Re: [RFC PATCH 1/5] x86/cet/shstk: Modify ARCH_X86_CET_ALLOC_SHSTK for 32-bit address range

[Kees Cook]

On Thu, May 21, 2020 at 02:17:16PM -0700, Yu-cheng Yu wrote:
> Sometimes a 64-bit task might need to have a shadow stack allocated from
> within 32-bit address range.  One example is selftests/x86/sigreturn.
> 
> Currently arch_prctl(ARCH_X86_CET_ALLOC_SHSTK) takes a input parameter for
> the desired shadow stack size.  Modify it and use bit[0] of the parameter
> to indicate the desire to allocate from 32-bit address range.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/include/asm/cet.h        |  2 +-
>  arch/x86/include/uapi/asm/prctl.h |  2 ++
>  arch/x86/kernel/cet.c             | 19 ++++++++++++-------
>  arch/x86/kernel/cet_prctl.c       |  6 +++++-
>  4 files changed, 20 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
> index f163c805a559..534b02785a39 100644
> --- a/arch/x86/include/asm/cet.h
> +++ b/arch/x86/include/asm/cet.h
> @@ -22,7 +22,7 @@ struct cet_status {
>  int prctl_cet(int option, u64 arg2);
>  int cet_setup_shstk(void);
>  int cet_setup_thread_shstk(struct task_struct *p);
> -int cet_alloc_shstk(unsigned long *arg);
> +int cet_alloc_shstk(unsigned long *arg, int map_32bit);
>  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);
> diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
> index d962f0ec9ccf..e254c6a21475 100644
> --- a/arch/x86/include/uapi/asm/prctl.h
> +++ b/arch/x86/include/uapi/asm/prctl.h
> @@ -19,4 +19,6 @@
>  #define ARCH_X86_CET_LOCK		0x3003
>  #define ARCH_X86_CET_ALLOC_SHSTK	0x3004
>  
> +#define ARCH_X86_CET_ALLOC_SHSTK_32BIT	0x1UL

Perhaps declare a set of bits here to verify that they are zero into the
future?

> +
>  #endif /* _ASM_X86_PRCTL_H */
> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> index 92b8730c0b08..d6f93e1864b2 100644
> --- a/arch/x86/kernel/cet.c
> +++ b/arch/x86/kernel/cet.c
> @@ -57,14 +57,19 @@ static unsigned long cet_get_shstk_addr(void)
>  	return ssp;
>  }
>  
> -static unsigned long alloc_shstk(unsigned long size)
> +static unsigned long alloc_shstk(unsigned long size, int map_32bit)
>  {
>  	struct mm_struct *mm = current->mm;
>  	unsigned long addr, populate;
> +	unsigned long map_flags;
> +
> +	map_flags = MAP_ANONYMOUS | MAP_PRIVATE;
> +	if (map_32bit)
> +		map_flags |= MAP_32BIT;
>  
>  	down_write(&mm->mmap_sem);
> -	addr = do_mmap(NULL, 0, size, PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE,
> -		       VM_SHSTK, 0, &populate, NULL);
> +	addr = do_mmap(NULL, 0, size, PROT_READ, map_flags, VM_SHSTK, 0,
> +		       &populate, NULL);
>  	up_write(&mm->mmap_sem);
>  
>  	if (populate)
> @@ -147,14 +152,14 @@ static int create_rstor_token(bool ia32, unsigned long ssp,
>  	return 0;
>  }
>  
> -int cet_alloc_shstk(unsigned long *arg)
> +int cet_alloc_shstk(unsigned long *arg, int map_32bit)
>  {
>  	unsigned long len = *arg;
>  	unsigned long addr;
>  	unsigned long token;
>  	unsigned long ssp;
>  
> -	addr = alloc_shstk(round_up(len, PAGE_SIZE));
> +	addr = alloc_shstk(round_up(len, PAGE_SIZE), map_32bit);
>  
>  	if (IS_ERR((void *)addr))
>  		return PTR_ERR((void *)addr);
> @@ -185,7 +190,7 @@ int cet_setup_shstk(void)
>  		return -EOPNOTSUPP;
>  
>  	size = round_up(min(rlimit(RLIMIT_STACK), 1UL << 32), PAGE_SIZE);
> -	addr = alloc_shstk(size);
> +	addr = alloc_shstk(size, 0);
>  
>  	if (IS_ERR((void *)addr))
>  		return PTR_ERR((void *)addr);
> @@ -226,7 +231,7 @@ int cet_setup_thread_shstk(struct task_struct *tsk)
>  	if (in_compat_syscall())
>  		size /= 4;
>  	size = round_up(size, PAGE_SIZE);
> -	addr = alloc_shstk(size);
> +	addr = alloc_shstk(size, 0);
>  
>  	if (IS_ERR((void *)addr)) {
>  		cet->shstk_base = 0;
> diff --git a/arch/x86/kernel/cet_prctl.c b/arch/x86/kernel/cet_prctl.c
> index a8e68fefd524..364ed2420202 100644
> --- a/arch/x86/kernel/cet_prctl.c
> +++ b/arch/x86/kernel/cet_prctl.c
> @@ -35,12 +35,16 @@ static int handle_alloc_shstk(u64 arg2)
>  	unsigned long arg;
>  	unsigned long addr = 0;
>  	unsigned long size = 0;
> +	int map_32bit;
>  
>  	if (get_user(arg, (unsigned long __user *)arg2))
>  		return -EFAULT;
>  

i.e. reject arg if any bits besides ARCH_X86_CET_ALLOC_SHSTK_32BIT are
set in the mask you pick.

> +	map_32bit = (arg & ARCH_X86_CET_ALLOC_SHSTK_32BIT) ? 1 : 0;
> +	arg &= ~(ARCH_X86_CET_ALLOC_SHSTK_32BIT);

And then clear the whole mask here.

-Kees

> +
>  	size = arg;
> -	err = cet_alloc_shstk(&arg);
> +	err = cet_alloc_shstk(&arg, map_32bit);
>  	if (err)
>  		return err;
>  
> -- 
> 2.21.0
> 

-- 
Kees Cook