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Subject[PATCH v5 16/27] mm: Handle shadow stack page fault
Date
When a task does fork(), its shadow stack must be duplicated for
the child. However, the child may not actually use all pages of
of the copied shadow stack. This patch implements a flow that
is similar to copy-on-write of an anonymous page, but for shadow
stack memory. A shadow stack PTE needs to be RO and dirty. We
use this dirty bit requirement to effect the copying of shadow
stack pages.

In copy_one_pte(), we clear the dirty bit from the shadow stack
PTE. On the next shadow stack access to the PTE, a page fault
occurs. At that time, we then copy/re-use the page and fix the
PTE.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/mm/pgtable.c | 15 +++++++++++++++
include/asm-generic/pgtable.h | 8 ++++++++
mm/memory.c | 7 ++++++-
3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 089e78c4effd..e9ee4c86a477 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -881,3 +881,18 @@ 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 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;
+}
+
+inline bool arch_copy_pte_mapping(vm_flags_t vm_flags)
+{
+ return (vm_flags & VM_SHSTK);
+}
+#endif /* CONFIG_X86_INTEL_SHADOW_STACK_USER */
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index 88ebc6102c7c..015b769377a3 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -1127,4 +1127,12 @@ static inline bool arch_has_pfn_modify_check(void)
#endif
#endif

+#ifndef CONFIG_ARCH_HAS_SHSTK
+#define pte_set_vma_features(pte, vma) pte
+#define arch_copy_pte_mapping(vma_flags) false
+#else
+pte_t pte_set_vma_features(pte_t pte, struct vm_area_struct *vma);
+bool arch_copy_pte_mapping(vm_flags_t vm_flags);
+#endif
+
#endif /* _ASM_GENERIC_PGTABLE_H */
diff --git a/mm/memory.c b/mm/memory.c
index c467102a5cbc..1fb676ec7da2 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1022,7 +1022,8 @@ 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);
}
@@ -2462,6 +2463,7 @@ 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);
@@ -2535,6 +2537,7 @@ 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
@@ -3045,6 +3048,7 @@ 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;
@@ -3187,6 +3191,7 @@ 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);
--
2.17.1
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