Re: [PATCH v9 16/42] mm: Add guard pages around a shadow stack.

+++ b/include/linux/mm.h

@@ -342,7 +342,36 @@ extern unsigned int kobjsize(const void *objp);
 #endif /* CONFIG_ARCH_HAS_PKEYS */
 
 #ifdef CONFIG_X86_USER_SHADOW_STACK
-# define VM_SHADOW_STACK	VM_HIGH_ARCH_5 /* Should not be set with VM_SHARED */
+/*
+ * This flag should not be set with VM_SHARED because of lack of support
+ * core mm. It will also get a guard page. This helps userspace protect
+ * itself from attacks. The reasoning is as follows:
+ *
+ * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The
+ * INCSSP instruction can increment the shadow stack pointer. It is the
+ * shadow stack analog of an instruction like:
+ *
+ *   addq $0x80, %rsp
+ *
+ * However, there is one important difference between an ADD on %rsp
+ * and INCSSP. In addition to modifying SSP, INCSSP also reads from the
+ * memory of the first and last elements that were "popped". It can be
+ * thought of as acting like this:
+ *
+ * READ_ONCE(ssp);       // read+discard top element on stack
+ * ssp += nr_to_pop * 8; // move the shadow stack
+ * READ_ONCE(ssp-8);     // read+discard last popped stack element
+ *
+ * The maximum distance INCSSP can move the SSP is 2040 bytes, before
+ * it would read the memory. Therefore a single page gap will be enough
+ * to prevent any operation from shifting the SSP to an adjacent stack,
+ * since it would have to land in the gap at least once, causing a
+ * fault.
+ *
+ * Prevent using INCSSP to move the SSP between shadow stacks by
+ * having a PAGE_SIZE guard gap.
+ */
+# define VM_SHADOW_STACK	VM_HIGH_ARCH_5
 #else
 # define VM_SHADOW_STACK	VM_NONE
 #endif