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[2003:cb:c705:6d00:5870:9639:1c17:8162]) by smtp.gmail.com with ESMTPSA id g9-20020a5d5409000000b002c558228b6dsm3365811wrv.12.2023.02.13.06.56.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 13 Feb 2023 06:56:22 -0800 (PST) Message-ID: <7693247c-a55d-a375-3621-1b07115a9d99@redhat.com> Date: Mon, 13 Feb 2023 15:56:22 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [PATCH v1 RFC Zisslpcfi 11/20] mmu: maybe_mkwrite updated to manufacture shadow stack PTEs Content-Language: en-US To: Deepak Gupta Cc: linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, Andrew Morton , linux-mm@kvack.org References: <20230213045351.3945824-1-debug@rivosinc.com> <20230213045351.3945824-12-debug@rivosinc.com> <2d6eefb8-c7c5-7d32-9a75-ae716f828cd9@redhat.com> <20230213143754.GC3943238@debug.ba.rivosinc.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20230213143754.GC3943238@debug.ba.rivosinc.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 13.02.23 15:37, Deepak Gupta wrote: > On Mon, Feb 13, 2023 at 01:05:16PM +0100, David Hildenbrand wrote: >> On 13.02.23 05:53, Deepak Gupta wrote: >>> maybe_mkwrite creates PTEs with WRITE encodings for underlying arch if >>> VM_WRITE is turned on in vma->vm_flags. Shadow stack memory is a write- >>> able memory except it can only be written by certain specific >>> instructions. This patch allows maybe_mkwrite to create shadow stack PTEs >>> if vma is shadow stack VMA. Each arch can define which combination of VMA >>> flags means a shadow stack. >>> >>> Additionally pte_mkshdwstk must be provided by arch specific PTE >>> construction headers to create shadow stack PTEs. (in arch specific >>> pgtable.h). >>> >>> This patch provides dummy/stub pte_mkshdwstk if CONFIG_USER_SHADOW_STACK >>> is not selected. >>> >>> Signed-off-by: Deepak Gupta >>> --- >>> include/linux/mm.h | 23 +++++++++++++++++++++-- >>> include/linux/pgtable.h | 4 ++++ >>> 2 files changed, 25 insertions(+), 2 deletions(-) >>> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h >>> index 8f857163ac89..a7705bc49bfe 100644 >>> --- a/include/linux/mm.h >>> +++ b/include/linux/mm.h >>> @@ -1093,6 +1093,21 @@ static inline unsigned long thp_size(struct page *page) >>> void free_compound_page(struct page *page); >>> #ifdef CONFIG_MMU >>> + >>> +#ifdef CONFIG_USER_SHADOW_STACK >>> +bool arch_is_shadow_stack_vma(struct vm_area_struct *vma); >>> +#endif >>> + >>> +static inline bool >>> +is_shadow_stack_vma(struct vm_area_struct *vma) >>> +{ >>> +#ifdef CONFIG_USER_SHADOW_STACK >>> + return arch_is_shadow_stack_vma(vma); >>> +#else >>> + return false; >>> +#endif >>> +} >>> + >>> /* >>> * Do pte_mkwrite, but only if the vma says VM_WRITE. We do this when >>> * servicing faults for write access. In the normal case, do always want >>> @@ -1101,8 +1116,12 @@ void free_compound_page(struct page *page); >>> */ >>> static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma) >>> { >>> - if (likely(vma->vm_flags & VM_WRITE)) >>> - pte = pte_mkwrite(pte); >>> + if (likely(vma->vm_flags & VM_WRITE)) { >>> + if (unlikely(is_shadow_stack_vma(vma))) >>> + pte = pte_mkshdwstk(pte); >>> + else >>> + pte = pte_mkwrite(pte); >>> + } >>> return pte; >> >> Exactly what we are trying to avoid in the x86 approach right now. >> Please see the x86 series on details, we shouldn't try reinventing the >> wheel but finding a core-mm approach that fits multiple architectures. >> >> https://lkml.kernel.org/r/20230119212317.8324-1-rick.p.edgecombe@intel.com > > Thanks David for comment here. I looked at x86 approach. This patch > actually written in a way which is not re-inventing wheel and is following > a core-mm approach that fits multiple architectures. > > Change above checks `is_shadow_stack_vma` and if it returns true then only > it manufactures shadow stack pte else it'll make a regular writeable mapping. > > Now if we look at `is_shadow_stack_vma` implementation, it returns false if > `CONFIG_USER_SHADOW_STACK` is not defined. If `CONFIG_USER_SHADOW_STACK is > defined then it calls `arch_is_shadow_stack_vma` which should be implemented > by arch specific code. This allows each architecture to define their own vma > flag encodings for shadow stack (riscv chooses presence of only `VM_WRITE` > which is analogous to choosen PTE encodings on riscv W=1,R=0,X=0) > > Additionally pte_mkshdwstk will be nop if not implemented by architecture. > > Let me know if this make sense. If I am missing something here, let me know. See the discussion in that thread. The idea is to pass a VMA to pte_mkwrite() and let it handle how to actually set it writable. -- Thanks, David / dhildenb