linux-kernel - Re: [PATCH v3 15/16] mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page

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Message-ID: <b35374d1-858c-393a-7b63-8816efe9b175@suse.cz>
Date:   Tue, 19 Apr 2022 17:56:51 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>, Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH v3 15/16] mm/gup: trigger FAULT_FLAG_UNSHARE when
 R/O-pinning a possibly shared anonymous page

On 3/29/22 18:04, David Hildenbrand wrote:
> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
> on the page table entry will end up replacing the mapped anonymous page
> due to COW, resulting in the GUP pin no longer being consistent with the
> page actually mapped into the page table.
> 
> The possible ways to deal with this situation are:
>  (1) Ignore and pin -- what we do right now.
>  (2) Fail to pin -- which would be rather surprising to callers and
>      could break user space.
>  (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
>      pins.
> 
> Let's implement 3) because it provides the clearest semantics and
> allows for checking in unpin_user_pages() and friends for possible BUGs:
> when trying to unpin a page that's no longer exclusive, clearly
> something went very wrong and might result in memory corruptions that
> might be hard to debug. So we better have a nice way to spot such
> issues.
> 
> This change implies that whenever user space *wrote* to a private
> mapping (IOW, we have an anonymous page mapped), that GUP pins will
> always remain consistent: reliable R/O GUP pins of anonymous pages.
> 
> As a side note, this commit fixes the COW security issue for hugetlb with
> FOLL_PIN as documented in:
>   https://lore.kernel.org/r/3ae33b08-d9ef-f846-56fb-645e3b9b4c66@redhat.com
> The vmsplice reproducer still applies, because vmsplice uses FOLL_GET
> instead of FOLL_PIN.
> 
> Note that follow_huge_pmd() doesn't apply because we cannot end up in
> there with FOLL_PIN.
> 
> This commit is heavily based on prototype patches by Andrea.
> 
> Co-developed-by: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>