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Date: Thu, 12 Jul 2018 19:21:42 -0700 From: Andy Lutomirski <luto@...capital.net> To: Dave Hansen <dave.hansen@...el.com> Cc: Dave Hansen <dave.hansen@...ux.intel.com>, Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, linux-mm@...ck.org, linux-arch@...r.kernel.org, linux-api@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, Balbir Singh <bsingharora@...il.com>, Cyrill Gorcunov <gorcunov@...il.com>, Florian Weimer <fweimer@...hat.com>, "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omiun.org>, Mike Kravetz <mike.kravetz@...cle.com>, Nadav Amit <nadav.amit@...il.com>, Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>, Peter Zijlstra <peterz@...radead.org>, "Ravi V. Shankar" <ravi.v.shankar@...el.com>, Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com> Subject: Re: [RFC PATCH v2 18/27] x86/cet/shstk: Introduce WRUSS instruction > On Jul 12, 2018, at 6:50 PM, Dave Hansen <dave.hansen@...el.com> wrote: > > On 07/12/2018 04:49 PM, Dave Hansen wrote: >>>> That seems like something we need to call out if so. It also means we >>>> need to update the SDM because some of the text is wrong. >>> It needs to mention the WRUSS case. >> Ugh. The documentation for this is not pretty. But, I guess this is >> not fundamentally different from access to U=1 pages when SMAP is in >> place and we've set EFLAGS.AC=1. > > I was wrong and misread the docs. We do not get X86_PF_USER set when > EFLAGS.AC=1. > > But, we *do* get X86_PF_USER (otherwise defined to be set when in ring3) > when running in ring0 with the WRUSS instruction and some other various > shadow-stack-access-related things. I'm sure folks had a good reason > for this architecture, but it is a pretty fundamentally *new* > architecture that we have to account for. I think it makes (some) sense. The USER bit is set for a page fault that was done with user privilege. So a descriptor table fault at CPL 3 has USER clear (regardless of the cause of the fault) and WRUSS has USER set. > > This new architecture is also not spelled out or accounted for in the > SDM as of yet. It's only called out here as far as I know: > https://software.intel.com/sites/default/files/managed/4d/2a/control-flow-enforcement-technology-preview.pdf > > Which reminds me: Yu-cheng, do you have a link to the docs anywhere in > your set? If not, you really should. I am tempted to suggest that the whole series not be merged until there are actual docs. It’s not a fantastic precedent.
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