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Date: Mon, 24 Aug 2020 21:39:59 -0700 From: Sean Christopherson <sean.j.christopherson@...el.com> To: Andrew Cooper <andrew.cooper3@...rix.com> Cc: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>, x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>, Tom Lendacky <thomas.lendacky@....com>, Pu Wen <puwen@...on.cn>, Stephen Hemminger <sthemmin@...rosoft.com>, Sasha Levin <alexander.levin@...rosoft.com>, Dirk Hohndel <dirkhh@...are.com>, Jan Kiszka <jan.kiszka@...mens.com>, Tony W Wang-oc <TonyWWang-oc@...oxin.com>, "H. Peter Anvin" <hpa@...ux.intel.com>, Asit Mallick <asit.k.mallick@...el.com>, Gordon Tetlow <gordon@...lows.org>, David Kaplan <David.Kaplan@....com>, Tony Luck <tony.luck@...el.com>, Andy Lutomirski <luto@...nel.org> Subject: TDX #VE in SYSCALL gap (was: [RFD] x86: Curing the exception and syscall trainwreck in hardware) +Andy On Mon, Aug 24, 2020 at 02:52:01PM +0100, Andrew Cooper wrote: > And to help with coordination, here is something prepared (slightly) > earlier. > > https://docs.google.com/document/d/1hWejnyDkjRRAW-JEsRjA5c9CKLOPc6VKJQsuvODlQEI/edit?usp=sharing > > This identifies the problems from software's perspective, along with > proposing behaviour which ought to resolve the issues. > > It is still a work-in-progress. The #VE section still needs updating in > light of the publication of the recent TDX spec. For #VE on memory accesses in the SYSCALL gap (or NMI entry), is this something we (Linux) as the guest kernel actually want to handle gracefully (where gracefully means not panicking)? For TDX, a #VE in the SYSCALL gap would require one of two things: a) The guest kernel to not accept/validate the GPA->HPA mapping for the relevant pages, e.g. code or scratch data. b) The host VMM to remap the GPA (making the GPA->HPA pending again). (a) is only possible if there's a fatal buggy guest kernel (or perhaps vBIOS). (b) requires either a buggy or malicious host VMM. I ask because, if the answer is "no, panic at will", then we shouldn't need to burn an IST for TDX #VE. Exceptions won't morph to #VE and hitting an instruction based #VE in the SYSCALL gap would be a CPU bug or a kernel bug. Ditto for a #VE in NMI entry before it gets to a thread stack. Am I missing anything?
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