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Tue, 7 Mar 2023 16:14:26 +0000 Date: Tue, 7 Mar 2023 16:14:09 +0000 From: Szabolcs Nagy To: Florian Weimer Cc: "Edgecombe, Rick P" , "david@redhat.com" , "bsingharora@gmail.com" , "hpa@zytor.com" , "Syromiatnikov, Eugene" , "peterz@infradead.org" , "rdunlap@infradead.org" , "keescook@chromium.org" , "Yu, Yu-cheng" , "Eranian, Stephane" , "kirill.shutemov@linux.intel.com" , "dave.hansen@linux.intel.com" , "linux-mm@kvack.org" , "nadav.amit@gmail.com" , "jannh@google.com" , "dethoma@microsoft.com" , "broonie@kernel.org" , "kcc@google.com" , "linux-arch@vger.kernel.org" , "bp@alien8.de" , "oleg@redhat.com" , "hjl.tools@gmail.com" , "Yang, Weijiang" , "Lutomirski, Andy" , "jamorris@linux.microsoft.com" , "arnd@arndb.de" , "tglx@linutronix.de" , "Schimpe, Christina" , "debug@rivosinc.com" , "x86@kernel.org" , "linux-doc@vger.kernel.org" , "mike.kravetz@oracle.com" , "pavel@ucw.cz" , "andrew.cooper3@citrix.com" , "john.allen@amd.com" , "rppt@kernel.org" , "nd@arm.com" , "mingo@redhat.com" , "corbet@lwn.net" , "linux-kernel@vger.kernel.org" , "linux-api@vger.kernel.org" , "gorcunov@gmail.com" , "akpm@linux-foundation.org" Subject: Re: [PATCH v7 01/41] Documentation/x86: Add CET shadow stack description Message-ID: References: <636de4a28a42a082f182e940fbd8e63ea23895cc.camel@intel.com> <9714f724b53b04fdf69302c6850885f5dfbf3af5.camel@intel.com> <87wn3tsuxf.fsf@oldenburg.str.redhat.com> <87ilfcoe59.fsf@oldenburg.str.redhat.com> Content-Type: text/plain; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 07 Mar 2023 16:14:37.0759 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 15acc6e2-902d-4032-cc33-08db1f270c23 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[63.35.35.123];Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: DBAEUR03FT018.eop-EUR03.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PAWPR08MB9996 Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org The 03/07/2023 15:00, Florian Weimer wrote: > * szabolcs: > > > changing/disabling the alt stack is not valid while a handler is > > executing on it. if we don't allow jumping out and back to an > > alt stack (swapcontext) then there can be only one alt stack > > live per thread and change/disable can do the shadow stack free. > > > > if jump back is allowed (linux even makes it race-free with > > SS_AUTODISARM) then the life-time of alt stack is extended > > beyond change/disable (jump back to an unregistered alt stack). > > > > to support jump back to an alt stack the requirements are > > > > 1) user has to manage an alt shadow stack together with the alt > > stack (requies user code change, not just libc). > > > > 2) kernel has to push a restore token on the thread shadow stack > > on signal entry (at least in case of alt shadow stack, and > > deal with corner cases around shadow stack overflow). > > We need to have a story for stackful coroutine switching as well, not > just for sigaltstack. I hope that we can use OpenJDK (Project Loom) and > QEMU as guinea pigs. If we have something that works for both, > hopefully that covers a broad range of scenarios. Userspace > coordination can eventually be handled by glibc; we can deallocate > alternate stacks on thread exit fairly easily (at least compared to the > current stack 8-). for stackful coroutines we just need a way to - allocate a shadow stack with a restore token on it. - switch to a target shadow stack with a restore token on it, while leaving behind a restore token on the old shadow stack. this is supported via map_shadow_stack syscall and the rstoressp, saveprevssp instruction pair. otoh there can be many alt shadow stacks per thread alive if we allow jump back (only one of them registered at a time) in fact they can be jumped to even from another thread, so their life-time is not tied to the thread (at least if we allow swapcontext across threads) so i think the libc cannot manage the alt shadow stacks, only user code can in the general case. and in case a signal runs on an alt shadow stack, the restore token can only be placed by the kernel on the old shadow stack. thanks.