linux-kernel - Re: [PATCH v2 28/39] x86/cet/shstk: Introduce map_shadow

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Date:   Mon, 10 Oct 2022 13:13:05 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com, rppt@...nel.org,
        jamorris@...ux.microsoft.com, dethoma@...rosoft.com
Subject: Re: [PATCH v2 28/39] x86/cet/shstk: Introduce map_shadow_stack syscall

* Rick Edgecombe:

> 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(adrr, stack_size, SHADOW_STACK_SET_TOKEN);

Jason has recently been working on vDSO-based getrandom acceleration.
It needs a way for a userspace thread to allocate userspace memory in a
specific way.  Jason proposed to use a vDSO call as the interface, not a
system call.

Maybe this approach is applicable here as well?  Or we can come up with
a more general interface for such per-thread allocations?

Thanks,
Florian