linux-kernel - Re: [PATCH v5 1/6] mm/gup: remove unused vmas parameter from get_user

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Message-ID: <ZGOTadDG/b0904YI@google.com>
Date:   Tue, 16 May 2023 07:30:01 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>, x86@...nel.org,
        linux-sgx@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, kvm@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, Xinhui Pan <Xinhui.Pan@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Christian Konig <christian.koenig@....com>,
        Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v5 1/6] mm/gup: remove unused vmas parameter from get_user_pages()

On Tue, May 16, 2023, David Hildenbrand wrote:
> On 15.05.23 21:07, Sean Christopherson wrote:
> > On Sun, May 14, 2023, Lorenzo Stoakes wrote:
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index cb5c13eee193..eaa5bb8dbadc 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > > @@ -2477,7 +2477,7 @@ static inline int check_user_page_hwpoison(unsigned long addr)
> > >   {
> > >   	int rc, flags = FOLL_HWPOISON | FOLL_WRITE;
> > > -	rc = get_user_pages(addr, 1, flags, NULL, NULL);
> > > +	rc = get_user_pages(addr, 1, flags, NULL);
> > >   	return rc == -EHWPOISON;
> > 
> > Unrelated to this patch, I think there's a pre-existing bug here.  If gup() returns
> > a valid page, KVM will leak the refcount and unintentionally pin the page.  That's
> 
> When passing NULL as "pages" to get_user_pages(), __get_user_pages_locked()
> won't set FOLL_GET. As FOLL_PIN is also not set, we won't be messing with
> the mapcount of the page.

Ah, that's what I'm missing.

> So even if get_user_pages() returns "1", we should be fine.
> 
> 
> Or am I misunderstanding your concern?

Nope, you covered everything.  I do think we can drop the extra gup() though,
AFAICT it's 100% redundant.  But it's not a bug.

Thanks!