Re: [NEEDS-REVIEW] Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

Yu-cheng Yu
2020-09-15T19:09:30Z

On Mon, 2020-09-14 at 17:12 -0700, Yu, Yu-cheng wrote:
> On 9/14/2020 7:50 AM, Dave Hansen wrote:
> > On 9/11/20 3:59 PM, Yu-cheng Yu wrote:
> > ...
> > > Here are the changes if we take the mprotect(PROT_SHSTK) approach.
> > > Any comments/suggestions?
> >
> > I still don't like it. :)
> >
> > I'll also be much happier when there's a proper changelog to accompany
> > this which also spells out the alternatives any why they suck so much.

[...]

I revised it. If this turns out needing more work/discussion, we can split it
out from the shadow stack series.

Thanks,
Yu-cheng

------

From 114f3108207e3eb15fe13b3ff43d1ac273b5072a Mon Sep 17 00:00:00 2001
From: Yu-cheng Yu <yu-cheng.yu@intel.com>
Date: Thu, 10 Sep 2020 16:41:30 -0700
Subject: [PATCH] mm: Introduce PROT_SHSTK for shadow stack

There are three possible options to create a shadow stack allocation API:
an arch_prctl, a new syscall, or adding PROT_SHSTK to mmap()/mprotect().
Each has its advantages and compromises.

An arch_prctl() is the least intrusive. However, the existing x86
arch_prctl() takes only two parameters. Multiple parameters must be
passed in a memory buffer. There is a proposal to pass more parameters in
registers [1], but no active discussion on that.

A new syscall minimizes compatibility issues and offers an extensible frame
work to other architectures, but this will likely result in some overlap of
mmap()/mprotect().

The introduction of PROT_SHSTK to mmap()/mprotect() takes advantage of
existing APIs. The x86-specific PROT_SHSTK is translated to VM_SHSTK, and
a shadow stack mapping is created without reinventing the wheel. There are
potential pitfalls though. The most obvious one would be using this as a
bypass to shadow stack protection. However, the attacker would have to get
to the syscall first.

Since arch_calc_vm_prot_bits() is modified, I have moved arch_vm_get_page
_prot() and arch_calc_vm_prot_bits() to x86/include/asm/mman.h.
This will be more consistent with other architectures.

[1] https://lore.kernel.org/lkml/20200828121624.108243-1-hjl.tools@gmail.com/

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/include/asm/mman.h | 63 ++++++++++++++++++++++++++++++++
arch/x86/include/uapi/asm/mman.h | 28 ++------------
mm/mprotect.c | 10 +++++
3 files changed, 77 insertions(+), 24 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..2f19e429c9e2
--- /dev/null
+++ b/arch/x86/include/asm/mman.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_MMAN_H
+#define _ASM_X86_MMAN_H
+
+#include <asm/cpufeature.h>
+#include <uapi/asm/mman.h>
+
+#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
+/*
+ * Take the 4 protection key bits out of the vma->vm_flags
+ * value and turn them in to the bits that we can put in
+ * to a pte.
+ *
+ * Only override these if Protection Keys are available
+ * (which is only on 64-bit).
+ */
+#define arch_vm_get_page_prot(vm_flags) __pgprot( \
+ ((vm_flags) & VM_PKEY_BIT0 ? _PAGE_PKEY_BIT0 : 0) | \
+ ((vm_flags) & VM_PKEY_BIT1 ? _PAGE_PKEY_BIT1 : 0) | \
+ ((vm_flags) & VM_PKEY_BIT2 ? _PAGE_PKEY_BIT2 : 0) | \
+ ((vm_flags) & VM_PKEY_BIT3 ? _PAGE_PKEY_BIT3 : 0))
+
+#define pkey_vm_prot_bits(prot, key) ( \
+ ((key) & 0x1 ? VM_PKEY_BIT0 : 0) | \
+ ((key) & 0x2 ? VM_PKEY_BIT1 : 0) | \
+ ((key) & 0x4 ? VM_PKEY_BIT2 : 0) | \
+ ((key) & 0x8 ? VM_PKEY_BIT3 : 0))
+#else
+#define pkey_vm_prot_bits(prot, key) (0)
+#endif
+
+static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
+ unsigned long pkey)
+{
+ unsigned long vm_prot_bits = pkey_vm_prot_bits(prot, pkey);
+
+ vm_prot_bits |= ((prot & PROT_SHSTK) ? VM_SHSTK : 0);
+ return vm_prot_bits;
+}
+#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
+
+static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
+{
+ unsigned long supported = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM;
+
+ if (IS_ENABLED(CONFIG_X86_INTEL_SHADOW_STACK_USER) &&
+ static_cpu_has(X86_FEATURE_SHSTK) && (prot & PROT_SHSTK)) {
+ supported |= PROT_SHSTK;
+
+ /*
+ * A shadow stack mapping is indirectly writable by only
+ * the CALL and WRUSS instructions, but not other write
+ * instructions). PROT_SHSTK and PROT_WRITE are mutually
+ * exclusive.
+ */
+ supported &= ~PROT_WRITE;
+ }
+
+ return (prot & ~supported) == 0;
+}
+#define arch_validate_prot arch_validate_prot
+
+#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..39bb7db344a6 100644
--- a/arch/x86/include/uapi/asm/mman.h
+++ b/arch/x86/include/uapi/asm/mman.h
@@ -1,31 +1,11 @@
/* 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 */

-#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
-/*
- * Take the 4 protection key bits out of the vma->vm_flags
- * value and turn them in to the bits that we can put in
- * to a pte.
- *
- * Only override these if Protection Keys are available
- * (which is only on 64-bit).
- */
-#define arch_vm_get_page_prot(vm_flags) __pgprot( \
- ((vm_flags) & VM_PKEY_BIT0 ? _PAGE_PKEY_BIT0 : 0) | \
- ((vm_flags) & VM_PKEY_BIT1 ? _PAGE_PKEY_BIT1 : 0) | \
- ((vm_flags) & VM_PKEY_BIT2 ? _PAGE_PKEY_BIT2 : 0) | \
- ((vm_flags) & VM_PKEY_BIT3 ? _PAGE_PKEY_BIT3 : 0))
-
-#define arch_calc_vm_prot_bits(prot, key) ( \
- ((key) & 0x1 ? VM_PKEY_BIT0 : 0) | \
- ((key) & 0x2 ? VM_PKEY_BIT1 : 0) | \
- ((key) & 0x4 ? VM_PKEY_BIT2 : 0) | \
- ((key) & 0x8 ? VM_PKEY_BIT3 : 0))
-#endif
+#define PROT_SHSTK 0x10 /* shadow stack pages */

#include <asm-generic/mman.h>

-#endif /* _ASM_X86_MMAN_H */
+#endif /* _UAPI_ASM_X86_MMAN_H */
diff --git a/mm/mprotect.c b/mm/mprotect.c
index a8edbcb3af99..27a1b7a4e89c 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -571,6 +571,16 @@ static int do_mprotect_pkey(unsigned long start, size_t
len,
goto out;
}
}
+
+ /*
+ * Only anonymous mapping is suitable for shadow stack. A function
+ * call stack should not be backed by a file.
+ */
+ if ((prot & PROT_SHSTK) && (vma->vm_file)) {
+ error = -EINVAL;
+ goto out;
+ }
+
if (start > vma->vm_start)
prev = vma;

--
2.21.0