linux-kernel - Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

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Date:   Tue, 5 Oct 2021 19:27:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Colin King <colin.king@...onical.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        David Stevens <stevensd@...omium.org>, kvm@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

On 05/10/21 17:41, Sean Christopherson wrote:
>>   			if (*gfn_track == NULL) {
>>   				mutex_unlock(&kvm->slots_arch_lock);
> Hrm, this fails to free the gfn_track allocations for previous memslots.  The
> on-demand rmaps code has the exact same bug (it frees rmaps for previous lpages
> in the_current_  slot, but does not free previous slots).

That's not a huge deal because the syscall is failing.  So as long as 
it's not leaked forever, it's okay.  The problem is the 
WARN_ON(slot->arch.rmap[i]), or the missing check in 
kvm_page_track_enable_mmu_write_tracking, but that's easily fixed.  I'd 
even remove the call to memslot_rmaps_free.

> And having two separate flows (and flags) for rmaps vs. gfn_track is pointless,
> and means we have to maintain two near-identical copies of non-obvious code.

I was thinking the separate flow (not so much the flag) is needed 
because, if KVMGT is enabled, gfn_track is allocated unconditionally. 
rmaps are added on top of that if shadow paging is enabled; but 
kvm_page_track_create_memslot will have already created the counter, 
including the one for KVM_PAGE_TRACK_WRITE.

But looking at the code again, I guess you could call 
kvm_page_track_enable_mmu_write_tracking inside alloc_all_memslots_rmaps 
(with a little bit of renaming), and with that the flag would go away.

I'll take a look tomorrow, but I'd rather avoid reverting the patch.

Thanks,

Paolo

> Paolo, is it too late to just drop the original deae4a10f166 ("KVM: x86: only
> allocate gfn_track when necessary")?
>