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From: Joerg Roedel <joro@8bytes.org>
To: Sean Christopherson <seanjc@google.com>
Cc: x86@kernel.org, Hyunwook Baek <baekhw@google.com>,
	Joerg Roedel <jroedel@suse.de>,
	stable@vger.kernel.org, hpa@zytor.com,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jiri Slaby <jslaby@suse.cz>,
	Dan Williams <dan.j.williams@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Juergen Gross <jgross@suse.com>,
	Kees Cook <keescook@chromium.org>,
	David Rientjes <rientjes@google.com>,
	Cfir Cohen <cfir@google.com>, Erdem Aktas <erdemaktas@google.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mike Stunes <mstunes@vmware.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 2/6] x86/sev-es: Forward page-faults which happen during emulation
Date: Wed, 19 May 2021 15:16:38 +0200	[thread overview]
Message-ID: <YKUPtrquQyImL3h5@8bytes.org> (raw)
In-Reply-To: <YJwQ1xsiDtv3LkBe@google.com>

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

WARNING: multiple messages have this Message-ID (diff)
From: Joerg Roedel <joro@8bytes.org>
To: Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	virtualization@lists.linux-foundation.org,
	Arvind Sankar <nivedita@alum.mit.edu>,
	hpa@zytor.com, Jiri Slaby <jslaby@suse.cz>,
	x86@kernel.org, David Rientjes <rientjes@google.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Joerg Roedel <jroedel@suse.de>, Kees Cook <keescook@chromium.org>,
	Cfir Cohen <cfir@google.com>, Hyunwook Baek <baekhw@google.com>,
	linux-coco@lists.linux.dev, Andy Lutomirski <luto@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Juergen Gross <jgross@suse.com>, Mike Stunes <mstunes@vmware.com>,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Erdem Aktas <erdemaktas@google.com>
Subject: Re: [PATCH 2/6] x86/sev-es: Forward page-faults which happen during emulation
Date: Wed, 19 May 2021 15:16:38 +0200	[thread overview]
Message-ID: <YKUPtrquQyImL3h5@8bytes.org> (raw)
In-Reply-To: <YJwQ1xsiDtv3LkBe@google.com>

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
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

  reply	other threads:[~2021-05-19 13:16 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-12  7:54 [PATCH 0/6] x86/sev-es: Fixes for SEV-ES guest support Joerg Roedel
2021-05-12  7:54 ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 1/6] x86/sev-es: Don't return NULL from sev_es_get_ghcb() Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 2/6] x86/sev-es: Forward page-faults which happen during emulation Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12 17:31   ` Sean Christopherson
2021-05-19 13:16     ` Joerg Roedel [this message]
2021-05-19 13:16       ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 3/6] x86/sev-es: Use __put_user()/__get_user Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12  8:04   ` David Laight
2021-05-12  8:04     ` David Laight
2021-05-12  8:16     ` Juergen Gross
2021-05-12  8:16       ` Juergen Gross via Virtualization
2021-05-12  8:50       ` 'Joerg Roedel'
2021-05-12  8:50         ` 'Joerg Roedel'
2021-05-12  8:58         ` Juergen Gross
2021-05-12  8:58           ` Juergen Gross via Virtualization
2021-05-12  9:31           ` David Laight
2021-05-12  9:31             ` David Laight
2021-05-12  9:32           ` Joerg Roedel
2021-05-12  9:32             ` Joerg Roedel
2021-05-19 11:33             ` 'Joerg Roedel'
2021-05-19 11:33               ` 'Joerg Roedel'
2021-05-12  8:37     ` 'Joerg Roedel'
2021-05-12  8:37       ` 'Joerg Roedel'
2021-05-12 15:59       ` Dave Hansen
2021-05-12 15:59         ` Dave Hansen
2021-05-12 15:57   ` Dave Hansen
2021-05-12 15:57     ` Dave Hansen
2021-05-12 16:00     ` Joerg Roedel
2021-05-12 16:00       ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 4/6] Revert "x86/sev-es: Handle string port IO to kernel memory properly" Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12 17:38   ` Sean Christopherson
2021-05-19 12:22     ` Joerg Roedel
2021-05-19 12:22       ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 5/6] x86/sev-es: Fix error message in runtime #VC handler Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel
2021-05-12  7:54 ` [PATCH 6/6] x86/sev-es: Leave NMI-mode before sending signals Joerg Roedel
2021-05-12  7:54   ` Joerg Roedel

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