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

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Message-ID: <YKUPtrquQyImL3h5@8bytes.org>
Date:   Wed, 19 May 2021 15:16:38 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     x86@...nel.org, Hyunwook Baek <baekhw@...gle.com>,
        Joerg Roedel <jroedel@...e.de>, stable@...r.kernel.org,
        hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 2/6] x86/sev-es: Forward page-faults which happen during
 emulation

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