From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yu-cheng Yu Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for shadow stack Date: Thu, 19 Jul 2018 10:06:03 -0700 Message-ID: <1532019963.16711.61.camel@intel.com> References: <20180710222639.8241-1-yu-cheng.yu@intel.com> <20180710222639.8241-17-yu-cheng.yu@intel.com> <1531328731.15351.3.camel@intel.com> <45a85b01-e005-8cb6-af96-b23ce9b5fca7@linux.intel.com> <1531868610.3541.21.camel@intel.com> <1531944882.10738.1.camel@intel.com> <3f158401-f0b6-7bf7-48ab-2958354b28ad@linux.intel.com> <1531955428.12385.30.camel@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann , Andy Lutomirski , Balbir Singh , Cyrill Gorcunov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek Peter List-Id: linux-api@vger.kernel.org On Wed, 2018-07-18 at 17:06 -0700, Dave Hansen wrote: > > > > > > > > > > > > > -static inline bool can_follow_write_pte(pte_t pte, unsigned > > > > int flags) > > > > +static inline bool can_follow_write(pte_t pte, unsigned int > > > > flags, > > > > +     struct vm_area_struct > > > > *vma) > > > >  { > > > > - return pte_write(pte) || > > > > - ((flags & FOLL_FORCE) && (flags & FOLL_COW) > > > > && pte_dirty(pte)); > > > > + if (!is_shstk_mapping(vma->vm_flags)) { > > > > + if (pte_write(pte)) > > > > + return true; > > > Let me see if I can say this another way. > > > > > > The bigger issue is that these patches change the semantics of > > > pte_write().  Before these patches, it meant that you *MUST* > > > have this > > > bit set to write to the page controlled by the PTE.  Now, it > > > means: you > > > can write if this bit is set *OR* the shadowstack bit > > > combination is set. > > Here, we only figure out (1) if the page is pointed by a writable > > PTE; or > > (2) if the page is pointed by a RO PTE (data or SHSTK) and it has > > been > > copied and it still exists.  We are not trying to > > determine if the > > SHSTK PTE is writable (we know it is not). > Please think about the big picture.  I'm not just talking about this > patch, but about every use of pte_write() in the kernel. > > > > > > > > > That's the fundamental problem.  We need some code in the kernel > > > that > > > logically represents the concept of "is this PTE a shadowstack > > > PTE or a > > > PTE with the write bit set", and we will call that pte_write(), > > > or maybe > > > pte_writable(). > > > > > > You *have* to somehow rectify this situation.  We can absolutely > > > no > > > leave pte_write() in its current, ambiguous state where it has > > > no real > > > meaning or where it is used to mean _both_ things depending on > > > context. > > True, the processor can always write to a page through a shadow > > stack > > PTE, but it must do that with a CALL instruction.  Can we define > > a  > > write operation as: MOV r1, *(r2).  Then we don't have any doubt > > on > > pte_write() any more. > No, we can't just move the target. :) > > You can define it this way, but then you also need to go to every > spot > in the kernel that calls pte_write() (and _PAGE_RW in fact) and > audit it > to ensure it means "mov ..." and not push. Which pte_write() do you think is right? bool is_shstk_pte(pte) { return (_PAGE_RW not set) && (_PAGE_DIRTY_HW set); } int pte_write_1(pte) { return (_PAGE_RW set) && !is_shstk_pte(pte); } int pte_write_2(pte) { return (_PAGE_RW set) || is_shstk_pte(pte); } Yu-cheng