linux-kernel - Re: [PATCH MANUALSEL 5.14 1/5] KVM: X86: fix lazy allocation of rmaps

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Date:   Tue, 26 Oct 2021 18:14:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.14 1/5] KVM: X86: fix lazy allocation of rmaps

On 25/10/21 22:38, Sasha Levin wrote:
> From: Paolo Bonzini <pbonzini@...hat.com>
> 
> [ Upstream commit fa13843d1565d4c5b3aeb9be3343b313416bef46 ]
> 
> If allocation of rmaps fails, but some of the pointers have already been written,
> those pointers can be cleaned up when the memslot is freed, or even reused later
> for another attempt at allocating the rmaps.  Therefore there is no need to
> WARN, as done for example in memslot_rmap_alloc, but the allocation *must* be
> skipped lest KVM will overwrite the previous pointer and will indeed leak memory.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   arch/x86/kvm/x86.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4b0e866e9f08..60d9aa0ab389 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11341,7 +11341,8 @@ static int memslot_rmap_alloc(struct kvm_memory_slot *slot,
>   		int lpages = gfn_to_index(slot->base_gfn + npages - 1,
>   					  slot->base_gfn, level) + 1;
>   
> -		WARN_ON(slot->arch.rmap[i]);
> +		if (slot->arch.rmap[i])
> +			continue;
>   
>   		slot->arch.rmap[i] = kvcalloc(lpages, sz, GFP_KERNEL_ACCOUNT);
>   		if (!slot->arch.rmap[i]) {
> 

NACK

There is no lazy allocation of rmaps in 5.14, and any failure to 
allocate goes straight to memslot_rmap_free followed by return -ENOMEM. 
  So the WARN_ON is justified there.

Paolo