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Date: Fri, 24 Jan 2025 13:54:04 -0700
From: Keith Busch <kbusch@kernel.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Keith Busch <kbusch@meta.com>, kvm@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org, Vlad Poenaru <thevlad@meta.com>,
	tj@kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Alyssa Ross <hi@alyssa.is>
Subject: Re: [PATCH] kvm: defer huge page recovery vhost task to later
Message-ID: <Z5P97NyK9Rb_cU1z@kbusch-mbp>
References: <20250123153543.2769928-1-kbusch@meta.com>
 <Z5Py_JYc8nYHNgZS@google.com>
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In-Reply-To: <Z5Py_JYc8nYHNgZS@google.com>

On Fri, Jan 24, 2025 at 12:07:24PM -0800, Sean Christopherson wrote:
> This is broken.  If the module param is toggled before the first KVM_RUN, KVM
> will hit a NULL pointer deref due to trying to start a non-existent vhost task:
> 
>   BUG: kernel NULL pointer dereference, address: 0000000000000040
>   #PF: supervisor read access in kernel mode
>   #PF: error_code(0x0000) - not-present page
>   PGD 0 P4D 0 
>   Oops: Oops: 0000 [#1] SMP
>   CPU: 16 UID: 0 PID: 1190 Comm: bash Not tainted 6.13.0-rc3-9bb02e874121-x86/xen_msr_fixes-vm #2382
>   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
>   RIP: 0010:vhost_task_wake+0x5/0x10
>   Call Trace:
>    <TASK>
>    set_nx_huge_pages+0xcc/0x1e0 [kvm]

Thanks for pointing out this gap. It looks like we'd have to hold the
kvm_lock in kvm_mmu_post_init_vm(), and add NULL checks in
set_nx_huge_pages() and set_nx_huge_pages_recovery_param() to prevent
the NULL deref. Is that okay?