linux-kernel - Re: [PATCH v5 5/7] KVM: selftests: Make arm64's MMIO ucall multi-VM friendly

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Date:   Mon, 29 Aug 2022 18:19:41 +0200
From:   Andrew Jones <andrew.jones@...ux.dev>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Atish Patra <atishp@...shpatra.org>,
        David Hildenbrand <david@...hat.com>,
        Tom Rix <trix@...hat.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Colton Lewis <coltonlewis@...gle.com>,
        Peter Gonda <pgonda@...gle.com>
Subject: Re: [PATCH v5 5/7] KVM: selftests: Make arm64's MMIO ucall multi-VM
 friendly

On Thu, Aug 25, 2022 at 11:25:20PM +0000, Sean Christopherson wrote:
> Fix a mostly-theoretical bug where ARM's ucall MMIO setup could result in
> different VMs stomping on each other by cloberring the global pointer.
> 
> Fix the most obvious issue by saving the MMIO gpa into the VM.
> 
> A more subtle bug is that creating VMs in parallel (on multiple tasks)
> could result in a VM using the wrong address.  Synchronizing a global to
> a guest effectively snapshots the value on a per-VM basis, i.e. the
> "global" is already prepped to work with multiple VMs, but setting the
> global in the host is not thread-safe.  To fix that bug, add
> write_guest_global() to allow stuffing a VM's copy of a "global" without
> modifying the host value.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  .../selftests/kvm/include/kvm_util_base.h     | 15 +++++++++++++++
>  .../testing/selftests/kvm/lib/aarch64/ucall.c | 19 ++++++++++++++-----
>  2 files changed, 29 insertions(+), 5 deletions(-)
>

Reviewed-by: Andrew Jones <andrew.jones@...ux.dev>