linux-kernel - [PATCH 22/35] x86/mm: Prevent VM

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Date:   Sun, 30 Jan 2022 13:18:25 -0800
From:   Rick Edgecombe <rick.p.edgecombe@...el.com>
To:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com
Cc:     rick.p.edgecombe@...el.com
Subject: [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@...el.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