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Date: Thu, 30 Aug 2018 10:26:00 -0700 From: Yu-cheng Yu <yu-cheng.yu@...el.com> To: Dave Hansen <dave.hansen@...ux.intel.com>, Jann Horn <jannh@...gle.com> Cc: 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, 2018-08-30 at 10:19 -0700, Dave Hansen wrote: > On 08/30/2018 09:23 AM, Jann Horn wrote: > > > > Three threads (A, B, C) run with the same CR3. > > > > 1. a dirty+writable PTE is placed directly in front of B's shadow > > stack. > > (this can happen, right? or is there a guard page?) > > 2. C's TLB caches the dirty+writable PTE. > > 3. A performs some syscall that triggers ptep_set_wrprotect(). > > 4. A's syscall calls clear_bit(). > > 5. B's TLB caches the transient shadow stack. > > [now C has write access to B's transiently-extended shadow stack] > > 6. B recurses into the transiently-extended shadow stack > > 7. C overwrites the transiently-extended shadow stack area. > > 8. B returns through the transiently-extended shadow stack, giving > > the attacker instruction pointer control in B. > > 9. A's syscall broadcasts a TLB flush. > Heh, that's a good point. The shadow stack permissions are *not* > strictly reduced because a page getting marked as shadow-stack has > *increased* permissions when being used as a shadow stack. Fun. > > For general hardening, it seems like we want to ensure that there's > a > guard page at the bottom of the shadow stack. Yu-cheng, do we have > a > guard page? We don't have the guard page now, but there is a shadow stack token there, which cannot be used as a return address. Yu-cheng
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