[PATCH 25/35] x86/cet/shstk: Add user-mode shadow stack support

Rick Edgecombe
2022-01-30T21:25:40Z

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).

Add the user shadow stack MSRs to the xsave helpers, so they can be used
to implement the functionality.

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.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Kees Cook <keescook@chromium.org>
---

v1:
- Switch to xsave helpers.
- Expand commit log.

Yu-cheng v30:
- Remove superfluous comments for struct thread_shstk.
- Replace 'populate' with 'unused'.

Yu-cheng v28:
- Update shstk_setup() with wrmsrl_safe(), returns success when shadow
stack feature is not present (since this is a setup function).

Yu-cheng 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 | 29 ++++++
arch/x86/include/asm/processor.h | 5 ++
arch/x86/kernel/Makefile | 1 +
arch/x86/kernel/fpu/xstate.c | 5 +-
arch/x86/kernel/process_64.c | 2 +
arch/x86/kernel/shstk.c | 149 +++++++++++++++++++++++++++++++
6 files changed, 190 insertions(+), 1 deletion(-)
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..de90e4ae083a
--- /dev/null
+++ b/arch/x86/include/asm/cet.h
@@ -0,0 +1,29 @@
+/* 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;
+
+struct thread_shstk {
+ u64 base;
+ u64 size;
+};
+
+#ifdef CONFIG_X86_SHADOW_STACK
+int shstk_setup(void);
+void shstk_free(struct task_struct *p);
+int shstk_disable(void);
+void reset_thread_shstk(void);
+#else
+static inline void shstk_setup(void) {}
+static inline void shstk_free(struct task_struct *p) {}
+static inline void shstk_disable(void) {}
+static inline void reset_thread_shstk(void) {}
+#endif /* CONFIG_X86_SHADOW_STACK */
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_X86_CET_H */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 2c5f12ae7d04..a9f4e9c4ca81 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>
@@ -528,6 +529,10 @@ struct thread_struct {
*/
u32 pkru;

+#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 6aef9ee28a39..d60ae6c365c7 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -153,6 +153,7 @@ obj-$(CONFIG_AMD_MEM_ENCRYPT) += sev.o

obj-$(CONFIG_ARCH_HAS_CC_PLATFORM) += cc_platform.o

+obj-$(CONFIG_X86_SHADOW_STACK) += shstk.o
###
# 64 bit specific files
ifeq ($(CONFIG_X86_64),y)
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index c5e20e0d0725..25b1b0c417fd 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1871,7 +1871,10 @@ int proc_pid_arch_status(struct seq_file *m, struct pid_namespace *ns,
static u64 *__get_xsave_member(void *xstate, u32 msr)
{
switch (msr) {
- /* Currently there are no MSR's supported */
+ case MSR_IA32_PL3_SSP:
+ return &((struct cet_user_state *)xstate)->user_ssp;
+ case MSR_IA32_U_CET:
+ return &((struct cet_user_state *)xstate)->user_cet;
default:
WARN_ONCE(1, "x86/fpu: unsupported xstate msr (%u)\n", msr);
return NULL;
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 3402edec236c..f05fe27d4967 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -514,6 +514,8 @@ start_thread_common(struct pt_regs *regs, unsigned long new_ip,
load_gs_index(__USER_DS);
}

+ reset_thread_shstk();
+
loadsegment(fs, 0);
loadsegment(es, _ds);
loadsegment(ds, _ds);
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
new file mode 100644
index 000000000000..4e8686ed885f
--- /dev/null
+++ b/arch/x86/kernel/shstk.c
@@ -0,0 +1,149 @@
+// 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>
+#include <asm/special_insns.h>
+#include <asm/fpu/api.h>
+
+static unsigned long alloc_shstk(unsigned long size)
+{
+ int flags = MAP_ANONYMOUS | MAP_PRIVATE;
+ struct mm_struct *mm = current->mm;
+ unsigned long addr, unused;
+
+ mmap_write_lock(mm);
+ addr = do_mmap(NULL, 0, size, PROT_READ, flags, VM_SHADOW_STACK, 0,
+ &unused, NULL);
+ mmap_write_unlock(mm);
+
+ return addr;
+}
+
+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;
+ }
+}
+
+int shstk_setup(void)
+{
+ struct thread_shstk *shstk = &current->thread.shstk;
+ unsigned long addr, size;
+ void *xstate;
+ int err;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
+ shstk->size ||
+ shstk->base)
+ return 1;
+
+ size = PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G));
+ addr = alloc_shstk(size);
+ if (IS_ERR_VALUE(addr))
+ return 1;
+
+ xstate = start_update_xsave_msrs(XFEATURE_CET_USER);
+ err = xsave_wrmsrl(xstate, MSR_IA32_PL3_SSP, addr + size);
+ if (!err)
+ err = xsave_wrmsrl(xstate, MSR_IA32_U_CET, CET_SHSTK_EN);
+ end_update_xsave_msrs();
+
+ if (err) {
+ /*
+ * Don't leak shadow stack if something went wrong with writing the
+ * msrs. Warn about it because things may be in a weird state.
+ */
+ WARN_ON_ONCE(1);
+ unmap_shadow_stack(addr, size);
+ return 1;
+ }
+
+ shstk->base = addr;
+ shstk->size = size;
+ return 0;
+}
+
+void reset_thread_shstk(void)
+{
+ memset(&current->thread.shstk, 0, sizeof(struct thread_shstk));
+}
+
+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;
+
+ unmap_shadow_stack(shstk->base, shstk->size);
+
+ shstk->base = 0;
+ shstk->size = 0;
+}
+
+int shstk_disable(void)
+{
+ struct thread_shstk *shstk = &current->thread.shstk;
+ void *xstate;
+ int err;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
+ !shstk->size ||
+ !shstk->base)
+ return 1;
+
+ xstate = start_update_xsave_msrs(XFEATURE_CET_USER);
+ err = xsave_set_clear_bits_msrl(xstate, MSR_IA32_U_CET, 0, CET_SHSTK_EN);
+ if (!err)
+ err = xsave_wrmsrl(xstate, MSR_IA32_PL3_SSP, 0);
+ end_update_xsave_msrs();
+
+ if (err)
+ return 1;
+
+ shstk_free(current);
+ return 0;
+}
--
2.17.1