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Date:   Fri, 18 Jun 2021 08:00:01 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Jim Mattson <jmattson@google.com>
Cc:     Denis Efremov <efremov@linux.com>,
        Paolo Bonzini <pbonzini@redhat.com>, joe@perches.com,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] KVM: Use vmemdup_user()
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On Thu 17-06-21 17:25:04, Jim Mattson wrote:
> On Wed, Jun 3, 2020 at 3:10 AM Denis Efremov <efremov@linux.com> wrote:
> >
> > Replace opencoded alloc and copy with vmemdup_user().
> >
> > Signed-off-by: Denis Efremov <efremov@linux.com>
> > ---
> > Looks like these are the only places in KVM that are suitable for
> > vmemdup_user().
> >
> >  arch/x86/kvm/cpuid.c | 17 +++++++----------
> >  virt/kvm/kvm_main.c  | 19 ++++++++-----------
> >  2 files changed, 15 insertions(+), 21 deletions(-)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 901cd1fdecd9..27438a2bdb62 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -182,17 +182,14 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
> >         r = -E2BIG;
> >         if (cpuid->nent > KVM_MAX_CPUID_ENTRIES)
> >                 goto out;
> > -       r = -ENOMEM;
> >         if (cpuid->nent) {
> > -               cpuid_entries =
> > -                       vmalloc(array_size(sizeof(struct kvm_cpuid_entry),
> > -                                          cpuid->nent));
> > -               if (!cpuid_entries)
> > -                       goto out;
> > -               r = -EFAULT;
> > -               if (copy_from_user(cpuid_entries, entries,
> > -                                  cpuid->nent * sizeof(struct kvm_cpuid_entry)))
> > +               cpuid_entries = vmemdup_user(entries,
> > +                                            array_size(sizeof(struct kvm_cpuid_entry),
> > +                                                       cpuid->nent));
> 
> Does this break memcg accounting? I ask, because I'm really not sure.

What do you mean by that? The original code uses plain vmalloc so the
allocation is not memcg accounted (please note that __GFP_ACCOUNT needs
to be specified explicitly). vmemdup_user is the same in that regards.

-- 
Michal Hocko
SUSE Labs