From: Will Deacon <will@kernel.org>
To: Marc Zyngier <maz@kernel.org>
Cc: kvmarm@lists.linux.dev, kvm@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
James Morse <james.morse@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Oliver Upton <oliver.upton@linux.dev>,
Zenghui Yu <yuzenghui@huawei.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Quentin Perret <qperret@google.com>,
Mostafa Saleh <smostafa@google.com>,
stable@vger.kernel.org
Subject: Re: [PATCH v2] KVM: arm64: Make vcpu flag updates non-preemptible
Date: Wed, 19 Apr 2023 09:58:15 +0100 [thread overview]
Message-ID: <20230419085814.GA928@willie-the-truck> (raw)
In-Reply-To: <20230418125737.2327972-1-maz@kernel.org>
On Tue, Apr 18, 2023 at 01:57:37PM +0100, Marc Zyngier wrote:
> Per-vcpu flags are updated using a non-atomic RMW operation.
> Which means it is possible to get preempted between the read and
> write operations.
>
> Another interesting thing to note is that preemption also updates
> flags, as we have some flag manipulation in both the load and put
> operations.
>
> It is thus possible to lose information communicated by either
> load or put, as the preempted flag update will overwrite the flags
> when the thread is resumed. This is specially critical if either
> load or put has stored information which depends on the physical
> CPU the vcpu runs on.
>
> This results in really elusive bugs, and kudos must be given to
> Mostafa for the long hours of debugging, and finally spotting
> the problem.
>
> Fix it by disabling preemption during the RMW operation, which
> ensures that the state stays consistent. Also upgrade vcpu_get_flag
> path to use READ_ONCE() to make sure the field is always atomically
> accessed.
>
> Fixes: e87abb73e594 ("KVM: arm64: Add helpers to manipulate vcpu flags among a set")
> Reported-by: Mostafa Saleh <smostafa@google.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org
> ---
>
> Notes:
> v2: add READ_ONCE() on the read path, expand commit message
Acked-by: Will Deacon <will@kernel.org>
Will
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prev parent reply other threads:[~2023-04-19 8:59 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-18 12:57 [PATCH v2] KVM: arm64: Make vcpu flag updates non-preemptible Marc Zyngier
2023-04-19 8:41 ` Oliver Upton
2023-04-19 8:58 ` Will Deacon [this message]
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