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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id i2sm3881450pjs.21.2020.02.25.12.20.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Feb 2020 12:20:53 -0800 (PST) Date: Tue, 25 Feb 2020 12:20:52 -0800 From: Kees Cook To: Yu-cheng Yu Cc: 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 , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V. Shankar" , Vedvyas Shanbhogue , Dave Martin , x86-patch-review@intel.com Subject: Re: [RFC PATCH v9 14/27] mm: Handle Shadow Stack page fault Message-ID: <202002251218.F919026@keescook> References: <20200205181935.3712-1-yu-cheng.yu@intel.com> <20200205181935.3712-15-yu-cheng.yu@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200205181935.3712-15-yu-cheng.yu@intel.com> Sender: linux-api-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Wed, Feb 05, 2020 at 10:19:22AM -0800, Yu-cheng Yu wrote: > When a task does fork(), its Shadow Stack (SHSTK) must be duplicated for > the child. This patch implements a flow similar to copy-on-write of an > anonymous page, but for SHSTK. > > A SHSTK PTE must be RO and Dirty. This Dirty bit requirement is used to > effect the copying. In copy_one_pte(), clear the Dirty bit from a SHSTK > PTE to cause a page fault upon the next SHSTK access. At that time, fix > the PTE and copy/re-use the page. Just to confirm, during the fork, it's really not a SHSTK for a moment (it's still RO, but not dirty). Can other racing threads muck this up, or is this bit removed only on the copied side? -Kees > > Signed-off-by: Yu-cheng Yu > --- > arch/x86/mm/pgtable.c | 15 +++++++++++++++ > include/asm-generic/pgtable.h | 17 +++++++++++++++++ > mm/memory.c | 7 ++++++- > 3 files changed, 38 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c > index 7bd2c3a52297..2eb33794c08d 100644 > --- a/arch/x86/mm/pgtable.c > +++ b/arch/x86/mm/pgtable.c > @@ -872,3 +872,18 @@ int pmd_free_pte_page(pmd_t *pmd, unsigned long addr) > > #endif /* CONFIG_X86_64 */ > #endif /* CONFIG_HAVE_ARCH_HUGE_VMAP */ > + > +#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER > +inline bool arch_copy_pte_mapping(vm_flags_t vm_flags) > +{ > + return (vm_flags & VM_SHSTK); > +} > + > +inline pte_t pte_set_vma_features(pte_t pte, struct vm_area_struct *vma) > +{ > + if (vma->vm_flags & VM_SHSTK) > + return pte_mkdirty_shstk(pte); > + else > + return pte; > +} > +#endif /* CONFIG_X86_INTEL_SHADOW_STACK_USER */ > diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h > index 798ea36a0549..9cb2f9ba5895 100644 > --- a/include/asm-generic/pgtable.h > +++ b/include/asm-generic/pgtable.h > @@ -1190,6 +1190,23 @@ static inline bool arch_has_pfn_modify_check(void) > } > #endif /* !_HAVE_ARCH_PFN_MODIFY_ALLOWED */ > > +#ifdef CONFIG_MMU > +#ifndef CONFIG_ARCH_HAS_SHSTK > +static inline bool arch_copy_pte_mapping(vm_flags_t vm_flags) > +{ > + return false; > +} > + > +static inline pte_t pte_set_vma_features(pte_t pte, struct vm_area_struct *vma) > +{ > + return pte; > +} > +#else > +bool arch_copy_pte_mapping(vm_flags_t vm_flags); > +pte_t pte_set_vma_features(pte_t pte, struct vm_area_struct *vma); > +#endif > +#endif /* CONFIG_MMU */ > + > /* > * Architecture PAGE_KERNEL_* fallbacks > * > diff --git a/mm/memory.c b/mm/memory.c > index 45442d9a4f52..6daa28614327 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -772,7 +772,8 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, > * If it's a COW mapping, write protect it both > * in the parent and the child > */ > - if (is_cow_mapping(vm_flags) && pte_write(pte)) { > + if ((is_cow_mapping(vm_flags) && pte_write(pte)) || > + arch_copy_pte_mapping(vm_flags)) { > ptep_set_wrprotect(src_mm, addr, src_pte); > pte = pte_wrprotect(pte); > } > @@ -2417,6 +2418,7 @@ static inline void wp_page_reuse(struct vm_fault *vmf) > flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte)); > entry = pte_mkyoung(vmf->orig_pte); > entry = maybe_mkwrite(pte_mkdirty(entry), vma); > + entry = pte_set_vma_features(entry, vma); > if (ptep_set_access_flags(vma, vmf->address, vmf->pte, entry, 1)) > update_mmu_cache(vma, vmf->address, vmf->pte); > pte_unmap_unlock(vmf->pte, vmf->ptl); > @@ -2504,6 +2506,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf) > flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte)); > entry = mk_pte(new_page, vma->vm_page_prot); > entry = maybe_mkwrite(pte_mkdirty(entry), vma); > + entry = pte_set_vma_features(entry, vma); > /* > * Clear the pte entry and flush it first, before updating the > * pte with the new entry. This will avoid a race condition > @@ -3023,6 +3026,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > pte = mk_pte(page, vma->vm_page_prot); > if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page, NULL)) { > pte = maybe_mkwrite(pte_mkdirty(pte), vma); > + pte = pte_set_vma_features(pte, vma); > vmf->flags &= ~FAULT_FLAG_WRITE; > ret |= VM_FAULT_WRITE; > exclusive = RMAP_EXCLUSIVE; > @@ -3165,6 +3169,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf) > entry = mk_pte(page, vma->vm_page_prot); > if (vma->vm_flags & VM_WRITE) > entry = pte_mkwrite(pte_mkdirty(entry)); > + entry = pte_set_vma_features(entry, vma); > > vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address, > &vmf->ptl); > -- > 2.21.0 > > -- Kees Cook