[PATCH 22/35] x86/mm: Prevent VM_WRITE shadow stacks

Shadow stack accesses are writes from handle_mm_fault() perspective. So to
generate the correct PTE, maybe_mkwrite() will rely on the presence of
VM_SHADOW_STACK or VM_WRITE in the vma.

In future patches, when VM_SHADOW_STACK is actually creatable by
userspace, a problem could happen if a user calls
mprotect( , , PROT_WRITE) on VM_SHADOW_STACK shadow stack memory. The code
would then be confused in the event of shadow stack accesses, and create a
writable PTE for a shadow stack access. Then the process would fault in a
loop.

Prevent this from happening by blocking this kind of memory (VM_WRITE and
VM_SHADOW_STACK) from being created, instead of complicating the fault
handler logic to handle it.

Add an x86 arch_validate_flags() implementation to handle the check.
Rename the uapi/asm/mman.h header guard to be able to use it for
arch/x86/include/asm/mman.h where the arch_validate_flags() will be.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---

v1:
 - New patch.

 arch/x86/include/asm/mman.h      | 21 +++++++++++++++++++++
 arch/x86/include/uapi/asm/mman.h |  6 +++---
 2 files changed, 24 insertions(+), 3 deletions(-)
 create mode 100644 arch/x86/include/asm/mman.h

diff --git a/arch/x86/include/asm/mman.h b/arch/x86/include/asm/mman.h
new file mode 100644
index 000000000000..b44fe31deb3a
--- /dev/null
+++ b/arch/x86/include/asm/mman.h

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_MMAN_H
+#define _ASM_X86_MMAN_H
+
+#include <linux/mm.h>
+#include <uapi/asm/mman.h>
+
+#ifdef CONFIG_X86_SHADOW_STACK
+static inline bool arch_validate_flags(unsigned long vm_flags)
+{
+	if ((vm_flags & VM_SHADOW_STACK) && (vm_flags & VM_WRITE))
+		return false;
+
+	return true;
+}
+
+#define arch_validate_flags(vm_flags) arch_validate_flags(vm_flags)
+
+#endif /* CONFIG_X86_SHADOW_STACK */
+
+#endif /* _ASM_X86_MMAN_H */

diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h
index d4a8d0424bfb..9704e27c4d24 100644
--- a/arch/x86/include/uapi/asm/mman.h
+++ b/arch/x86/include/uapi/asm/mman.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
-#ifndef _ASM_X86_MMAN_H
-#define _ASM_X86_MMAN_H
+#ifndef _UAPI_ASM_X86_MMAN_H
+#define _UAPI_ASM_X86_MMAN_H
 
 #define MAP_32BIT	0x40		/* only give out 32bit addresses */
 

@@ -28,4 +28,4 @@
 
 #include <asm-generic/mman.h>
 
-#endif /* _ASM_X86_MMAN_H */
+#endif /* _UAPI_ASM_X86_MMAN_H */

-- 
2.17.1