linux-kernel - Re: [PATCH v2 4/4] mm: page_table_check: Ensure user pages are not slab pages

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Date:   Tue, 16 May 2023 14:54:04 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ruihan Li <lrh2000@....edu.cn>,
        Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     linux-mm@...ck.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        syzbot+fcf1a817ceb50935ce99@...kaller.appspotmail.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 4/4] mm: page_table_check: Ensure user pages are not
 slab pages

On 16.05.23 13:51, Ruihan Li wrote:
> On Mon, May 15, 2023 at 12:28:54PM -0400, Pasha Tatashin wrote:
>>
>> On Mon, May 15, 2023 at 9:10 AM Ruihan Li <lrh2000@....edu.cn> wrote:
>>>
>>> The current uses of PageAnon in page table check functions can lead to
>>> type confusion bugs between struct page and slab [1], if slab pages are
>>> accidentally mapped into the user space. This is because slab reuses the
>>> bits in struct page to store its internal states, which renders PageAnon
>>> ineffective on slab pages.
>>>
>>> Since slab pages are not expected to be mapped into the user space, this
>>> patch adds BUG_ON(PageSlab(page)) checks to make sure that slab pages
>>> are not inadvertently mapped. Otherwise, there must be some bugs in the
>>> kernel.
>>>
>>> Reported-by: syzbot+fcf1a817ceb50935ce99@...kaller.appspotmail.com
>>> Closes: https://lore.kernel.org/lkml/000000000000258e5e05fae79fc1@google.com/ [1]
>>> Fixes: df4e817b7108 ("mm: page table check")
>>> Cc: <stable@...r.kernel.org> # 5.17
>>> Signed-off-by: Ruihan Li <lrh2000@....edu.cn>
>>
>> Acked-by: Pasha Tatashin <pasha.tatashin@...een.com>
>>
>> I would also update order in mm/memory.c
>> static int validate_page_before_insert(struct page *page)
>> {
>> if (PageAnon(page) || PageSlab(page) || page_has_type(page))
>>
>> It is not strictly a bug there, as it works by accident, but
>> PageSlab() should go before PageAnon(), because without checking if
>> this is PageSlab() we should not be testing for PageAnon().
> 
> Right. Perhaps it would be better to send another patch for this
> separately.

Probably not really worth it IMHO. With PageSlab() we might have 
PageAnon() false-positives. Either will take the same path here ...

On a related note, stable_page_flags() checks PageKsm()/PageAnon() 
without caring about PageSlab().

At least it's just a debugging interface and will indicate KPF_SLAB in 
any case as well ...

-- 
Thanks,

David / dhildenb