linux-kernel - Re: [PATCH v4 27/39] x86/shstk: Handle thread shadow stack

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Date:   Fri, 2 Dec 2022 18:44:21 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.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>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        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>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v4 27/39] x86/shstk: Handle thread shadow stack

On Fri, Dec 02, 2022 at 04:35:54PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> When a process is duplicated, but the child shares the address space with
> the parent, there is potential for the threads sharing a single stack to
> cause conflicts for each other. In the normal non-cet case this is handled
> in two ways.
> 
> With regular CLONE_VM a new stack is provided by userspace such that the
> parent and child have different stacks.
> 
> For vfork, the parent is suspended until the child exits. So as long as
> the child doesn't return from the vfork()/CLONE_VFORK calling function and
> sticks to a limited set of operations, the parent and child can share the
> same stack.
> 
> For shadow stack, these scenarios present similar sharing problems. For the
> CLONE_VM case, the child and the parent must have separate shadow stacks.
> Instead of changing clone to take a shadow stack, have the kernel just
> allocate one and switch to it.
> 
> Use stack_size passed from clone3() syscall for thread shadow stack size. A
> compat-mode thread shadow stack size is further reduced to 1/4. This
> allows more threads to run in a 32-bit address space. The clone() does not
> pass stack_size, which was added to clone3(). In that case, use
> RLIMIT_STACK size and cap to 4 GB.
> 
> For shadow stack enabled vfork(), the parent and child can share the same
> shadow stack, like they can share a normal stack. Since the parent is
> suspended until the child terminates, the child will not interfere with
> the parent while executing as long as it doesn't return from the vfork()
> and overwrite up the shadow stack. The child can safely overwrite down
> the shadow stack, as the parent can just overwrite this later. So CET does
> not add any additional limitations for vfork().
> 
> Userspace implementing posix vfork() can actually prevent the child from
> returning from the vfork() calling function, using CET. Glibc does this
> by adjusting the shadow stack pointer in the child, so that the child
> receives a #CP if it tries to return from vfork() calling function.
> 
> Free the shadow stack on thread exit by doing it in mm_release(). Skip
> this when exiting a vfork() child since the stack is shared in the
> parent.
> 
> During this operation, the shadow stack pointer of the new thread needs
> to be updated to point to the newly allocated shadow stack. Since the
> ability to do this is confined to the FPU subsystem, change
> fpu_clone() to take the new shadow stack pointer, and update it
> internally inside the FPU subsystem. This part was suggested by Thomas
> Gleixner.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook