Hi Sean,

On Wed, May 12, 2021 at 05:31:03PM +0000, Sean Christopherson wrote:

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;

Your observations are all correct. I put some changes onto this
patch-set to fix these problems.

Regards,

	Joerg