[PATCH v12 25/26] x86/cet/shstk: Add arch_prctl functions for shadow stack

Yu-cheng Yu
2020-09-18T19:24:47Z

arch_prctl(ARCH_X86_CET_STATUS, u64 *args)
Get CET feature status.

The parameter 'args' is a pointer to a user buffer. The kernel returns
the following information:

*args = shadow stack/IBT status
*(args + 1) = shadow stack base address
*(args + 2) = shadow stack size

arch_prctl(ARCH_X86_CET_DISABLE, unsigned int features)
Disable CET features specified in 'features'. Return -EPERM if CET is
locked.

arch_prctl(ARCH_X86_CET_LOCK)
Lock in CET features.

Also change do_arch_prctl_common()'s parameter 'cpuid_enabled' to
'arg2', as it is now also passed to prctl_cet().

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
v12:
- Remove ARCH_X86_CET_MMAP_SHSTK.

v11:
- Check input for invalid features.
- Fix prctl_cet() return values.
- Change ARCH_X86_CET_ALLOC_SHSTK to ARCH_X86_CET_MMAP_SHSTK to take
MAP_32BIT, MAP_POPULATE as inputs.

v10:
- Verify CET is enabled before handling arch_prctl.
- Change input parameters from unsigned long to u64, to make it clear they
are 64-bit.

arch/x86/include/asm/cet.h | 3 ++
arch/x86/include/uapi/asm/prctl.h | 4 ++
arch/x86/kernel/Makefile | 2 +-
arch/x86/kernel/cet_prctl.c | 68 +++++++++++++++++++++++++
arch/x86/kernel/process.c | 6 +--
tools/arch/x86/include/uapi/asm/prctl.h | 4 ++
6 files changed, 83 insertions(+), 4 deletions(-)
create mode 100644 arch/x86/kernel/cet_prctl.c

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index 7a416c8af791..1ebe82416196 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h
@@ -14,9 +14,11 @@ struct sc_ext;
struct cet_status {
unsigned long shstk_base;
unsigned long shstk_size;
+ unsigned int locked:1;
};

#ifdef CONFIG_X86_INTEL_CET
+int prctl_cet(int option, u64 arg2);
int cet_setup_shstk(void);
int cet_setup_thread_shstk(struct task_struct *p, unsigned long clone_flags);
void cet_disable_shstk(void);
@@ -25,6 +27,7 @@ int cet_verify_rstor_token(bool ia32, unsigned long ssp, unsigned long *new_ssp)
void cet_restore_signal(struct sc_ext *sc);
int cet_setup_signal(bool ia32, unsigned long rstor, struct sc_ext *sc);
#else
+static inline int prctl_cet(int option, u64 arg2) { return -EINVAL; }
static inline int cet_setup_thread_shstk(struct task_struct *p,
unsigned long clone_flags) { return 0; }
static inline void cet_disable_shstk(void) {}
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 5a6aac9fa41f..9245bf629120 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -14,4 +14,8 @@
#define ARCH_MAP_VDSO_32 0x2002
#define ARCH_MAP_VDSO_64 0x2003

+#define ARCH_X86_CET_STATUS 0x3001
+#define ARCH_X86_CET_DISABLE 0x3002
+#define ARCH_X86_CET_LOCK 0x3003
+
#endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 76f27f518266..97556e4204d6 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -145,7 +145,7 @@ obj-$(CONFIG_UNWINDER_ORC) += unwind_orc.o
obj-$(CONFIG_UNWINDER_FRAME_POINTER) += unwind_frame.o
obj-$(CONFIG_UNWINDER_GUESS) += unwind_guess.o

-obj-$(CONFIG_X86_INTEL_CET) += cet.o
+obj-$(CONFIG_X86_INTEL_CET) += cet.o cet_prctl.o

###
# 64 bit specific files
diff --git a/arch/x86/kernel/cet_prctl.c b/arch/x86/kernel/cet_prctl.c
new file mode 100644
index 000000000000..3e1d049e65c3
--- /dev/null
+++ b/arch/x86/kernel/cet_prctl.c
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/errno.h>
+#include <linux/uaccess.h>
+#include <linux/prctl.h>
+#include <linux/compat.h>
+#include <linux/mman.h>
+#include <linux/elfcore.h>
+#include <asm/processor.h>
+#include <asm/prctl.h>
+#include <asm/cet.h>
+
+/* See Documentation/x86/intel_cet.rst. */
+
+static int copy_status_to_user(struct cet_status *cet, u64 arg2)
+{
+ u64 buf[3] = {0, 0, 0};
+
+ if (cet->shstk_size) {
+ buf[0] |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
+ buf[1] = (u64)cet->shstk_base;
+ buf[2] = (u64)cet->shstk_size;
+ }
+
+ return copy_to_user((u64 __user *)arg2, buf, sizeof(buf));
+}
+
+int prctl_cet(int option, u64 arg2)
+{
+ struct cet_status *cet;
+ unsigned int features;
+
+ /*
+ * GLIBC's ENOTSUPP == EOPNOTSUPP == 95, and it does not recognize
+ * the kernel's ENOTSUPP (524). So return EOPNOTSUPP here.
+ */
+ if (!IS_ENABLED(CONFIG_X86_INTEL_CET))
+ return -EOPNOTSUPP;
+
+ cet = &current->thread.cet;
+
+ if (option == ARCH_X86_CET_STATUS)
+ return copy_status_to_user(cet, arg2);
+
+ if (!static_cpu_has(X86_FEATURE_SHSTK))
+ return -EOPNOTSUPP;
+
+ switch (option) {
+ case ARCH_X86_CET_DISABLE:
+ if (cet->locked)
+ return -EPERM;
+
+ features = (unsigned int)arg2;
+
+ if (features & GNU_PROPERTY_X86_FEATURE_1_INVAL)
+ return -EINVAL;
+ if (features & GNU_PROPERTY_X86_FEATURE_1_SHSTK)
+ cet_disable_shstk();
+ return 0;
+
+ case ARCH_X86_CET_LOCK:
+ cet->locked = 1;
+ return 0;
+
+ default:
+ return -ENOSYS;
+ }
+}
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 1d292d0cfe8a..bd773036ed6b 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -976,14 +976,14 @@ unsigned long get_wchan(struct task_struct *p)
}

long do_arch_prctl_common(struct task_struct *task, int option,
- unsigned long cpuid_enabled)
+ unsigned long arg2)
{
switch (option) {
case ARCH_GET_CPUID:
return get_cpuid_mode();
case ARCH_SET_CPUID:
- return set_cpuid_mode(task, cpuid_enabled);
+ return set_cpuid_mode(task, arg2);
}

- return -EINVAL;
+ return prctl_cet(option, arg2);
}
diff --git a/tools/arch/x86/include/uapi/asm/prctl.h b/tools/arch/x86/include/uapi/asm/prctl.h
index 5a6aac9fa41f..9245bf629120 100644
--- a/tools/arch/x86/include/uapi/asm/prctl.h
+++ b/tools/arch/x86/include/uapi/asm/prctl.h
@@ -14,4 +14,8 @@
#define ARCH_MAP_VDSO_32 0x2002
#define ARCH_MAP_VDSO_64 0x2003

+#define ARCH_X86_CET_STATUS 0x3001
+#define ARCH_X86_CET_DISABLE 0x3002
+#define ARCH_X86_CET_LOCK 0x3003
+
#endif /* _ASM_X86_PRCTL_H */
--
2.21.0