Re: [PATCH 2/6] KVM: x86: Stack size is overridden by __linearize

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/11/2014 16:43, Nadav Amit wrote:
> When performing segmented-read/write in the emulator for stack operations, it
> ignores the stack size, and uses the ad_bytes as indication for the pointer
> size. As a result, a wrong address may be accessed.
> 
> To fix this behavior, we can remove the masking of address in __linearize and
> perform it beforehand.  It is already done for the operands (so currently it is
> inefficiently done twice). It is missing in two cases:
> 1. When using rip_relative
> 2. On fetch_bit_operand that changes the address.
> 
> This patch masks the address on these two occassions, and removes the masking
> from __linearize.
> 
> Note that it does not mask EIP during fetch. In protected/legacy mode code
> fetch when RIP >= 2^32 should result in #GP and not wrap-around. Since we make
> limit checks within __linearize, this is the expected behavior.

Partial revert of commit 518547b32ab4 (KVM: x86: Emulator does not
calculate address correctly, 2014-09-30).

Paolo


> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
> ---
>  arch/x86/kvm/emulate.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 5d47714..1317560 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -665,8 +665,7 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
>  	u16 sel;
>  	unsigned cpl;
>  
> -	la = seg_base(ctxt, addr.seg) +
> -	    (fetch || ctxt->ad_bytes == 8 ? addr.ea : (u32)addr.ea);
> +	la = seg_base(ctxt, addr.seg) + addr.ea;
>  	*max_size = 0;
>  	switch (ctxt->mode) {
>  	case X86EMUL_MODE_PROT64:
> @@ -1289,7 +1288,8 @@ static void fetch_bit_operand(struct x86_emulate_ctxt *ctxt)
>  		else
>  			sv = (s64)ctxt->src.val & (s64)mask;
>  
> -		ctxt->dst.addr.mem.ea += (sv >> 3);
> +		ctxt->dst.addr.mem.ea = address_mask(ctxt,
> +					   ctxt->dst.addr.mem.ea + (sv >> 3));
>  	}
>  
>  	/* only subword offset */
> @@ -4638,7 +4638,8 @@ done_prefixes:
>  	rc = decode_operand(ctxt, &ctxt->dst, (ctxt->d >> DstShift) & OpMask);
>  
>  	if (ctxt->rip_relative)
> -		ctxt->memopp->addr.mem.ea += ctxt->_eip;
> +		ctxt->memopp->addr.mem.ea = address_mask(ctxt,
> +					ctxt->memopp->addr.mem.ea + ctxt->_eip);
>  
>  done:
>  	return (rc != X86EMUL_CONTINUE) ? EMULATION_FAILED : EMULATION_OK;
>