[PATCH v13 20/26] x86/cet/shstk: User-mode shadow stack support

From: Yu-cheng Yu
Date: Fri Sep 25 2020 - 11:10:31 EST

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

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
v12:
- Split cet_disable_free_shstk() into disable and free parts, because
only the disable shadow stack arch_prctl() needs the disable part.
Other cases need only the free part. In an exit syscall, there is
no need to write CET MSRs.

v11:
- Modify alloc_shstk() to take flags and pass to do_mmap().
This is to be used by an arch_prctl() introduced later.

v10:
- Change no_cet_shstk to no_user_shstk.
- Limit shadow stack size to 4 GB, and round_up to PAGE_SIZE.
- Replace checking shstk_enabled with shstk_size being zero.
- WARN_ON_ONCE() when vm_munmap() fails.

v9:
- Change cpu_feature_enabled() to static_cpu_has().
- Merge cet_disable_shstk to cet_disable_free_shstk.
- Remove the empty slot at the top of the shadow stack, as it is not
needed.
- Move do_mmap_locked() to alloc_shstk(), which is a static function.

v6:
- Create a function do_mmap_locked() for shadow stack allocation.

v2:
- Change noshstk to no_cet_shstk.

arch/x86/include/asm/cet.h | 28 ++++
arch/x86/include/asm/disabled-features.h | 8 +-
arch/x86/include/asm/processor.h | 5 +
arch/x86/kernel/Makefile | 2 +
arch/x86/kernel/cet.c | 147 ++++++++++++++++++
arch/x86/kernel/cpu/common.c | 28 ++++
arch/x86/kernel/process.c | 1 +
.../arch/x86/include/asm/disabled-features.h | 8 +-
8 files changed, 225 insertions(+), 2 deletions(-)
create mode 100644 arch/x86/include/asm/cet.h
create mode 100644 arch/x86/kernel/cet.c

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
new file mode 100644
index 000000000000..5750fbcbb952
--- /dev/null
+++ b/arch/x86/include/asm/cet.h
@@ -0,0 +1,28 @@
+/* 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 cet_status {
+ unsigned long shstk_base;
+ unsigned long shstk_size;
+};
+
+#ifdef CONFIG_X86_CET
+int cet_setup_shstk(void);
+void cet_disable_shstk(void);
+void cet_free_shstk(struct task_struct *p);
+#else
+static inline void cet_disable_shstk(void) {}
+static inline void cet_free_shstk(struct task_struct *p) {}
+#endif
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_X86_CET_H */
diff --git a/arch/x86/include/asm/disabled-features.h b/arch/x86/include/asm/disabled-features.h
index 4ea8584682f9..edac76ed75e7 100644
--- a/arch/x86/include/asm/disabled-features.h
+++ b/arch/x86/include/asm/disabled-features.h
@@ -56,6 +56,12 @@
# define DISABLE_PTI (1 << (X86_FEATURE_PTI & 31))
#endif

+#ifdef CONFIG_X86_SHADOW_STACK_USER
+#define DISABLE_SHSTK 0
+#else
+#define DISABLE_SHSTK (1<<(X86_FEATURE_SHSTK & 31))
+#endif
+
/*
* Make sure to add features to the correct mask
*/
@@ -75,7 +81,7 @@
#define DISABLED_MASK13 0
#define DISABLED_MASK14 0
#define DISABLED_MASK15 0
-#define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP)
+#define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP|DISABLE_SHSTK)
#define DISABLED_MASK17 0
#define DISABLED_MASK18 0
#define DISABLED_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 19)
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 01acbd63cad8..139cb99c7076 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>
@@ -542,6 +543,10 @@ struct thread_struct {

unsigned int sig_on_uaccess_err:1;

+#ifdef CONFIG_X86_CET
+ struct cet_status cet;
+#endif
+
/* Floating point and extended processor state */
struct fpu fpu;
/*
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index e77261db2391..1fb85595afa7 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -145,6 +145,8 @@ obj-$(CONFIG_UNWINDER_ORC) += unwind_orc.o
obj-$(CONFIG_UNWINDER_FRAME_POINTER) += unwind_frame.o
obj-$(CONFIG_UNWINDER_GUESS) += unwind_guess.o

+obj-$(CONFIG_X86_CET) += cet.o
+
###
# 64 bit specific files
ifeq ($(CONFIG_X86_64),y)
diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
new file mode 100644
index 000000000000..f8b0a077594f
--- /dev/null
+++ b/arch/x86/kernel/cet.c
@@ -0,0 +1,147 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * cet.c - Control-flow Enforcement (CET)
+ *
+ * Copyright (c) 2019, Intel Corporation.
+ * Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
+ */
+
+#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 <asm/msr.h>
+#include <asm/user.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 cet_get_shstk_addr(void)
+{
+ struct fpu *fpu = &current->thread.fpu;
+ unsigned long ssp = 0;
+
+ fpregs_lock();
+
+ if (fpregs_state_valid(fpu, smp_processor_id())) {
+ rdmsrl(MSR_IA32_PL3_SSP, ssp);
+ } else {
+ struct cet_user_state *p;
+
+ p = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
+ if (p)
+ ssp = p->user_ssp;
+ }
+
+ fpregs_unlock();
+ return ssp;
+}
+
+static unsigned long alloc_shstk(unsigned long size, int flags)
+{
+ struct mm_struct *mm = current->mm;
+ unsigned long addr, populate;
+
+ /* VM_SHSTK requires MAP_ANONYMOUS, MAP_PRIVATE */
+ flags |= MAP_ANONYMOUS | MAP_PRIVATE;
+
+ mmap_write_lock(mm);
+ addr = do_mmap(NULL, 0, size, PROT_READ, flags, VM_SHSTK, 0,
+ &populate, NULL);
+ mmap_write_unlock(mm);
+
+ if (populate)
+ mm_populate(addr, populate);
+
+ return addr;
+}
+
+int cet_setup_shstk(void)
+{
+ unsigned long addr, size;
+ struct cet_status *cet = &current->thread.cet;
+
+ if (!static_cpu_has(X86_FEATURE_SHSTK))
+ return -EOPNOTSUPP;
+
+ size = round_up(min(rlimit(RLIMIT_STACK), 1UL << 32), PAGE_SIZE);
+ addr = alloc_shstk(size, 0);
+
+ if (IS_ERR_VALUE(addr))
+ return PTR_ERR((void *)addr);
+
+ cet->shstk_base = addr;
+ cet->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 cet_disable_shstk(void)
+{
+ struct cet_status *cet = &current->thread.cet;
+ u64 msr_val;
+
+ if (!static_cpu_has(X86_FEATURE_SHSTK) ||
+ !cet->shstk_size || !cet->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();
+
+ cet_free_shstk(current);
+}
+
+void cet_free_shstk(struct task_struct *tsk)
+{
+ struct cet_status *cet = &tsk->thread.cet;
+
+ if (!static_cpu_has(X86_FEATURE_SHSTK) ||
+ !cet->shstk_size || !cet->shstk_base)
+ return;
+
+ if (!tsk->mm || (tsk->mm != current->mm))
+ return;
+
+ while (1) {
+ int r;
+
+ r = vm_munmap(cet->shstk_base, cet->shstk_size);
+
+ /*
+ * Retry if mmap_lock is not available.
+ */
+ if (r == -EINTR) {
+ cond_resched();
+ continue;
+ }
+
+ WARN_ON_ONCE(r);
+ break;
+ }
+
+ cet->shstk_base = 0;
+ cet->shstk_size = 0;
+}
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index c5d6f17d9b9d..084480f975aa 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -56,6 +56,7 @@
#include <asm/microcode_intel.h>
#include <asm/intel-family.h>
#include <asm/cpu_device_id.h>
+#include <asm/cet.h>
#include <asm/uv/uv.h>

#include "cpu.h"
@@ -509,6 +510,32 @@ static __init int setup_disable_pku(char *arg)
__setup("nopku", setup_disable_pku);
#endif /* CONFIG_X86_64 */

+static __always_inline void setup_cet(struct cpuinfo_x86 *c)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK) &&
+ !cpu_feature_enabled(X86_FEATURE_IBT))
+ return;
+
+ cr4_set_bits(X86_CR4_CET);
+}
+
+#ifdef CONFIG_X86_SHADOW_STACK_USER
+static __init int setup_disable_shstk(char *s)
+{
+ /* require an exact match without trailing characters */
+ if (s[0] != '\0')
+ return 0;
+
+ if (!boot_cpu_has(X86_FEATURE_SHSTK))
+ return 1;
+
+ setup_clear_cpu_cap(X86_FEATURE_SHSTK);
+ pr_info("x86: 'no_user_shstk' specified, disabling user Shadow Stack\n");
+ return 1;
+}
+__setup("no_user_shstk", setup_disable_shstk);
+#endif
+
/*
* Some CPU features depend on higher CPUID levels, which may not always
* be available due to CPUID level capping or broken virtualization
@@ -1544,6 +1571,7 @@ static void identify_cpu(struct cpuinfo_x86 *c)

x86_init_rdrand(c);
setup_pku(c);
+ setup_cet(c);

/*
* Clear/Set all flags overridden by options, need do it
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index ba4593a913fa..ff3b44d6740b 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -43,6 +43,7 @@
#include <asm/io_bitmap.h>
#include <asm/proto.h>
#include <asm/frame.h>
+#include <asm/cet.h>

#include "process.h"

diff --git a/tools/arch/x86/include/asm/disabled-features.h b/tools/arch/x86/include/asm/disabled-features.h
index 4ea8584682f9..edac76ed75e7 100644
--- a/tools/arch/x86/include/asm/disabled-features.h
+++ b/tools/arch/x86/include/asm/disabled-features.h
@@ -56,6 +56,12 @@
# define DISABLE_PTI (1 << (X86_FEATURE_PTI & 31))
#endif

+#ifdef CONFIG_X86_SHADOW_STACK_USER
+#define DISABLE_SHSTK 0
+#else
+#define DISABLE_SHSTK (1<<(X86_FEATURE_SHSTK & 31))
+#endif
+
/*
* Make sure to add features to the correct mask
*/
@@ -75,7 +81,7 @@
#define DISABLED_MASK13 0
#define DISABLED_MASK14 0
#define DISABLED_MASK15 0
-#define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP)
+#define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP|DISABLE_SHSTK)
#define DISABLED_MASK17 0
#define DISABLED_MASK18 0
#define DISABLED_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 19)
--
2.21.0