Date:   Fri, 10 Mar 2023 17:11:19 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
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Subject: Re: [PATCH v7 33/41] x86/shstk: Introduce map_shadow_stack syscall

On Mon, Feb 27, 2023 at 02:29:49PM -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

"stacks"

> 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

"threads"

> 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:
					     ^
					     of


> 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

"its"

> 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);

...

> 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

Yeah, this'll need a manpage too, I presume. But later.

> +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 -EOPNOTSUPP;
> +
> +	if (flags & ~SHADOW_STACK_SET_TOKEN)
> +		return -EINVAL;
> +
> +	/* If there isn't space for a token */
> +	if (set_tok && size < 8)
> +		return -EINVAL;
> +
> +	if (addr && addr <= 0xFFFFFFFF)

			< SZ_4G

> +		return -EINVAL;

Can we use distinct negative retvals in each case so that it is clear to
userspace where it fails, *if* it fails?

> +	/*
> +	 * 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);
> +}

-- 
Regards/Gruss,
    Boris.

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