Re: [PATCH 3/5] KVM: x86: Decoding guest instructions which cross page boundary may fail

["Radim Krčmář" <rkrcmar@redhat.com>]

2014-10-03 01:10+0300, Nadav Amit:
> Once an instruction crosses a page boundary, the size read from the second page
> disregards the common case that part of the operand resides on the first page.
> As a result, fetch of long insturctions may fail, and thereby cause the
> decoding to fail as well.
> 
> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
> ---

Good catch, was it thanks to an exhaustive test-suite?

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

>  arch/x86/kvm/emulate.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 694dfa7..b2e8aba 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -751,8 +751,10 @@ static int __do_insn_fetch_bytes(struct x86_emulate_ctxt *ctxt, int op_size)
>  static __always_inline int do_insn_fetch_bytes(struct x86_emulate_ctxt *ctxt,
>  					       unsigned size)
>  {
> -	if (unlikely(ctxt->fetch.end - ctxt->fetch.ptr < size))
> -		return __do_insn_fetch_bytes(ctxt, size);
> +	unsigned done_size = ctxt->fetch.end - ctxt->fetch.ptr;
> +
> +	if (unlikely(done_size < size))
> +		return __do_insn_fetch_bytes(ctxt, size - done_size);
>  	else
>  		return X86EMUL_CONTINUE;
>  }
> -- 
> 1.9.1
> 
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