[PATCH v6 29/41] x86/shstk: Add user-mode shadow stack support

From: Rick Edgecombe
Date: Sat Feb 18 2023 - 16:21:47 EST

From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Introduce basic shadow stack enabling/disabling/allocation routines.
A task's shadow stack is allocated from memory with VM_SHADOW_STACK flag
and has a fixed size of min(RLIMIT_STACK, 4GB).

Keep the task's shadow stack address and size in thread_struct. This will
be copied when cloning new threads, but needs to be cleared during exec,
so add a function to do this.

Do not support IA32 emulation or x32.

Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>

---
v5:
- Switch to EOPNOTSUPP
- Use MAP_ABOVE4G
- Move set_clr_bits_msrl() to patch where it is first used

v4:
- Just set MSR_IA32_U_CET when disabling shadow stack, since we don't
have IBT yet. (Peterz)

v3:
- Use define for set_clr_bits_msrl() (Kees)
- Make some functions static (Kees)
- Change feature_foo() to features_foo() (Kees)
- Centralize shadow stack size rlimit checks (Kees)
- Disable x32 support

v2:
- Get rid of unnecessary shstk->base checks
- Don't support IA32 emulation
---
arch/x86/include/asm/processor.h | 2 +
arch/x86/include/asm/shstk.h | 7 ++
arch/x86/include/uapi/asm/prctl.h | 3 +
arch/x86/kernel/shstk.c | 145 ++++++++++++++++++++++++++++++
4 files changed, 157 insertions(+)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index bd16e012b3e9..ff98cd6d5af2 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -479,6 +479,8 @@ struct thread_struct {
#ifdef CONFIG_X86_USER_SHADOW_STACK
unsigned long features;
unsigned long features_locked;
+
+ struct thread_shstk shstk;
#endif

/* Floating point and extended processor state */
diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
index ec753809f074..2b1f7c9b9995 100644
--- a/arch/x86/include/asm/shstk.h
+++ b/arch/x86/include/asm/shstk.h
@@ -8,12 +8,19 @@
struct task_struct;

#ifdef CONFIG_X86_USER_SHADOW_STACK
+struct thread_shstk {
+ u64 base;
+ u64 size;
+};
+
long shstk_prctl(struct task_struct *task, int option, unsigned long features);
void reset_thread_features(void);
+void shstk_free(struct task_struct *p);
#else
static inline long shstk_prctl(struct task_struct *task, int option,
unsigned long arg2) { return -EINVAL; }
static inline void reset_thread_features(void) {}
+static inline void shstk_free(struct task_struct *p) {}
#endif /* CONFIG_X86_USER_SHADOW_STACK */

#endif /* __ASSEMBLY__ */
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index b2b3b7200b2d..7dfd9dc00509 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -26,4 +26,7 @@
#define ARCH_SHSTK_DISABLE 0x5002
#define ARCH_SHSTK_LOCK 0x5003

+/* ARCH_SHSTK_ features bits */
+#define ARCH_SHSTK_SHSTK (1ULL << 0)
+
#endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 41ed6552e0a5..3cb85224d856 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -8,14 +8,159 @@

#include <linux/sched.h>
#include <linux/bitops.h>
+#include <linux/types.h>
+#include <linux/mm.h>
+#include <linux/mman.h>
+#include <linux/slab.h>
+#include <linux/uaccess.h>
+#include <linux/sched/signal.h>
+#include <linux/compat.h>
+#include <linux/sizes.h>
+#include <linux/user.h>
+#include <asm/msr.h>
+#include <asm/fpu/xstate.h>
+#include <asm/fpu/types.h>
+#include <asm/shstk.h>
+#include <asm/special_insns.h>
+#include <asm/fpu/api.h>
#include <asm/prctl.h>

+static bool features_enabled(unsigned long features)
+{
+ return current->thread.features & features;
+}
+
+static void features_set(unsigned long features)
+{
+ current->thread.features |= features;
+}
+
+static void features_clr(unsigned long features)
+{
+ current->thread.features &= ~features;
+}
+
+static unsigned long alloc_shstk(unsigned long size)
+{
+ int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_ABOVE4G;
+ struct mm_struct *mm = current->mm;
+ unsigned long addr, unused;
+
+ mmap_write_lock(mm);
+ addr = do_mmap(NULL, addr, size, PROT_READ, flags,
+ VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL);
+
+ mmap_write_unlock(mm);
+
+ return addr;
+}
+
+static unsigned long adjust_shstk_size(unsigned long size)
+{
+ if (size)
+ return PAGE_ALIGN(size);
+
+ return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G));
+}
+
+static void unmap_shadow_stack(u64 base, u64 size)
+{
+ while (1) {
+ int r;
+
+ r = vm_munmap(base, size);
+
+ /*
+ * vm_munmap() returns -EINTR when mmap_lock is held by
+ * something else, and that lock should not be held for a
+ * long time. Retry it for the case.
+ */
+ if (r == -EINTR) {
+ cond_resched();
+ continue;
+ }
+
+ /*
+ * For all other types of vm_munmap() failure, either the
+ * system is out of memory or there is bug.
+ */
+ WARN_ON_ONCE(r);
+ break;
+ }
+}
+
+static int shstk_setup(void)
+{
+ struct thread_shstk *shstk = &current->thread.shstk;
+ unsigned long addr, size;
+
+ /* Already enabled */
+ if (features_enabled(ARCH_SHSTK_SHSTK))
+ return 0;
+
+ /* Also not supported for 32 bit and x32 */
+ if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) || in_32bit_syscall())
+ return -EOPNOTSUPP;
+
+ size = adjust_shstk_size(0);
+ addr = alloc_shstk(size);
+ if (IS_ERR_VALUE(addr))
+ return PTR_ERR((void *)addr);
+
+ fpregs_lock_and_load();
+ wrmsrl(MSR_IA32_PL3_SSP, addr + size);
+ wrmsrl(MSR_IA32_U_CET, CET_SHSTK_EN);
+ fpregs_unlock();
+
+ shstk->base = addr;
+ shstk->size = size;
+ features_set(ARCH_SHSTK_SHSTK);
+
+ return 0;
+}
+
void reset_thread_features(void)
{
+ memset(&current->thread.shstk, 0, sizeof(struct thread_shstk));
current->thread.features = 0;
current->thread.features_locked = 0;
}

+void shstk_free(struct task_struct *tsk)
+{
+ struct thread_shstk *shstk = &tsk->thread.shstk;
+
+ if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) ||
+ !features_enabled(ARCH_SHSTK_SHSTK))
+ return;
+
+ if (!tsk->mm)
+ return;
+
+ unmap_shadow_stack(shstk->base, shstk->size);
+}
+
+static int shstk_disable(void)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+ return -EOPNOTSUPP;
+
+ /* Already disabled? */
+ if (!features_enabled(ARCH_SHSTK_SHSTK))
+ return 0;
+
+ fpregs_lock_and_load();
+ /* Disable WRSS too when disabling shadow stack */
+ wrmsrl(MSR_IA32_U_CET, 0);
+ wrmsrl(MSR_IA32_PL3_SSP, 0);
+ fpregs_unlock();
+
+ shstk_free(current);
+ features_clr(ARCH_SHSTK_SHSTK);
+
+ return 0;
+}
+
long shstk_prctl(struct task_struct *task, int option, unsigned long features)
{
if (option == ARCH_SHSTK_LOCK) {
--
2.17.1