[PATCH v12 26/26] mm: Introduce PROT_SHSTK for shadow stack

Yu-cheng Yu
2020-09-18T19:24:34Z

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 | 81 ++++++++++++++++++++++++++++++++
arch/x86/include/uapi/asm/mman.h | 28 ++---------
include/linux/mman.h | 8 ++++
mm/mmap.c | 8 +++-
mm/mprotect.c | 4 ++
5 files changed, 104 insertions(+), 25 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..4e73dcc0fea2
--- /dev/null
+++ b/arch/x86/include/asm/mman.h
@@ -0,0 +1,81 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_MMAN_H
+#define _ASM_X86_MMAN_H
+
+#include <asm/cpufeature.h>
+#include <linux/mm.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);
+
+ if (!(prot & PROT_WRITE) && (prot & PROT_SHSTK))
+ vm_prot_bits |= VM_SHSTK;
+
+ 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
+
+static inline bool arch_vma_can_mprot(struct vm_area_struct *vma,
+ unsigned long prot)
+{
+ bool can_mprot;
+
+ /*
+ * Function call stack should not be backed by a file or shared.
+ */
+ can_mprot = !(prot & PROT_SHSTK) ||
+ !(vma->vm_file || (vma->vm_flags & VM_SHARED));
+ return can_mprot;
+}
+#define arch_vma_can_mprot arch_vma_can_mprot
+
+#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/include/linux/mman.h b/include/linux/mman.h
index 6f34c33075f9..4d776adb0fdf 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -103,6 +103,14 @@ static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
#define arch_validate_prot arch_validate_prot
#endif

+#ifndef arch_vma_can_mprot
+/*
+ * Allow architectures to check if the vma can support the new
+ * protection.
+ */
+#define arch_vma_can_mprot(vma, prot) true
+#endif
+
/*
* Optimisation macro. It is equivalent to:
* (x & bit1) ? bit2 : 0
diff --git a/mm/mmap.c b/mm/mmap.c
index 81d4a00092da..4c403dfccff0 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1445,6 +1445,12 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
struct inode *inode = file_inode(file);
unsigned long flags_mask;

+ /*
+ * Call stack cannot be backed by a file.
+ */
+ if (vm_flags & VM_SHSTK)
+ return -EINVAL;
+
if (!file_mmap_ok(file, inode, pgoff, len))
return -EOVERFLOW;

@@ -1509,7 +1515,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
} else {
switch (flags & MAP_TYPE) {
case MAP_SHARED:
- if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP))
+ if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP|VM_SHSTK))
return -EINVAL;
/*
* Ignore pgoff.
diff --git a/mm/mprotect.c b/mm/mprotect.c
index a8edbcb3af99..cf73b59a36da 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -553,6 +553,10 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
error = -ENOMEM;
if (!vma)
goto out;
+ if (!arch_vma_can_mprot(vma, prot)) {
+ error = -EINVAL;
+ goto out;
+ }
prev = vma->vm_prev;
if (unlikely(grows & PROT_GROWSDOWN)) {
if (vma->vm_start >= end)
--
2.21.0