linux-kernel - Re: [PATCH] powerpc/kvm/cma: Fix panic introduces by signed shift operation

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Message-ID: <5406D293.3060404@redhat.com>
Date:	Wed, 03 Sep 2014 10:34:27 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org
CC:	kvm@...r.kernel.org, kvm-ppc@...r.kernel.org,
	linux-kernel@...r.kernel.org, Alexey Kardashevskiy <aik@...abs.ru>,
	Paul Mackerras <paulus@...ba.org>,
	Alexander Graf <agraf@...e.de>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH] powerpc/kvm/cma: Fix panic introduces by signed shift
 operation

Il 02/09/2014 18:13, Laurent Dufour ha scritto:
> fc95ca7284bc54953165cba76c3228bd2cdb9591 introduces a memset in
> kvmppc_alloc_hpt since the general CMA doesn't clear the memory it
> allocates.
> 
> However, the size argument passed to memset is computed from a signed value
> and its signed bit is extended by the cast the compiler is doing. This lead
> to extremely large size value when dealing with order value >= 31, and
> almost all the memory following the allocated space is cleaned. As a
> consequence, the system is panicing and may even fail spawning the kdump
> kernel.
> 
> This fix makes use of an unsigned value for the memset's size argument to
> avoid sign extension. Among this fix, another shift operation which may
> lead to signed extended value too is also fixed.
> 
> Cc: Alexey Kardashevskiy <aik@...abs.ru>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Alexander Graf <agraf@...e.de>
> Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Signed-off-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
> ---
>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 72c20bb16d26..79294c4c5015 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -62,10 +62,10 @@ long kvmppc_alloc_hpt(struct kvm *kvm, u32 *htab_orderp)
>  	}
>  
>  	kvm->arch.hpt_cma_alloc = 0;
> -	page = kvm_alloc_hpt(1 << (order - PAGE_SHIFT));
> +	page = kvm_alloc_hpt(1ul << (order - PAGE_SHIFT));
>  	if (page) {
>  		hpt = (unsigned long)pfn_to_kaddr(page_to_pfn(page));
> -		memset((void *)hpt, 0, (1 << order));
> +		memset((void *)hpt, 0, (1ul << order));
>  		kvm->arch.hpt_cma_alloc = 1;
>  	}
>  
> 

Thanks, applied to kvm/master.

Paolo
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