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    SubjectRe: [PATCH v1 RFC Zisslpcfi 11/20] mmu: maybe_mkwrite updated to manufacture shadow stack PTEs
    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 <debug@rivosinc.com>
    >>---
    >> 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.

    >
    >--
    >Thanks,
    >
    >David / dhildenb
    >

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