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Subject[RFC PATCH v3 14/24] 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 | 10 ++++++++++
include/asm-generic/pgtable.h | 7 +++++++
mm/memory.c | 3 +++
3 files changed, 20 insertions(+)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index e848a4811785..c63261128ac3 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -872,3 +872,13 @@ 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;
+}
+#endif /* CONFIG_X86_INTEL_SHADOW_STACK_USER */
diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index aa5271717126..558a485617cd 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -1146,6 +1146,13 @@ static inline bool pmd_dirty_hw(pmd_t pmd)
{
return false;
}
+
+static inline pte_t pte_set_vma_features(pte_t pte, struct vm_area_struct *vma)
+{
+ return pte;
+}
+#else
+pte_t pte_set_vma_features(pte_t pte, struct vm_area_struct *vma);
#endif

#endif /* _ASM_GENERIC_PGTABLE_H */
diff --git a/mm/memory.c b/mm/memory.c
index c467102a5cbc..9b4e11944b5d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2462,6 +2462,7 @@ 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 +2536,7 @@ 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
@@ -3187,6 +3189,7 @@ 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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