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Subject[PATCH v19 14/25] x86/mm: Update maybe_mkwrite() for shadow stack
Date
When serving a page fault, maybe_mkwrite() makes a PTE writable if its vma
has VM_WRITE.

A shadow stack vma has VM_SHSTK. Its PTEs have _PAGE_DIRTY, but not
_PAGE_WRITE. In fork(), _PAGE_DIRTY is cleared to effect copy-on-write,
and in page fault, _PAGE_DIRTY is restored and the shadow stack page is
writable again.

Update maybe_mkwrite() by introducing arch_maybe_mkwrite(), which sets
_PAGE_DIRTY for a shadow stack PTE.

Apply the same changes to maybe_pmd_mkwrite().

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/Kconfig | 4 ++++
arch/x86/mm/pgtable.c | 18 ++++++++++++++++++
include/linux/mm.h | 2 ++
include/linux/pgtable.h | 24 ++++++++++++++++++++++++
mm/huge_memory.c | 2 ++
5 files changed, 50 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 074b3c0e6bf6..35de64559a59 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1954,6 +1954,9 @@ config X86_SGX
config ARCH_HAS_SHADOW_STACK
def_bool n

+config ARCH_MAYBE_MKWRITE
+ def_bool n
+
config X86_CET
prompt "Intel Control-flow protection for user-mode"
def_bool n
@@ -1961,6 +1964,7 @@ config X86_CET
depends on AS_WRUSS
select ARCH_USES_HIGH_VMA_FLAGS
select ARCH_HAS_SHADOW_STACK
+ select ARCH_MAYBE_MKWRITE
help
Control-flow protection is a set of hardware features which place
additional restrictions on indirect branches. These help
diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index f6a9e2e36642..0f4fbf51a9fc 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -610,6 +610,24 @@ int pmdp_clear_flush_young(struct vm_area_struct *vma,
}
#endif

+#ifdef CONFIG_ARCH_MAYBE_MKWRITE
+pte_t arch_maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
+{
+ if (likely(vma->vm_flags & VM_SHSTK))
+ pte = pte_mkwrite_shstk(pte);
+ return pte;
+}
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+pmd_t arch_maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
+{
+ if (likely(vma->vm_flags & VM_SHSTK))
+ pmd = pmd_mkwrite_shstk(pmd);
+ return pmd;
+}
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
+#endif /* CONFIG_ARCH_MAYBE_MKWRITE */
+
/**
* reserve_top_address - reserves a hole in the top of kernel address space
* @reserve - size of hole to reserve
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7be5f8b874aa..abd756e426fc 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -993,6 +993,8 @@ 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);
+ else
+ pte = arch_maybe_mkwrite(pte, vma);
return pte;
}

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 8fcdfa52eb4b..d8452218d09b 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1439,6 +1439,30 @@ static inline bool arch_has_pfn_modify_check(void)
}
#endif /* !_HAVE_ARCH_PFN_MODIFY_ALLOWED */

+#ifdef CONFIG_MMU
+#ifdef CONFIG_ARCH_MAYBE_MKWRITE
+pte_t arch_maybe_mkwrite(pte_t pte, struct vm_area_struct *vma);
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+pmd_t arch_maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma);
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
+
+#else /* !CONFIG_ARCH_MAYBE_MKWRITE */
+static inline pte_t arch_maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
+{
+ return pte;
+}
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static inline pmd_t arch_maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
+{
+ return pmd;
+}
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
+
+#endif /* CONFIG_ARCH_MAYBE_MKWRITE */
+#endif /* CONFIG_MMU */
+
/*
* Architecture PAGE_KERNEL_* fallbacks
*
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9237976abe72..bfec65c9308b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -478,6 +478,8 @@ pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
{
if (likely(vma->vm_flags & VM_WRITE))
pmd = pmd_mkwrite(pmd);
+ else
+ pmd = arch_maybe_pmd_mkwrite(pmd, vma);
return pmd;
}

--
2.21.0
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