linux-kernel - Re: [PATCH 2/3] KVM: Always flush async #PF workqueue when vCPU is being destroyed

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: <ZTa6TxJS2erGBxgp@google.com>
Date:   Mon, 23 Oct 2023 11:24:15 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 2/3] KVM: Always flush async #PF workqueue when vCPU is
 being destroyed

On 2023-10-18 01:46 PM, Sean Christopherson wrote:
> Always flush the per-vCPU async #PF workqueue when a vCPU is clearing its
> completion queue, i.e. when a VM and all its vCPUs is being destroyed.

nit:

  ... or when the guest toggles CR0.PG or async_pf support.

> KVM must ensure that none of its workqueue callbacks is running when the
> last reference to the KVM _module_ is put.  Gifting a reference to the
> associated VM prevents the workqueue callback from dereferencing freed
> vCPU/VM memory, but does not prevent the KVM module from being unloaded
> before the callback completes.
> 
> Drop the misguided VM refcount gifting, as calling kvm_put_kvm() from
> async_pf_execute() if kvm_put_kvm() flushes the async #PF workqueue will
> result in deadlock.  async_pf_execute() can't return until kvm_put_kvm()
> finishes, and kvm_put_kvm() can't return until async_pf_execute() finishes:
>
[...]
> 
> Note, commit 5f6de5cbebee ("KVM: Prevent module exit until all VMs are
> freed") *tried* to fix the module refcounting issue by having VMs grab a
> reference to the module, but that only made the bug slightly harder to hit
> as it gave async_pf_execute() a bit more time to complete before the KVM
> module could be unloaded.

Blegh! Thanks for the fix.

> 
> Fixes: af585b921e5d ("KVM: Halt vcpu if page it tries to access is swapped out")
> Cc: stable@...r.kernel.org
> Cc: David Matlack <dmatlack@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: David Matlack <dmatlack@...gle.com>