@@ -20,11 +20,68 @@
((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_SHSTK)
+ vm_prot_bits |= VM_SHADOW_STACK;
+
+ return vm_prot_bits;
+}
+
+#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
+
+#ifdef CONFIG_X86_SHADOW_STACK
+static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
+{
+ unsigned long valid = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM |
+ PROT_SHSTK;
+
+ if (prot & ~valid)
+ return false;
+
+ if (prot & PROT_SHSTK) {
+ 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;
+ }
+
+ return true;
+}
+
+#define arch_validate_prot arch_validate_prot
+
+static inline bool arch_validate_flags(unsigned long vm_flags, bool is_anon)
+{
+ if (vm_flags & VM_SHADOW_STACK) {
+ if ((vm_flags & VM_SHARED) || !is_anon)
+ return false;
+ }
+
+ return true;
+}
+
+#define arch_validate_flags(vm_flags, is_anon) arch_validate_flags(vm_flags, is_anon)
+
+#endif /* CONFIG_X86_SHADOW_STACK */
+
#endif /* _ASM_X86_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>
@@ -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_SHADOW_STACK
#elif defined(CONFIG_PPC)
# define VM_SAO VM_ARCH_1 /* Strong Access Ordering (powerpc) */
#elif defined(CONFIG_PARISC)
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_SHADOW_STACK 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> Cc: Kees Cook <keescook@chromium.org> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> --- v24: - Update arch_calc_vm_prot_bits(), leave PROT* checking to arch_validate_prot(). - Update arch_validate_prot(), leave vma flags checking to arch_validate_flags(). - Add arch_validate_flags(). arch/x86/include/asm/mman.h | 59 +++++++++++++++++++++++++++++++- arch/x86/include/uapi/asm/mman.h | 1 + include/linux/mm.h | 1 + 3 files changed, 60 insertions(+), 1 deletion(-)