linux-kernel - Re: [PATCH 1/3] KVM: Properly unwind VM creation if creating debugfs fails

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Date:   Tue, 16 Aug 2022 17:52:32 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+744e173caec2e1627ee0@...kaller.appspotmail.com,
        David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 1/3] KVM: Properly unwind VM creation if creating debugfs
 fails

On Tue, Aug 16, 2022 at 05:39:35AM +0000, Sean Christopherson wrote:
> Properly unwind VM creation if kvm_create_vm_debugfs() fails.  A recent
> change to invoke kvm_create_vm_debug() in kvm_create_vm() was led astray

typo: kvm_create_vm_debugfs()

> by buggy try_get_module() handling adding by commit 5f6de5cbebee ("KVM:
> Prevent module exit until all VMs are freed").  The debugfs error path
> effectively inherits the bad error path of try_module_get(), e.g. KVM
> leaves the to-be-free VM on vm_list even though KVM appears to do the
> right thing by calling module_put() and falling through.
> 
> Opportunistically hoist kvm_create_vm_debugfs() above the call to
> kvm_arch_post_init_vm() so that the "post-init" arch hook is actually
> invoked after the VM is initialized (ignoring kvm_coalesced_mmio_init()
> for the moment).  x86 is the only non-nop implementation of the post-init
> hook, and it doesn't allocate/initialize any objects that are reachable
> via debugfs code (spawns a kthread worker for the NX huge page mitigation).
> 
> Leave the buggy try_get_module() alone for now, it will be fixed in a
> separate commit.
> 
> Fixes: b74ed7a68ec1 ("KVM: Actually create debugfs in kvm_create_vm()")
> Reported-by: syzbot+744e173caec2e1627ee0@...kaller.appspotmail.com
> Cc: Oliver Upton <oliver.upton@...ux.dev>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Fun times! Thanks for the fix Sean.

Reviewed-by: Oliver Upton <oliver.upton@...ux.dev>

--
Best,
Oliver