Re: [PATCH 2/6] x86/sev-es: Forward page-faults which happen during emulation

[Sean Christopherson]

On Wed, May 12, 2021, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@xxxxxxx>
> 
> When emulating guest instructions for MMIO or IOIO accesses the #VC
> handler might get a page-fault and will not be able to complete. Forward
> the page-fault in this case to the correct handler instead of killing
> the machine.
> 
> Fixes: 0786138c78e7 ("x86/sev-es: Add a Runtime #VC Exception Handler")
> Cc: stable@xxxxxxxxxxxxxxx # v5.10+
> Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
> ---
>  arch/x86/kernel/sev.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index c49270c7669e..6530a844eb61 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -1265,6 +1265,10 @@ static __always_inline void vc_forward_exception(struct es_em_ctxt *ctxt)
>  	case X86_TRAP_UD:
>  		exc_invalid_op(ctxt->regs);
>  		break;
> +	case X86_TRAP_PF:
> +		write_cr2(ctxt->fi.cr2);
> +		exc_page_fault(ctxt->regs, error_code);
> +		break;

This got me looking at the flows that "inject" #PF, and I'm pretty sure there
are bugs in __vc_decode_user_insn() + insn_get_effective_ip().

Problem #1: __vc_decode_user_insn() assumes a #PF if insn_fetch_from_user_inatomic()
fails, but the majority of failure cases in insn_get_seg_base() are #GPs, not #PF.

	res = insn_fetch_from_user_inatomic(ctxt->regs, buffer);
	if (!res) {
		ctxt->fi.vector     = X86_TRAP_PF;
		ctxt->fi.error_code = X86_PF_INSTR | X86_PF_USER;
		ctxt->fi.cr2        = ctxt->regs->ip;
		return ES_EXCEPTION;
	}

Problem #2: Using '0' as an error code means a legitimate effective IP of '0'
will be misinterpreted as a failure.  Practically speaking, I highly doubt anyone
will ever actually run code at address 0, but it's technically possible.  The
most robust approach would be to pass a pointer to @ip and return an actual error
code.  Using a non-canonical magic value might also work, but that could run afoul
of future shenanigans like LAM.

	ip = insn_get_effective_ip(regs);
	if (!ip)
		return 0;