Re: [PATCH v20 25/25] mm: Introduce PROT_SHSTK for shadow stack

From: Kees Cook <keescook@chromium.org>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
	linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Andy Lutomirski <luto@kernel.org>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Weijiang Yang <weijiang.yang@intel.com>,
	Pengfei Xu <pengfei.xu@intel.com>,
	haitao.huang@intel.com
Subject: Re: [PATCH v20 25/25] mm: Introduce PROT_SHSTK for shadow stack
Date: Wed, 10 Feb 2021 11:46:30 -0800	[thread overview]
Message-ID: <202102101145.7DE8B381@keescook> (raw)
In-Reply-To: <20210210175703.12492-26-yu-cheng.yu@intel.com>

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@intel.com>

With that done:

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

-- 
Kees Cook