[PATCH v21 12/26] mm: Introduce VM_SHSTK for shadow stack memory

Yu-cheng Yu
2021-02-17T22:38:49Z

A shadow stack PTE must be read-only and have _PAGE_DIRTY set. However,
read-only and Dirty PTEs also exist for copy-on-write (COW) pages. These
two cases are handled differently for page faults. Introduce VM_SHSTK to
track shadow stack VMAs.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
Documentation/filesystems/proc.rst | 1 +
arch/x86/mm/mmap.c | 2 ++
fs/proc/task_mmu.c | 3 +++
include/linux/mm.h | 8 ++++++++
4 files changed, 14 insertions(+)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 2fa69f710e2a..fc533bfb9391 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -549,6 +549,7 @@ encoded manner. The codes are the following:
mg mergable advise flag
bt arm64 BTI guarded page
mt arm64 MTE allocation tags are enabled
+ ss shadow stack page
== =======================================

Note that there is no guarantee that every flag and associated mnemonic will
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index c90c20904a60..a22c6b6fc607 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -165,6 +165,8 @@ unsigned long get_mmap_base(int is_legacy)

const char *arch_vma_name(struct vm_area_struct *vma)
{
+ if (vma->vm_flags & VM_SHSTK)
+ return "[shadow stack]";
return NULL;
}

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 602e3a52884d..59623dcd92bb 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -661,6 +661,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
[ilog2(VM_PKEY_BIT4)] = "",
#endif
#endif /* CONFIG_ARCH_HAS_PKEYS */
+#ifdef CONFIG_X86_CET
+ [ilog2(VM_SHSTK)] = "ss",
+#endif
};
size_t i;

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ecdf8a8cd6ae..7be5f8b874aa 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -312,11 +312,13 @@ extern unsigned int kobjsize(const void *objp);
#define VM_HIGH_ARCH_BIT_2 34 /* bit only usable on 64-bit architectures */
#define VM_HIGH_ARCH_BIT_3 35 /* bit only usable on 64-bit architectures */
#define VM_HIGH_ARCH_BIT_4 36 /* bit only usable on 64-bit architectures */
+#define VM_HIGH_ARCH_BIT_5 37 /* bit only usable on 64-bit architectures */
#define VM_HIGH_ARCH_0 BIT(VM_HIGH_ARCH_BIT_0)
#define VM_HIGH_ARCH_1 BIT(VM_HIGH_ARCH_BIT_1)
#define VM_HIGH_ARCH_2 BIT(VM_HIGH_ARCH_BIT_2)
#define VM_HIGH_ARCH_3 BIT(VM_HIGH_ARCH_BIT_3)
#define VM_HIGH_ARCH_4 BIT(VM_HIGH_ARCH_BIT_4)
+#define VM_HIGH_ARCH_5 BIT(VM_HIGH_ARCH_BIT_5)
#endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */

#ifdef CONFIG_ARCH_HAS_PKEYS
@@ -332,6 +334,12 @@ extern unsigned int kobjsize(const void *objp);
#endif
#endif /* CONFIG_ARCH_HAS_PKEYS */

+#ifdef CONFIG_X86_CET
+# define VM_SHSTK VM_HIGH_ARCH_5
+#else
+# define VM_SHSTK VM_NONE
+#endif
+
#if defined(CONFIG_X86)
# define VM_PAT VM_ARCH_1 /* PAT reserves whole VMA at once (x86) */
#elif defined(CONFIG_PPC)
--
2.21.0