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

[Eugene Syromiatnikov]

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@xxxxxxxxx>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  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.