap_count). The page count is only stable if it's 1. Any other value
>> means that it can fluctuate due to concurrent lookups, some of which
>> can be done locklessly under RCU.
> 
> I'm pretty sure the patch as is will reintroduce the CVE. So I think in

Actually, I wonder how reuse_swap_page() in this patch can reintroduce CVE,
I think the invoking logic here is as same as that in do_swap_page(). 
So, could you give me some hint about this? Thanks :)

> addition to the reuse_swap_page() check we need more.
> 
> I'm wondering if we can get rid of the mapcount checks in
> reuse_swap_page() and instead check for page_count() and swapcount only.
> 
> We don't care if it's unstable in a sense than it will be bigger than
> what we expect. In that case we COW as we would already do.
> 
> Thoughts?
> 

-- 
Best Regards,
Liang Zhang