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Date: Mon, 3 Oct 2022 21:05:04 -0700 From: Kees Cook <keescook@...omium.org> To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com> Cc: "bsingharora@...il.com" <bsingharora@...il.com>, "hpa@...or.com" <hpa@...or.com>, "Syromiatnikov, Eugene" <esyr@...hat.com>, "peterz@...radead.org" <peterz@...radead.org>, "rdunlap@...radead.org" <rdunlap@...radead.org>, "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>, "Eranian, Stephane" <eranian@...gle.com>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "fweimer@...hat.com" <fweimer@...hat.com>, "nadav.amit@...il.com" <nadav.amit@...il.com>, "jannh@...gle.com" <jannh@...gle.com>, "dethoma@...rosoft.com" <dethoma@...rosoft.com>, "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>, "kcc@...gle.com" <kcc@...gle.com>, "bp@...en8.de" <bp@...en8.de>, "oleg@...hat.com" <oleg@...hat.com>, "hjl.tools@...il.com" <hjl.tools@...il.com>, "Yang, Weijiang" <weijiang.yang@...el.com>, "Lutomirski, Andy" <luto@...nel.org>, "pavel@....cz" <pavel@....cz>, "arnd@...db.de" <arnd@...db.de>, "Moreira, Joao" <joao.moreira@...el.com>, "tglx@...utronix.de" <tglx@...utronix.de>, "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>, "x86@...nel.org" <x86@...nel.org>, "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>, "jamorris@...ux.microsoft.com" <jamorris@...ux.microsoft.com>, "john.allen@....com" <john.allen@....com>, "rppt@...nel.org" <rppt@...nel.org>, "mingo@...hat.com" <mingo@...hat.com>, "Shankar, Ravi V" <ravi.v.shankar@...el.com>, "corbet@....net" <corbet@....net>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>, "gorcunov@...il.com" <gorcunov@...il.com> Subject: Re: [PATCH v2 06/39] x86/fpu: Add helper for modifying xstate On Mon, Oct 03, 2022 at 08:05:13PM +0000, Edgecombe, Rick P wrote: > On Mon, 2022-10-03 at 10:48 -0700, Kees Cook wrote: > > > The easiest way to modify supervisor xfeature data is to force > > > restore > > > the registers and write directly to the MSRs. Often times this is > > > just fine > > > anyway as the registers need to be restored before returning to > > > userspace. > > > Do this for now, leaving buffer writing optimizations for the > > > future. > > > > Just for my own clarity, does this mean lock/load _needs_ to happen > > before MSR access, or is it just a convenient place to do it? From > > later > > patches it seems it's a requirement during MSR access, which might be > > a > > good idea to detail here. It answers the question "when is this > > function > > needed?" > > The CET state is xsaves managed. It gets lazily restored before > returning to userspace with the rest of the fpu stuff. This function > will force restore all the fpu state to the registers early and lock > them from being automatically saved/restored. Then the tasks CET state > can be modified in the MSRs, before unlocking the fpregs. Last time I > tried to modify the state directly in the xsave buffer when it was > efficient, but it had issues and Thomas suggested this. Okay, gotcha. Thanks! -- Kees Cook
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