linux-kernel - Re: [PATCH v2 6/8] mm: prevent get_user_pages() from overflowing page refcount

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Date:   Thu, 17 Oct 2019 16:28:56 +0000
From:   Ajay Kaher <akaher@...are.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "punit.agrawal@....com" <punit.agrawal@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "mszeredi@...hat.com" <mszeredi@...hat.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Srivatsa Bhat <srivatsab@...are.com>,
        "srivatsa@...il.mit.edu" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        Srinidhi Rao <srinidhir@...are.com>,
        Vikash Bansal <bvikas@...are.com>,
        Anish Swaminathan <anishs@...are.com>,
        Vasavi Sirnapalli <vsirnapalli@...are.com>,
        Steven Rostedt <srostedt@...are.com>,
        "stable@...nel.org" <stable@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>
Subject: Re: [PATCH v2 6/8] mm: prevent get_user_pages() from overflowing page
 refcount


On 09/10/19, 6:43 PM, "Vlastimil Babka" <vbabka@...e.cz> wrote:

>> Reported-by: Jann Horn <jannh@...gle.com>
>> Acked-by: Matthew Wilcox <willy@...radead.org>
>> Cc: stable@...nel.org
>> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
>> [ 4.4.y backport notes:
>>   Ajay: Added local variable 'err' with-in follow_hugetlb_page()
>>         from 2be7cfed995e, to resolve compilation error
>>   Srivatsa: Replaced call to get_page_foll() with try_get_page_foll() ]
>> Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@...il.mit.edu>
>> Signed-off-by: Ajay Kaher <akaher@...are.com>
>> ---
>>  mm/gup.c     | 43 ++++++++++++++++++++++++++++++++-----------
>>  mm/hugetlb.c | 16 +++++++++++++++-
>>  2 files changed, 47 insertions(+), 12 deletions(-)
>    
> This seems to have the same issue as the 4.9 stable version [1], in not
> touching the arch-specific gup.c variants.
>    
> [1]
> https://lore.kernel.org/lkml/6650323f-dbc9-f069-000b-f6b0f941a065@suse.cz/

Thanks Vlastimil for highlighting this here.

Yes, arch-specific gup.c variants also need to handle not only for 4.4.y,
however it should be handled till 4.19.y. I believe it's better to start
from 4.19.y and then backport those changes till 4.4.y.

Affected areas of gup.c (where page->count have been used) are:
#1: get_page() used in these files and this is safe as
       it's defined in mm.h (here it's already taken care of)
#2: get_head_page_multiple() has following:
               VM_BUG_ON_PAGE(page_count(page) == 0, page);
           Need to change this to:
               VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(page), page);
#3: Some of the files have used page_cache_get_speculative(),
       page_cache_add_speculative() with combination of compound_head(),
       this scenario needs to be handled as it was handled here:
           https://lore.kernel.org/stable/1570581863-12090-7-git-send-email-akaher@vmware.com/

Please share with me any suggestions or patches if you have already  
worked on this.

Could we handle arch-specific gup.c in different patch sets and 
let these patches to merge to 4.4.y?

- Ajay