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

[Rick Edgecombe <rick.p.edgecombe@intel.com>]

From: Yu-cheng Yu <yu-cheng.yu@intel.com>

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@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Kees Cook <keescook@chromium.org>

---
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