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Date: Thu, 30 Aug 2018 23:01:26 +0200 From: Jann Horn <jannh@...gle.com> To: yu-cheng.yu@...el.com Cc: Dave Hansen <dave.hansen@...ux.intel.com>, "the arch/x86 maintainers" <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, kernel list <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>, linux-arch <linux-arch@...r.kernel.org>, Linux API <linux-api@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>, Andy Lutomirski <luto@...capital.net>, Balbir Singh <bsingharora@...il.com>, Cyrill Gorcunov <gorcunov@...il.com>, Florian Weimer <fweimer@...hat.com>, hjl.tools@...il.com, Jonathan Corbet <corbet@....net>, 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@...el.com, vedvyas.shanbhogue@...el.com Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW On Thu, Aug 30, 2018 at 10:57 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote: > > On Thu, 2018-08-30 at 22:44 +0200, Jann Horn wrote: > > On Thu, Aug 30, 2018 at 10:25 PM Yu-cheng Yu <yu-cheng.yu@...el.com> > > wrote: > ... > > > In the flow you described, if C writes to the overflow page before > > > B > > > gets in with a 'call', the return address is still correct for > > > B. To > > > make an attack, C needs to write again before the TLB flush. I > > > agree > > > that is possible. > > > > > > Assume we have a guard page, can someone in the short window do > > > recursive calls in B, move ssp to the end of the guard page, and > > > trigger the same again? He can simply take the incssp route. > > I don't understand what you're saying. If the shadow stack is > > between > > guard pages, you should never be able to move SSP past that area's > > guard pages without an appropriate shadow stack token (not even with > > INCSSP, since that has a maximum range of PAGE_SIZE/2), and > > therefore, > > it shouldn't matter whether memory outside that range is incorrectly > > marked as shadow stack. Am I missing something? > > INCSSP has a range of 256, but we can do multiple of that. > But I realize the key is not to have the transient SHSTK page at all. > The guard page is !pte_write() and even we have flaws in > ptep_set_wrprotect(), there will not be any transient SHSTK pages. I > will add guard pages to both ends. > > Still thinking how to fix ptep_set_wrprotect(). cmpxchg loop? Or is that slow?
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