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

From: Ajay Kaher <akaher@vmware.com>
To: Vlastimil Babka <vbabka@suse.cz>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
Cc: "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"punit.agrawal@arm.com" <punit.agrawal@arm.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"kirill.shutemov@linux.intel.com"
	<kirill.shutemov@linux.intel.com>,
	"willy@infradead.org" <willy@infradead.org>,
	"will.deacon@arm.com" <will.deacon@arm.com>,
	"mszeredi@redhat.com" <mszeredi@redhat.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Srivatsa Bhat <srivatsab@vmware.com>,
	"srivatsa@csail.mit.edu" <srivatsa@csail.mit.edu>,
	Alexey Makhalov <amakhalov@vmware.com>,
	Srinidhi Rao <srinidhir@vmware.com>,
	Vikash Bansal <bvikas@vmware.com>,
	Anish Swaminathan <anishs@vmware.com>,
	Vasavi Sirnapalli <vsirnapalli@vmware.com>,
	Steven Rostedt <srostedt@vmware.com>,
	"stable@kernel.org" <stable@kernel.org>,
	Ben Hutchings <ben@decadent.org.uk>
Subject: Re: [PATCH v2 6/8] mm: prevent get_user_pages() from overflowing page refcount
Date: Thu, 17 Oct 2019 16:28:56 +0000	[thread overview]
Message-ID: <BF0587E3-D104-4DB2-B972-9BC4FD4CA014@vmware.com> (raw)
In-Reply-To: <f899be71-4bc0-d07b-f650-d85a335cdebb@suse.cz>

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

>> Reported-by: Jann Horn <jannh@google.com>
>> Acked-by: Matthew Wilcox <willy@infradead.org>
>> Cc: stable@kernel.org
>> Signed-off-by: Linus Torvalds <torvalds@linux-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@csail.mit.edu>
>> Signed-off-by: Ajay Kaher <akaher@vmware.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