[RFC PATCH v9 27/27] x86/cet/shstk: Add arch_prctl functions for Shadow Stack

From: Yu-cheng Yu
Date: Wed Feb 05 2020 - 13:20:53 EST

arch_prctl(ARCH_X86_CET_STATUS, unsigned long *addr)
Return CET feature status.

The parameter 'addr' is a pointer to a user buffer. On returning to
the caller, the kernel fills the following information:

*addr = SHSTK/IBT status
*(addr + 1) = SHSTK base address
*(addr + 2) = SHSTK size

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

arch_prctl(ARCH_X86_CET_LOCK)
Lock in CET feature.

arch_prctl(ARCH_X86_CET_ALLOC_SHSTK, unsigned long *addr)
Allocate a new SHSTK.

The parameter 'addr' is a pointer to a user buffer and indicates the
desired SHSTK size to allocate. On returning to the caller the buffer
contains the address of the new SHSTK.

There is no CET enabling arch_prctl function. By design, CET is enabled
automatically if the binary and the system can support it.

The parameters passed are always unsigned 64-bit. When an IA32 application
passing pointers, it should only use the lower 32 bits.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
arch/x86/include/asm/cet.h | 4 ++
arch/x86/include/uapi/asm/prctl.h | 5 ++
arch/x86/kernel/Makefile | 2 +-
arch/x86/kernel/cet.c | 29 +++++++++++
arch/x86/kernel/cet_prctl.c | 84 +++++++++++++++++++++++++++++++
arch/x86/kernel/process.c | 4 +-
6 files changed, 125 insertions(+), 3 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 9a3e2da9c1c4..b64f6d810ae0 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h
@@ -14,16 +14,20 @@ struct sc_ext;
struct cet_status {
unsigned long shstk_base;
unsigned long shstk_size;
+ unsigned int locked:1;
unsigned int shstk_enabled:1;
};

#ifdef CONFIG_X86_INTEL_CET
+int prctl_cet(int option, unsigned long arg2);
int cet_setup_shstk(void);
int cet_setup_thread_shstk(struct task_struct *p);
+int cet_alloc_shstk(unsigned long *arg);
void cet_disable_free_shstk(struct task_struct *p);
int cet_restore_signal(bool ia32, 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, unsigned long arg2) { return -EINVAL; }
static inline int cet_setup_thread_shstk(struct task_struct *p) { return 0; }
static inline void cet_disable_free_shstk(struct task_struct *p) {}
static inline int cet_restore_signal(bool ia32, struct sc_ext *sc) { return -EINVAL; }
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 5a6aac9fa41f..d962f0ec9ccf 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -14,4 +14,9 @@
#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
+#define ARCH_X86_CET_ALLOC_SHSTK 0x3004
+
#endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index b8c1ea4ab7eb..69a19957e200 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -142,7 +142,7 @@ 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.c b/arch/x86/kernel/cet.c
index 5b45abda80a1..01aa24c40a5d 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c
@@ -145,6 +145,35 @@ static int create_rstor_token(bool ia32, unsigned long ssp,
return 0;
}

+int cet_alloc_shstk(unsigned long *arg)
+{
+ unsigned long len = *arg;
+ unsigned long addr;
+ unsigned long token;
+ unsigned long ssp;
+
+ addr = alloc_shstk(len);
+
+ if (IS_ERR((void *)addr))
+ return PTR_ERR((void *)addr);
+
+ /* Restore token is 8 bytes and aligned to 8 bytes */
+ ssp = addr + len;
+ token = ssp;
+
+ if (!in_ia32_syscall())
+ token |= TOKEN_MODE_64;
+ ssp -= 8;
+
+ if (write_user_shstk_64(ssp, token)) {
+ vm_munmap(addr, len);
+ return -EINVAL;
+ }
+
+ *arg = addr;
+ return 0;
+}
+
int cet_setup_shstk(void)
{
unsigned long addr, size;
diff --git a/arch/x86/kernel/cet_prctl.c b/arch/x86/kernel/cet_prctl.c
new file mode 100644
index 000000000000..6cf8f87e3d98
--- /dev/null
+++ b/arch/x86/kernel/cet_prctl.c
@@ -0,0 +1,84 @@
+/* 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 handle_get_status(unsigned long arg2)
+{
+ struct cet_status *cet = &current->thread.cet;
+ unsigned int features = 0;
+ unsigned long buf[3];
+
+ if (cet->shstk_enabled)
+ features |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
+
+ buf[0] = (unsigned long)features;
+ buf[1] = cet->shstk_base;
+ buf[2] = cet->shstk_size;
+ return copy_to_user((unsigned long __user *)arg2, buf,
+ sizeof(buf));
+}
+
+static int handle_alloc_shstk(unsigned long arg2)
+{
+ int err = 0;
+ unsigned long arg;
+ unsigned long addr = 0;
+ unsigned long size = 0;
+
+ if (get_user(arg, (unsigned long __user *)arg2))
+ return -EFAULT;
+
+ size = arg;
+ err = cet_alloc_shstk(&arg);
+ if (err)
+ return err;
+
+ addr = arg;
+ if (put_user(addr, (unsigned long __user *)arg2)) {
+ vm_munmap(addr, size);
+ return -EFAULT;
+ }
+
+ return 0;
+}
+
+int prctl_cet(int option, unsigned long arg2)
+{
+ struct cet_status *cet = &current->thread.cet;
+
+ if (!cpu_x86_cet_enabled())
+ return -EINVAL;
+
+ switch (option) {
+ case ARCH_X86_CET_STATUS:
+ return handle_get_status(arg2);
+
+ case ARCH_X86_CET_DISABLE:
+ if (cet->locked)
+ return -EPERM;
+ if (arg2 & GNU_PROPERTY_X86_FEATURE_1_SHSTK)
+ cet_disable_free_shstk(current);
+
+ return 0;
+
+ case ARCH_X86_CET_LOCK:
+ cet->locked = 1;
+ return 0;
+
+ case ARCH_X86_CET_ALLOC_SHSTK:
+ return handle_alloc_shstk(arg2);
+
+ default:
+ return -EINVAL;
+ }
+}
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 7098618142f2..63dc88070923 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -998,7 +998,7 @@ long do_arch_prctl_common(struct task_struct *task, int option,
return get_cpuid_mode();
case ARCH_SET_CPUID:
return set_cpuid_mode(task, cpuid_enabled);
+ default:
+ return prctl_cet(option, cpuid_enabled);
}
-
- return -EINVAL;
}
--
2.21.0