Re: [RFC PATCH v4 26/27] x86/cet/shstk: Add arch_prctl functions for Shadow Stack

Eugene Syromiatnikov
2018-10-03T17:57:13Z

On Fri, Sep 21, 2018 at 08:03:50AM -0700, Yu-cheng Yu wrote:
> arch_prctl(ARCH_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

The subtle detail here is that x32 binaries will get 64-bit value, which
is not entirely obvious. I think, it might be better to define
a structure type for it as a part of UAPI, for example:

struct user_cet_status {
__u32 struct_size;
__u32 features;
__kernel_ulong_t shstk_base;
__kernel_ulong_t shstk_size;
};

Adding "struct_size" field along with appropriate checks will also
allow for possible extensions, if they ever appear.

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

While x86_64 and x32 will have 64-bit space for feature bits, IA-32 will
have only 32 bits.

> arch_prctl(ARCH_CET_LOCK)
> Lock in CET feature.
>
> arch_prctl(ARCH_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.

Again, on x32 that will be a pointer to a 64-bit value, which is not
entirely obvious from this description.

It's not clear whether inability to enable some CET feature in runtime
is unavailable by design or by omission; same for setting (an allocated)
shadow stack as task's shadow stack.

>
> Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> arch/x86/include/asm/cet.h | 5 ++
> arch/x86/include/uapi/asm/prctl.h | 5 ++
> arch/x86/kernel/Makefile | 2 +-
> arch/x86/kernel/cet.c | 27 +++++++++++
> arch/x86/kernel/cet_prctl.c | 79 +++++++++++++++++++++++++++++++
> arch/x86/kernel/process.c | 5 ++
> 6 files changed, 122 insertions(+), 1 deletion(-)
> 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 b7b33e1026bb..212bd68e31d3 100644
> --- a/arch/x86/include/asm/cet.h
> +++ b/arch/x86/include/asm/cet.h
> @@ -12,19 +12,24 @@ struct task_struct;
> 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_shstk(void);
> void cet_disable_free_shstk(struct task_struct *p);
> int cet_restore_signal(unsigned long ssp);
> int cet_setup_signal(bool ia32, unsigned long rstor, unsigned long *new_ssp);
> #else
> +static inline int prctl_cet(int option, unsigned long arg2) { return 0; }

Why 0 and not -EINVAL?

> static inline int cet_setup_shstk(void) { return 0; }

0 here also looks strange.

> static inline int cet_setup_thread_shstk(struct task_struct *p) { return 0; }

And here.

> +static inline int cet_alloc_shstk(unsigned long *arg) { return -EINVAL; }
> static inline void cet_disable_shstk(void) {}
> static inline void cet_disable_free_shstk(struct task_struct *p) {}
> static inline int cet_restore_signal(unsigned long ssp) { return 0; }
> diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
> index 5a6aac9fa41f..3aec1088e01d 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_CET_STATUS 0x3001
> +#define ARCH_CET_DISABLE 0x3002
> +#define ARCH_CET_LOCK 0x3003
> +#define ARCH_CET_ALLOC_SHSTK 0x3004
> +
> #endif /* _ASM_X86_PRCTL_H */
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 36b14ef410c8..b9e6cdc6b4f7 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -139,7 +139,7 @@ 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-$(CONFIG_X86_INTEL_CET) += cet.o cet_prctl.o
>
> obj-$(CONFIG_ARCH_HAS_PROGRAM_PROPERTIES) += elf.o
>
> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> index ce0b3b7b1160..1c2689738604 100644
> --- a/arch/x86/kernel/cet.c
> +++ b/arch/x86/kernel/cet.c
> @@ -110,6 +110,33 @@ 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 = do_mmap_locked(0, len, PROT_READ,
> + MAP_ANONYMOUS | MAP_PRIVATE, VM_SHSTK);
> + if (addr >= TASK_SIZE_MAX)
> + return -ENOMEM;
> +
> + /* Restore token is 8 bytes and aligned to 8 bytes */
> + ssp = addr + len;
> + token = ssp;
> +
> + if (!in_ia32_syscall())
> + token |= 1;

This pair of check and bit or'ing definitely asks for a macro or a
wrapper function.

> + ssp -= 8;
> +
> + if (write_user_shstk_64(ssp, token))
> + return -EINVAL;

Shouldn't addr be unmapped on error?

> + *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..c4b7c19f5040
> --- /dev/null
> +++ b/arch/x86/kernel/cet_prctl.c
> @@ -0,0 +1,79 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#include <linux/errno.h>
> +#include <linux/uaccess.h>
> +#include <linux/prctl.h>
> +#include <linux/compat.h>
> +#include <asm/processor.h>
> +#include <asm/prctl.h>
> +#include <asm/elf.h>
> +#include <asm/elf_property.h>
> +#include <asm/cet.h>
> +
> +/* See Documentation/x86/intel_cet.txt. */
> +
> +static int handle_get_status(unsigned long arg2)
> +{
> + unsigned int features = 0;
> + unsigned long shstk_base, shstk_size;
> + unsigned long buf[3];
> +
> + if (current->thread.cet.shstk_enabled)
> + features |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
> +
> + shstk_base = current->thread.cet.shstk_base;
> + shstk_size = current->thread.cet.shstk_size;
> +
> + buf[0] = (unsigned long)features;
> + buf[1] = shstk_base;
> + buf[2] = 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 shstk_size = 0;
> +
> + if (get_user(shstk_size, (unsigned long __user *)arg2))
> + return -EFAULT;
> +
> + err = cet_alloc_shstk(&shstk_size);
> + if (err)
> + return err;
> +
> + if (put_user(shstk_size, (unsigned long __user *)arg2))

Again, leaking allocated stack.

> + return -EFAULT;
> +
> + return 0;
> +}
> +
> +int prctl_cet(int option, unsigned long arg2)
> +{
> + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> + return -EINVAL;
> +
> + switch (option) {
> + case ARCH_CET_STATUS:
> + return handle_get_status(arg2);
> +
> + case ARCH_CET_DISABLE:
> + if (current->thread.cet.locked)
> + return -EPERM;
> + if (arg2 & GNU_PROPERTY_X86_FEATURE_1_SHSTK)
> + cet_disable_free_shstk(current);

The rest of bits in arg2 should be 0, otherwise this interface won't be
possible to extend.

> + return 0;
> +
> + case ARCH_CET_LOCK:
> + current->thread.cet.locked = 1;
> + return 0;
> +
> + case ARCH_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 440f012ef925..251b8714f9a3 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -792,6 +792,11 @@ 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);
> + case ARCH_CET_STATUS:
> + case ARCH_CET_DISABLE:
> + case ARCH_CET_LOCK:
> + case ARCH_CET_ALLOC_SHSTK:
> + return prctl_cet(option, cpuid_enabled);

It's probably a good opportunity to change the strange name for an argument
of a dispatch call.