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Message-ID: <202102101805.0B98ACA743@keescook> Date: Wed, 10 Feb 2021 18:05:48 -0800 From: Kees Cook <keescook@...omium.org> To: "Yu, Yu-cheng" <yu-cheng.yu@...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>, "Ravi V. Shankar" <ravi.v.shankar@...el.com>, Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>, Dave Martin <Dave.Martin@....com>, Weijiang Yang <weijiang.yang@...el.com>, Pengfei Xu <pengfei.xu@...el.com>, haitao.huang@...el.com Subject: Re: [PATCH v20 21/25] x86/cet/shstk: Handle signals for shadow stack On Wed, Feb 10, 2021 at 01:38:10PM -0800, Yu, Yu-cheng wrote: > On 2/10/2021 11:58 AM, Kees Cook wrote: > > On Wed, Feb 10, 2021 at 09:56:59AM -0800, Yu-cheng Yu wrote: > > > To deliver a signal, create a shadow stack restore token and put the token > > > and the signal restorer address on the shadow stack. For sigreturn, verify > > > the token and restore from it the shadow stack pointer. > > > > > > A shadow stack restore token marks a restore point of the shadow stack. > > > The token is distinctively different from any shadow stack address. > > > > How is it different? It seems like it just has the last 2 bits > > masked/set? > > > > For example, for 64-bit apps, > > A shadow stack pointer value (*ssp) has to be in some code area, but for a > token, (*ptr_of_token) = (ptr_of_token + 8), which has to be within the same > shadow stack area. In cet_verify_rstor_token(), this is checked. > > > > In sigreturn, restoring from a token ensures the target address is the > > > location pointed by the token. > > > > As in, a token (real stack address with 2-bit mask) is checked against > > the real stack address? I don't see a comparison -- it only checks that > > it is < TASK_SIZE. > > > > How does cet_restore_signal() figure into this? (As in, the MSR writes?) > > > > The kernel takes the restore address from the token. It will not mistakenly > take a wrong address from the shadow stack. I will put this in my commit > logs. Ah-ha, okay, got it now. Thank you! Reviewed-by: Kees Cook <keescook@...omium.org> -- Kees Cook
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