[PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

Yu-cheng Yu
2020-08-25T00:30:29Z

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, u64 features)
Disable CET features specified in 'features'. Return -EPERM if CET is
locked.

arch_prctl(ARCH_X86_CET_LOCK)
Lock in CET features.

arch_prctl(ARCH_X86_CET_MMAP_SHSTK, u64 *args)
Allocate a new shadow stack.

The parameter 'args' is a pointer to a user buffer.

*args = desired size
*(args + 1) = MAP_32BIT or MAP_POPULATE

On returning, *args is the allocated shadow stack address.

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>
---
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 | 4 +
arch/x86/include/uapi/asm/prctl.h | 5 ++
arch/x86/kernel/Makefile | 2 +-
arch/x86/kernel/cet.c | 26 +++++++
arch/x86/kernel/cet_prctl.c | 98 +++++++++++++++++++++++++
arch/x86/kernel/process.c | 6 +-
tools/arch/x86/include/uapi/asm/prctl.h | 5 ++
7 files changed, 142 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 71dc92acd2f2..f7eb197998ad 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;
};

#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 cet_alloc_shstk(unsigned long size, int flags);
void cet_disable_free_shstk(struct task_struct *p);
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) { return 0; }
static inline void cet_disable_free_shstk(struct task_struct *p) {}
static inline void cet_restore_signal(struct sc_ext *sc) { return; }
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 5a6aac9fa41f..3aaac13cdc87 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_MMAP_SHSTK 0x3004
+
#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.c b/arch/x86/kernel/cet.c
index b30c61a66c8e..2bf1a6b6abb6 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c
@@ -148,6 +148,32 @@ static int create_rstor_token(bool ia32, unsigned long ssp,
return 0;
}

+unsigned long cet_alloc_shstk(unsigned long len, int flags)
+{
+ unsigned long token;
+ unsigned long addr, ssp;
+
+ addr = alloc_shstk(round_up(len, PAGE_SIZE), flags);
+
+ if (IS_ERR_VALUE(addr))
+ return 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;
+ }
+
+ return addr;
+}
+
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..cc49eef08ab0
--- /dev/null
+++ b/arch/x86/kernel/cet_prctl.c
@@ -0,0 +1,98 @@
+// 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));
+}
+
+static int handle_mmap_shstk(u64 arg2)
+{
+ u64 buf[3];
+ unsigned long addr, size;
+ int allowed_flags;
+
+ if (copy_from_user(buf, (unsigned long __user *)arg2, sizeof(buf)))
+ return -EFAULT;
+
+ size = buf[0];
+
+ /*
+ * Check invalid flags
+ */
+ allowed_flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_32BIT | MAP_POPULATE;
+
+ if (buf[1] & ~allowed_flags)
+ return -EINVAL;
+
+ addr = cet_alloc_shstk(size, buf[1]);
+ if (IS_ERR_VALUE(addr))
+ return PTR_ERR((void *)addr);
+
+ if (put_user(addr, (u64 __user *)arg2)) {
+ vm_munmap(addr, size);
+ return -EFAULT;
+ }
+
+ return 0;
+}
+
+int prctl_cet(int option, u64 arg2)
+{
+ struct cet_status *cet;
+
+ /*
+ * 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;
+ if (arg2 & GNU_PROPERTY_X86_FEATURE_1_INVAL)
+ return -EINVAL;
+ 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_MMAP_SHSTK:
+ return handle_mmap_shstk(arg2);
+
+ default:
+ return -ENOSYS;
+ }
+}
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index b6fe5b061841..5a657a9774dc 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -980,14 +980,14 @@ 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..3aaac13cdc87 100644
--- a/tools/arch/x86/include/uapi/asm/prctl.h
+++ b/tools/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_MMAP_SHSTK 0x3004
+
#endif /* _ASM_X86_PRCTL_H */
--
2.21.0