linux-kernel - Re: [PATCH] KVM: x86: do mask out upper bits of PAE CR3

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Date:   Mon, 24 Jul 2017 20:28:04 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH] KVM: x86: do mask out upper bits of PAE CR3

2017-07-24 18:59+0200, Paolo Bonzini:
> This reverts the change of commit f85c758dbee54cc3612a6e873ef7cecdb66ebee5,
> as the behavior it modified was intended.
> 
> The VM is running in 32-bit PAE mode, and Table 4-7 of the Intel manual
> says:
> 
> Table 4-7. Use of CR3 with PAE Paging
> Bit Position(s)	Contents
> 4:0		Ignored
> 31:5		Physical address of the 32-Byte aligned
> 		page-directory-pointer table used for linear-address
> 		translation
> 63:32		Ignored (these bits exist only on processors supporting
> 		the Intel-64 architecture)
> 
> To placate the static checker, write the mask explicitly as an
> unsigned long constant instead of using a 32-bit unsigned constant.
> 
> Cc: Dan Carpenter <dan.carpenter@...cle.com>
> Fixes: f85c758dbee54cc3612a6e873ef7cecdb66ebee5
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---

Thanks for catching this,

Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 82a63c59f77b..6c97c82814c4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -597,8 +597,8 @@ bool pdptrs_changed(struct kvm_vcpu *vcpu)
>  		      (unsigned long *)&vcpu->arch.regs_avail))
>  		return true;
>  
> -	gfn = (kvm_read_cr3(vcpu) & ~31ul) >> PAGE_SHIFT;
> -	offset = (kvm_read_cr3(vcpu) & ~31ul) & (PAGE_SIZE - 1);
> +	gfn = (kvm_read_cr3(vcpu) & 0xffffffe0ul) >> PAGE_SHIFT;
> +	offset = (kvm_read_cr3(vcpu) & 0xffffffe0ul) & (PAGE_SIZE - 1);
>  	r = kvm_read_nested_guest_page(vcpu, gfn, pdpte, offset, sizeof(pdpte),
>  				       PFERR_USER_MASK | PFERR_WRITE_MASK);
>  	if (r < 0)
> -- 
> 1.8.3.1
>