[PATCH v27 23/31] x86/cet/shstk: Add user-mode shadow stack support

Yu-cheng Yu
2021-05-21T22:14:36Z

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).

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: Kees Cook <keescook@chromium.org>
---
v27:
- Change 'struct cet_status' to 'struct thread_shstk', and change member
types from unsigned long to u64.
- Re-order local variables in reverse order of length.
- WARN_ON_ONCE() when vm_munmap() fails.

arch/x86/include/asm/cet.h | 30 +++++++
arch/x86/include/asm/processor.h | 5 ++
arch/x86/kernel/Makefile | 1 +
arch/x86/kernel/shstk.c | 130 +++++++++++++++++++++++++++++++
4 files changed, 166 insertions(+)
create mode 100644 arch/x86/include/asm/cet.h
create mode 100644 arch/x86/kernel/shstk.c

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
new file mode 100644
index 000000000000..6432baf4de1f
--- /dev/null
+++ b/arch/x86/include/asm/cet.h
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_CET_H
+#define _ASM_X86_CET_H
+
+#ifndef __ASSEMBLY__
+#include <linux/types.h>
+
+struct task_struct;
+
+/*
+ * Per-thread CET status
+ */
+struct thread_shstk {
+ u64 base;
+ u64 size;
+};
+
+#ifdef CONFIG_X86_SHADOW_STACK
+int shstk_setup(void);
+void shstk_free(struct task_struct *p);
+void shstk_disable(void);
+#else
+static inline int shstk_setup(void) { return 0; }
+static inline void shstk_free(struct task_struct *p) {}
+static inline void shstk_disable(void) {}
+#endif
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_X86_CET_H */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 556b2b17c3e2..7eb56a837cfa 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -27,6 +27,7 @@ struct vm86;
#include <asm/unwind_hints.h>
#include <asm/vmxfeatures.h>
#include <asm/vdso/processor.h>
+#include <asm/cet.h>

#include <linux/personality.h>
#include <linux/cache.h>
@@ -518,6 +519,10 @@ struct thread_struct {

unsigned int sig_on_uaccess_err:1;

+#ifdef CONFIG_X86_SHADOW_STACK
+ struct thread_shstk shstk;
+#endif
+
/* Floating point and extended processor state */
struct fpu fpu;
/*
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 0f66682ac02a..b0e599102277 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -149,6 +149,7 @@ obj-$(CONFIG_UNWINDER_FRAME_POINTER) += unwind_frame.o
obj-$(CONFIG_UNWINDER_GUESS) += unwind_guess.o

obj-$(CONFIG_AMD_MEM_ENCRYPT) += sev.o
+obj-$(CONFIG_X86_SHADOW_STACK) += shstk.o
###
# 64 bit specific files
ifeq ($(CONFIG_X86_64),y)
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
new file mode 100644
index 000000000000..5ea2b494e9f9
--- /dev/null
+++ b/arch/x86/kernel/shstk.c
@@ -0,0 +1,130 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * shstk.c - Intel shadow stack support
+ *
+ * Copyright (c) 2021, Intel Corporation.
+ * Yu-cheng Yu <yu-cheng.yu@intel.com>
+ */
+
+#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/internal.h>
+#include <asm/fpu/xstate.h>
+#include <asm/fpu/types.h>
+#include <asm/cet.h>
+
+static void start_update_msrs(void)
+{
+ fpregs_lock();
+ if (test_thread_flag(TIF_NEED_FPU_LOAD))
+ __fpregs_load_activate();
+}
+
+static void end_update_msrs(void)
+{
+ fpregs_unlock();
+}
+
+static unsigned long alloc_shstk(unsigned long size)
+{
+ int flags = MAP_ANONYMOUS | MAP_PRIVATE;
+ struct mm_struct *mm = current->mm;
+ unsigned long addr, populate;
+
+ mmap_write_lock(mm);
+ addr = do_mmap(NULL, 0, size, PROT_READ, flags, VM_SHADOW_STACK, 0,
+ &populate, NULL);
+ mmap_write_unlock(mm);
+
+ return addr;
+}
+
+int shstk_setup(void)
+{
+ struct thread_shstk *shstk = &current->thread.shstk;
+ unsigned long addr, size;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return -EOPNOTSUPP;
+
+ size = round_up(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
+ addr = alloc_shstk(size);
+ if (IS_ERR_VALUE(addr))
+ return PTR_ERR((void *)addr);
+
+ shstk->base = addr;
+ shstk->size = size;
+
+ start_update_msrs();
+ wrmsrl(MSR_IA32_PL3_SSP, addr + size);
+ wrmsrl(MSR_IA32_U_CET, CET_SHSTK_EN);
+ end_update_msrs();
+
+ return 0;
+}
+
+void shstk_free(struct task_struct *tsk)
+{
+ struct thread_shstk *shstk = &tsk->thread.shstk;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
+ !shstk->size ||
+ !shstk->base)
+ return;
+
+ if (!tsk->mm)
+ return;
+
+ while (1) {
+ int r;
+
+ r = vm_munmap(shstk->base, shstk->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;
+ }
+
+ shstk->base = 0;
+ shstk->size = 0;
+}
+
+void shstk_disable(void)
+{
+ struct thread_shstk *shstk = &current->thread.shstk;
+ u64 msr_val;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
+ !shstk->size ||
+ !shstk->base)
+ return;
+
+ start_update_msrs();
+ rdmsrl(MSR_IA32_U_CET, msr_val);
+ wrmsrl(MSR_IA32_U_CET, msr_val & ~CET_SHSTK_EN);
+ wrmsrl(MSR_IA32_PL3_SSP, 0);
+ end_update_msrs();
+
+ shstk_free(current);
+}
--
2.21.0