linux-kernel - Re: [PATCH STABLE 4.4 5/8] mm: prevent get_user

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Date:   Fri, 6 Dec 2019 04:15:23 +0000
From:   Ajay Kaher <akaher@...are.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "stable@...nel.org" <stable@...nel.org>,
        Srivatsa Bhat <srivatsab@...are.com>,
        "srivatsa@...il.mit.edu" <srivatsa@...il.mit.edu>,
        Vasavi Sirnapalli <vsirnapalli@...are.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH STABLE 4.4 5/8] mm: prevent get_user_pages() from
 overflowing page refcount



On 03/12/19, 6:28 PM, "Vlastimil Babka" <vbabka@...e.cz> wrote:
>>>    
>>> [ 4.4 backport: there's get_page_foll(), so add try_get_page()-like checks
>>>                 in there, enabled by a new parameter, which is false where
>>>                 upstream patch doesn't replace get_page() with try_get_page()
>>>                 (the THP and hugetlb callers).
>> 
>> Could we have try_get_page_foll(), as in:
>> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fstable%2F1570581863-12090-3-git-send-email-akaher%40vmware.com%2F&amp;data=02%7C01%7Cakaher%40vmware.com%7Cb6592f0fbec040aa045f08d777f06a9f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637109746821395444&amp;sdata=cYBj3SvEikPbiHsVZj3zCys8t9ISLiHKzAlsSqiZRW8%3D&amp;reserved=0
>> 
>> + Code will be in sync as we have try_get_page()
>> + No need to add extra argument to try_get_page()
>> + No need to modify the callers of try_get_page()

Any reason for not using try_get_page_foll().

>>> 		In gup_pte_range(), we don't expect tail pages, so just check
>>>                 page ref count instead of try_get_compound_head()
>> 
>> Technically it's fine. If you want to keep the code of stable versions in sync
>> with latest versions then this could be done in following ways (without any
>> modification in upstream patch for gup_pte_range()):
>> 
>> Apply 7aef4172c7957d7e65fc172be4c99becaef855d4 before applying
>> 8fde12ca79aff9b5ba951fce1a2641901b8d8e64, as done here:
>> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fstable%2F1570581863-12090-4-git-send-email-akaher%40vmware.com%2F&amp;data=02%7C01%7Cakaher%40vmware.com%7Cb6592f0fbec040aa045f08d777f06a9f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637109746821395444&amp;sdata=gTJMJ3Yx6G0ng46TQsBzCS2DowwP7YtIjluKJuqvN6o%3D&amp;reserved=0
    
> Yup, I have considered that, and deliberately didn't add that commit
> 7aef4172c795 ("mm: handle PTE-mapped tail pages in gerneric fast gup
> implementaiton") as it's part of a large THP refcount rework. In 4.4 we
> don't expect to GUP tail pages so I wanted to keep it that way -
> minimally, the compound_head() operation is a unnecessary added cost,
> although it would also work.