linux-kernel - Re: [PATCH v5.5 02/30] KVM: Disallow user memslot with size that exceeds "unsigned long"

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Message-ID: <2f451590-bf89-d447-d92a-fe9f3f41ca74@oracle.com>
Date:   Tue, 9 Nov 2021 01:38:02 +0100
From:   "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Ben Gardon <bgardon@...gle.com>, Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v5.5 02/30] KVM: Disallow user memslot with size that
 exceeds "unsigned long"

On 04.11.2021 01:25, Sean Christopherson wrote:
> Reject userspace memslots whose size exceeds the storage capacity of an
> "unsigned long".  KVM's uAPI takes the size as u64 to support large slots
> on 64-bit hosts, but does not account for the size being truncated on
> 32-bit hosts in various flows.  The access_ok() check on the userspace
> virtual address in particular casts the size to "unsigned long" and will
> check the wrong number of bytes.
> 
> KVM doesn't actually support slots whose size doesn't fit in an "unsigned
> long", e.g. KVM's internal kvm_memory_slot.npages is an "unsigned long",
> not a "u64", and misc arch specific code follows that behavior.
> 
> Fixes: fa3d315a4ce2 ("KVM: Validate userspace_addr of memslot when registered")
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   virt/kvm/kvm_main.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 99e69375c4c9..83287730389f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1689,7 +1689,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
>   	id = (u16)mem->slot;
>   
>   	/* General sanity checks */
> -	if (mem->memory_size & (PAGE_SIZE - 1))
> +	if ((mem->memory_size & (PAGE_SIZE - 1)) ||
> +	    (mem->memory_size != (unsigned long)mem->memory_size))
>   		return -EINVAL;
>   	if (mem->guest_phys_addr & (PAGE_SIZE - 1))
>   		return -EINVAL;
> 

Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>