linux-kernel - Re: [PATCH 22/35] x86/mm: Prevent VM

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Message-ID: <3df8595d-46d9-aaee-dd33-3118102ef750@intel.com>
Date:   Fri, 11 Feb 2022 14:19:49 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>, 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>,
        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>,
        Dave Martin <Dave.Martin@....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
Subject: Re: [PATCH 22/35] x86/mm: Prevent VM_WRITE shadow stacks

On 1/30/22 13:18, Rick Edgecombe wrote:
> Shadow stack accesses are writes from handle_mm_fault() perspective. So to
> generate the correct PTE, maybe_mkwrite() will rely on the presence of
> VM_SHADOW_STACK or VM_WRITE in the vma.
> 
> In future patches, when VM_SHADOW_STACK is actually creatable by
> userspace, a problem could happen if a user calls
> mprotect( , , PROT_WRITE) on VM_SHADOW_STACK shadow stack memory. The code
> would then be confused in the event of shadow stack accesses, and create a
> writable PTE for a shadow stack access. Then the process would fault in a
> loop.
> 
> Prevent this from happening by blocking this kind of memory (VM_WRITE and
> VM_SHADOW_STACK) from being created, instead of complicating the fault
> handler logic to handle it.
> 
> Add an x86 arch_validate_flags() implementation to handle the check.
> Rename the uapi/asm/mman.h header guard to be able to use it for
> arch/x86/include/asm/mman.h where the arch_validate_flags() will be.

It would be great if this also said:

	There is an existing arch_validate_flags() hook for mmap() and
	mprotect() which allows architectures to reject unwanted
	->vm_flags combinations.  Add an implementation for x86.

That's somewhat implied from what is there already, but making it more
clear would be nice.  There's a much higher bar to add a new arch hook
than to just implement an existing one.


> diff --git a/arch/x86/include/asm/mman.h b/arch/x86/include/asm/mman.h
> new file mode 100644
> index 000000000000..b44fe31deb3a
> --- /dev/null
> +++ b/arch/x86/include/asm/mman.h
> @@ -0,0 +1,21 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_MMAN_H
> +#define _ASM_X86_MMAN_H
> +
> +#include <linux/mm.h>
> +#include <uapi/asm/mman.h>
> +
> +#ifdef CONFIG_X86_SHADOW_STACK
> +static inline bool arch_validate_flags(unsigned long vm_flags)
> +{
> +	if ((vm_flags & VM_SHADOW_STACK) && (vm_flags & VM_WRITE))
> +		return false;
> +
> +	return true;
> +}

The design decision here seems to be that VM_SHADOW_STACK is itself a
pseudo-VM_WRITE flag.  Like you said: "Shadow stack accesses are writes
from handle_mm_fault()".

Very early on, this series seems to have made the decision that shadow
stacks are writable and need lots of write handling behavior, *BUT*
shouldn't have VM_WRITE set.  As a whole, that seems odd.

The alternative would be *requiring* VM_WRITE and VM_SHADOW_STACK be set
together.  I guess the downside is that pte_mkwrite() would need to be
made to work on shadow stack PTEs.

That particular design decision was never discussed.  I think it has a
really big impact on the rest of the series.  What do you think?  Was it
a good idea?  Or would the alternative be more complicated than what you
have now?