Re: [PATCH v4 30/39] x86/shstk: Introduce map_shadow_stack syscall

[Kees Cook]

On Fri, Dec 02, 2022 at 04:35:57PM -0800, Rick Edgecombe wrote:
> When operating with shadow stacks enabled, the kernel will automatically
> allocate shadow stacks for new threads, however in some cases userspace
> will need additional shadow stacks. The main example of this is the
> ucontext family of functions, which require userspace allocating and
> pivoting to userspace managed stacks.
> 
> Unlike most other user memory permissions, shadow stacks need to be
> provisioned with special data in order to be useful. They need to be setup
> with a restore token so that userspace can pivot to them via the RSTORSSP
> instruction. But, the security design of shadow stack's is that they
> should not be written to except in limited circumstances. This presents a
> problem for userspace, as to how userspace can provision this special
> data, without allowing for the shadow stack to be generally writable.
> 
> Previously, a new PROT_SHADOW_STACK was attempted, which could be
> mprotect()ed from RW permissions after the data was provisioned. This was
> found to not be secure enough, as other thread's could write to the
> shadow stack during the writable window.
> 
> The kernel can use a special instruction, WRUSS, to write directly to
> userspace shadow stacks. So the solution can be that memory can be mapped
> as shadow stack permissions from the beginning (never generally writable
> in userspace), and the kernel itself can write the restore token.
> 
> First, a new madvise() flag was explored, which could operate on the
> PROT_SHADOW_STACK memory. This had a couple downsides:
> 1. Extra checks were needed in mprotect() to prevent writable memory from
>    ever becoming PROT_SHADOW_STACK.
> 2. Extra checks/vma state were needed in the new madvise() to prevent
>    restore tokens being written into the middle of pre-used shadow stacks.
>    It is ideal to prevent restore tokens being added at arbitrary
>    locations, so the check was to make sure the shadow stack had never been
>    written to.
> 3. It stood out from the rest of the madvise flags, as more of direct
>    action than a hint at future desired behavior.
> 
> So rather than repurpose two existing syscalls (mmap, madvise) that don't
> quite fit, just implement a new map_shadow_stack syscall to allow
> userspace to map and setup new shadow stacks in one step. While ucontext
> is the primary motivator, userspace may have other unforeseen reasons to
> setup it's own shadow stacks using the WRSS instruction. Towards this
> provide a flag so that stacks can be optionally setup securely for the
> common case of ucontext without enabling WRSS. Or potentially have the
> kernel set up the shadow stack in some new way.
> 
> The following example demonstrates how to create a new shadow stack with
> map_shadow_stack:
> void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
> 
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
> 
> v3:
>  - Change syscall common -> 64 (Kees)
>  - Use bit shift notation instead of 0x1 for uapi header (Kees)
>  - Call do_mmap() with MAP_FIXED_NOREPLACE (Kees)
>  - Block unsupported flags (Kees)
>  - Require size >= 8 to set token (Kees)
> 
> v2:
>  - Change syscall to take address like mmap() for CRIU's usage
> 
> v1:
>  - New patch (replaces PROT_SHADOW_STACK).
> 
>  arch/x86/entry/syscalls/syscall_64.tbl |  1 +
>  arch/x86/include/uapi/asm/mman.h       |  3 ++
>  arch/x86/kernel/shstk.c                | 56 ++++++++++++++++++++++----
>  include/linux/syscalls.h               |  1 +
>  include/uapi/asm-generic/unistd.h      |  2 +-
>  kernel/sys_ni.c                        |  1 +
>  6 files changed, 55 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index c84d12608cd2..f65c671ce3b1 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -372,6 +372,7 @@
>  448	common	process_mrelease	sys_process_mrelease
>  449	common	futex_waitv		sys_futex_waitv
>  450	common	set_mempolicy_home_node	sys_set_mempolicy_home_node
> +451	64	map_shadow_stack	sys_map_shadow_stack
>  
>  #
>  # Due to a historical design error, certain syscalls are numbered differently
> diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h
> index 775dbd3aff73..15c5a1c4fc29 100644
> --- a/arch/x86/include/uapi/asm/mman.h
> +++ b/arch/x86/include/uapi/asm/mman.h
> @@ -12,6 +12,9 @@
>  		((key) & 0x8 ? VM_PKEY_BIT3 : 0))
>  #endif
>  
> +/* Flags for map_shadow_stack(2) */
> +#define SHADOW_STACK_SET_TOKEN	(1ULL << 0)	/* Set up a restore token in the shadow stack */
> +
>  #include <asm-generic/mman.h>
>  
>  #endif /* _ASM_X86_MMAN_H */
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index e53225a8d39e..8f329c22728a 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -17,6 +17,7 @@
>  #include <linux/compat.h>
>  #include <linux/sizes.h>
>  #include <linux/user.h>
> +#include <linux/syscalls.h>
>  #include <asm/msr.h>
>  #include <asm/fpu/xstate.h>
>  #include <asm/fpu/types.h>
> @@ -71,19 +72,31 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr)
>  	return 0;
>  }
>  
> -static unsigned long alloc_shstk(unsigned long size)
> +static unsigned long alloc_shstk(unsigned long addr, unsigned long size,
> +				 unsigned long token_offset, bool set_res_tok)
>  {
>  	int flags = MAP_ANONYMOUS | MAP_PRIVATE;
>  	struct mm_struct *mm = current->mm;
> -	unsigned long addr, unused;
> +	unsigned long mapped_addr, unused;
>  
> -	mmap_write_lock(mm);
> -	addr = do_mmap(NULL, 0, size, PROT_READ, flags,
> -		       VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL);
> +	if (addr)
> +		flags |= MAP_FIXED_NOREPLACE;
>  
> +	mmap_write_lock(mm);
> +	mapped_addr = do_mmap(NULL, addr, size, PROT_READ, flags,
> +			      VM_SHADOW_STACK | VM_WRITE, 0, &unused, NULL);
>  	mmap_write_unlock(mm);
>  
> -	return addr;
> +	if (!set_res_tok || IS_ERR_VALUE(addr))

Should this be IS_ERR_VALUE(mapped_addr) (i.e. the result of the
do_mmap)?

> +		goto out;
> +
> +	if (create_rstor_token(mapped_addr + token_offset, NULL)) {
> +		vm_munmap(mapped_addr, size);
> +		return -EINVAL;
> +	}
> +
> +out:
> +	return mapped_addr;
>  }
>  
>  static unsigned long adjust_shstk_size(unsigned long size)
> @@ -134,7 +147,7 @@ static int shstk_setup(void)
>  		return -EOPNOTSUPP;
>  
>  	size = adjust_shstk_size(0);
> -	addr = alloc_shstk(size);
> +	addr = alloc_shstk(0, size, 0, false);
>  	if (IS_ERR_VALUE(addr))
>  		return PTR_ERR((void *)addr);
>  
> @@ -179,7 +192,7 @@ int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
>  
>  
>  	size = adjust_shstk_size(stack_size);
> -	addr = alloc_shstk(size);
> +	addr = alloc_shstk(0, size, 0, false);
>  	if (IS_ERR_VALUE(addr))
>  		return PTR_ERR((void *)addr);
>  
> @@ -373,6 +386,33 @@ static int shstk_disable(void)
>  	return 0;
>  }
>  
> +SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
> +{
> +	bool set_tok = flags & SHADOW_STACK_SET_TOKEN;
> +	unsigned long aligned_size;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
> +		return -ENOSYS;

Using -ENOSYS means there's no way to tell the difference between
"kernel doesn't support it" and "CPU doesn't support it". Should this,
perhaps return -ENOTSUP?

> +
> +	if (flags & ~SHADOW_STACK_SET_TOKEN)
> +		return -EINVAL;
> +
> +	/* If there isn't space for a token */
> +	if (set_tok && size < 8)
> +		return -EINVAL;
> +
> +	/*
> +	 * An overflow would result in attempting to write the restore token
> +	 * to the wrong location. Not catastrophic, but just return the right
> +	 * error code and block it.
> +	 */
> +	aligned_size = PAGE_ALIGN(size);
> +	if (aligned_size < size)
> +		return -EOVERFLOW;
> +
> +	return alloc_shstk(addr, aligned_size, size, set_tok);
> +}
> +
>  long shstk_prctl(struct task_struct *task, int option, unsigned long features)
>  {
>  	if (option == ARCH_SHSTK_LOCK) {
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 33a0ee3bcb2e..392dc11e3556 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -1058,6 +1058,7 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
>  asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
>  					    unsigned long home_node,
>  					    unsigned long flags);
> +asmlinkage long sys_map_shadow_stack(unsigned long addr, unsigned long size, unsigned int flags);
>  
>  /*
>   * Architecture-specific system calls
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 45fa180cc56a..b12940ec5926 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -887,7 +887,7 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
>  __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
>  
>  #undef __NR_syscalls
> -#define __NR_syscalls 451
> +#define __NR_syscalls 452
>  
>  /*
>   * 32 bit systems traditionally used different
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 860b2dcf3ac4..cb9aebd34646 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -381,6 +381,7 @@ COND_SYSCALL(vm86old);
>  COND_SYSCALL(modify_ldt);
>  COND_SYSCALL(vm86);
>  COND_SYSCALL(kexec_file_load);
> +COND_SYSCALL(map_shadow_stack);
>  
>  /* s390 */
>  COND_SYSCALL(s390_pci_mmio_read);
> -- 
> 2.17.1
> 

Otherwise, looks good!

-- 
Kees Cook