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Date: Fri, 2 Dec 2022 18:55:49 -0800 From: Kees Cook <keescook@...omium.org> To: Rick Edgecombe <rick.p.edgecombe@...el.com> Cc: 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>, Andy Lutomirski <luto@...nel.org>, Balbir Singh <bsingharora@...il.com>, Borislav Petkov <bp@...en8.de>, Cyrill Gorcunov <gorcunov@...il.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Eugene Syromiatnikov <esyr@...hat.com>, Florian Weimer <fweimer@...hat.com>, "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, 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>, Randy Dunlap <rdunlap@...radead.org>, Weijiang Yang <weijiang.yang@...el.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, John Allen <john.allen@....com>, kcc@...gle.com, eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com, dethoma@...rosoft.com, akpm@...ux-foundation.org, Andrew.Cooper3@...rix.com, christina.schimpe@...el.com, Yu-cheng Yu <yu-cheng.yu@...el.com> Subject: Re: [PATCH v4 37/39] x86: Add PTRACE interface for shadow stack On Fri, Dec 02, 2022 at 04:36:04PM -0800, Rick Edgecombe wrote: > From: Yu-cheng Yu <yu-cheng.yu@...el.com> > > Some applications (like GDB) would like to tweak shadow stack state via > ptrace. This allows for existing functionality to continue to work for > seized shadow stack applications. Provide an regset interface for > manipulating the shadow stack pointer (SSP). > > There is already ptrace functionality for accessing xstate, but this > does not include supervisor xfeatures. So there is not a completely > clear place for where to put the shadow stack state. Adding it to the > user xfeatures regset would complicate that code, as it currently shares > logic with signals which should not have supervisor features. > > Don't add a general supervisor xfeature regset like the user one, > because it is better to maintain flexibility for other supervisor > xfeatures to define their own interface. For example, an xfeature may > decide not to expose all of it's state to userspace, as is actually the > case for shadow stack ptrace functionality. A lot of enum values remain > to be used, so just put it in dedicated shadow stack regset. > > The only downside to not having a generic supervisor xfeature regset, > is that apps need to be enlightened of any new supervisor xfeature > exposed this way (i.e. they can't try to have generic save/restore > logic). But maybe that is a good thing, because they have to think > through each new xfeature instead of encountering issues when new a new > supervisor xfeature was added. > > By adding a shadow stack regset, it also has the effect of including the > shadow stack state in a core dump, which could be useful for debugging. > > The shadow stack specific xstate includes the SSP, and the shadow stack > and WRSS enablement status. Enabling shadow stack or wrss in the kernel > involves more than just flipping the bit. The kernel is made aware that > it has to do extra things when cloning or handling signals. That logic > is triggered off of separate feature enablement state kept in the task > struct. So the flipping on HW shadow stack enforcement without notifying > the kernel to change its behavior would severely limit what an application > could do without crashing, and the results would depend on kernel > internal implementation details. There is also no known use for controlling > this state via prtace today. So only expose the SSP, which is something > that userspace already has indirect control over. > > Tested-by: Pengfei Xu <pengfei.xu@...el.com> > Tested-by: John Allen <john.allen@....com> > Co-developed-by: Rick Edgecombe <rick.p.edgecombe@...el.com> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com> Reviewed-by: Kees Cook <keescook@...omium.org> -- Kees Cook
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