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

[Sasha Levin]

On Tue, Oct 26, 2021 at 06:14:34PM +0200, Paolo Bonzini wrote:
> On 25/10/21 22:38, Sasha Levin wrote:
> > From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> >
> > [ 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@xxxxxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > ---
> >  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.

I'll queue up the ones you've acked, thanks :)

--
Thanks,
Sasha