[PATCH v23 28/28] mm: Introduce PROT_SHSTK for shadow stack

Yu-cheng Yu
2021-03-16T15:28:07Z

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.

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

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
arch/x86/include/asm/mman.h | 57 +++++++++++++++++++++++++++++++-
arch/x86/include/uapi/asm/mman.h | 1 +
include/linux/mm.h | 1 +
mm/mmap.c | 8 ++++-
4 files changed, 65 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/mman.h b/arch/x86/include/asm/mman.h
index 629f6c81263a..bd94e30b5d34 100644
--- a/arch/x86/include/asm/mman.h
+++ b/arch/x86/include/asm/mman.h
@@ -20,11 +20,66 @@
((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) ( \
+#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)
+
+#ifdef CONFIG_X86_CET
+static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
+{
+ unsigned long valid = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM;
+
+ if (prot & ~(valid | PROT_SHSTK))
+ return false;
+
+ if (prot & PROT_SHSTK) {
+ struct vm_area_struct *vma;
+
+ if (!current->thread.cet.shstk_size)
+ return false;
+
+ /*
+ * 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.
+ */
+ if (prot & PROT_WRITE)
+ return false;
+
+ vma = find_vma(current->mm, addr);
+ if (!vma)
+ return false;
+
+ /*
+ * Shadow stack cannot be backed by a file or shared.
+ */
+ if (vma->vm_file || (vma->vm_flags & VM_SHARED))
+ return false;
+ }
+
+ return true;
+}
+
+#define arch_validate_prot arch_validate_prot
#endif

#endif /* _ASM_X86_MMAN_H */
diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h
index 3ce1923e6ed9..39bb7db344a6 100644
--- a/arch/x86/include/uapi/asm/mman.h
+++ b/arch/x86/include/uapi/asm/mman.h
@@ -4,6 +4,7 @@

#define MAP_32BIT 0x40 /* only give out 32bit addresses */

+#define PROT_SHSTK 0x10 /* shadow stack pages */

#include <asm-generic/mman.h>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index e178be052419..40c4b0fe7cc4 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -342,6 +342,7 @@ extern unsigned int kobjsize(const void *objp);

#if defined(CONFIG_X86)
# define VM_PAT VM_ARCH_1 /* PAT reserves whole VMA at once (x86) */
+# define VM_ARCH_CLEAR VM_SHSTK
#elif defined(CONFIG_PPC)
# define VM_SAO VM_ARCH_1 /* Strong Access Ordering (powerpc) */
#elif defined(CONFIG_PARISC)
diff --git a/mm/mmap.c b/mm/mmap.c
index 99077171010b..934cb3cbe952 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1481,6 +1481,12 @@ 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;

@@ -1545,7 +1551,7 @@ 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.
--
2.21.0