linux-kernel - Re: [PATCH v5 12/19] KVM: Move memslot deletion to helper function

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200206162818.GD13067@linux.intel.com>
Date:   Thu, 6 Feb 2020 08:28:18 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        Christoffer Dall <christoffer.dall@....com>,
        Philippe Mathieu-Daudé <f4bug@...at.org>
Subject: Re: [PATCH v5 12/19] KVM: Move memslot deletion to helper function

On Thu, Feb 06, 2020 at 11:14:15AM -0500, Peter Xu wrote:
> On Tue, Jan 21, 2020 at 02:31:50PM -0800, Sean Christopherson wrote:
> > Move memslot deletion into its own routine so that the success path for
> > other memslot updates does not need to use kvm_free_memslot(), i.e. can
> > explicitly destroy the dirty bitmap when necessary.  This paves the way
> > for dropping @dont from kvm_free_memslot(), i.e. all callers now pass
> > NULL for @dont.
> > 
> > Add a comment above the code to make a copy of the existing memslot
> > prior to deletion, it is not at all obvious that the pointer will become
> > stale during sorting and/or installation of new memslots.
> 
> Could you help explain a bit on this explicit comment?  I can follow
> up with the patch itself which looks all correct to me, but I failed
> to catch what this extra comment wants to emphasize...

It's tempting to write the code like this (I know, because I did it):

	if (!mem->memory_size)
		return kvm_delete_memslot(kvm, mem, slot, as_id);

	new = *slot;

Where @slot is a pointer to the memslot to be deleted.  At first, second,
and third glances, this seems perfectly sane.

The issue is that slot was pulled from struct kvm_memslots.memslots, e.g.

	slot = &slots->memslots[index];

Note that slots->memslots holds actual "struct kvm_memory_slot" objects,
not pointers to slots.  When update_memslots() sorts the slots, it swaps
the actual slot objects, not pointers.  I.e. after update_memslots(), even
though @slot points at the same address, it's could be pointing at a
different slot.  As a result kvm_free_memslot() in kvm_delete_memslot()
will free the dirty page info and arch-specific points for some random
slot, not the intended slot, and will set npages=0 for that random slot.