linux-kernel - Re: [PATCH v20 25/25] mm: Introduce PROT

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Message-ID: <202102101145.7DE8B381@keescook>
Date:   Wed, 10 Feb 2021 11:46:30 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Yu-cheng Yu <yu-cheng.yu@...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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>, haitao.huang@...el.com
Subject: Re: [PATCH v20 25/25] mm: Introduce PROT_SHSTK for shadow stack

On Wed, Feb 10, 2021 at 09:57:03AM -0800, Yu-cheng Yu wrote:
> There are three possible options to create a shadow stack allocation API:
> an arch_prctl, a new syscall, or adding PROT_SHSTK to mmap()/mprotect().
> Each has its advantages and compromises.
> 
> An arch_prctl() is the least intrusive.  However, the existing x86
> arch_prctl() takes only two parameters.  Multiple parameters must be
> passed in a memory buffer.  There is a proposal to pass more parameters in
> registers [1], but no active discussion on that.
> 
> A new syscall minimizes compatibility issues and offers an extensible frame
> work to other architectures, but this will likely result in some overlap of
> mmap()/mprotect().
> 
> The introduction of PROT_SHSTK to mmap()/mprotect() takes advantage of
> existing APIs.  The x86-specific PROT_SHSTK is translated to VM_SHSTK and
> a shadow stack mapping is created without reinventing the wheel.  There are
> potential pitfalls though.  The most obvious one would be using this as a
> bypass to shadow stack protection.  However, the attacker would have to get
> to the syscall first.
> 
> Since arch_calc_vm_prot_bits() is modified, I have moved arch_vm_get_page
> _prot() and arch_calc_vm_prot_bits() to x86/include/asm/mman.h.
> This will be more consistent with other architectures.

This portion of the patch seems logically separate from the PROT_SHSTK
implementation. Can you please separate it into its own patch?

> 
> [1] https://lore.kernel.org/lkml/20200828121624.108243-1-hjl.tools@gmail.com/
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>

With that done:

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

-- 
Kees Cook