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Date: Wed, 9 Sep 2020 15:08:45 -0700
From: "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To: Dave Hansen <dave.hansen@...el.com>,
Andy Lutomirski <luto@...nel.org>
Cc: Dave Martin <Dave.Martin@....com>, "H.J. Lu" <hjl.tools@...il.com>,
Florian Weimer <fweimer@...hat.com>, X86 ML <x86@...nel.org>,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
linux-arch <linux-arch@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>,
Arnd Bergmann <arnd@...db.de>,
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>,
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>,
Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
shadow stack
On 9/8/2020 11:25 AM, Yu, Yu-cheng wrote:
> On 9/8/2020 10:57 AM, Dave Hansen wrote:
>> On 9/8/20 10:50 AM, Yu, Yu-cheng wrote:
>>> What about this:
>>>
>>> - Do not add any new syscall or arch_prctl for creating a new shadow
>>> stack.
>>>
>>> - Add a new arch_prctl that can turn an anonymous mapping to a shadow
>>> stack mapping.
>>>
>>> This allows the application to do whatever is necessary. It can even
>>> allow GDB or JIT code to create or fix a call stack.
>>
>> Fine with me. But, it's going to effectively be
>>
>> arch_prctl(PR_CONVERT_TO_SHS..., addr, len);
>>
>> when it could just as easily be:
>>
>> madvise(addr, len, MADV_SHSTK...);
>>
>> Or a new syscall. The only question in my mind is whether we want to do
>> something generic that we can use for other similar things in the
>> future, like:
>>
>> madvise2(addr, len, flags, MADV2_SHSTK...);
>>
>> I don't really feel strongly about it, though. Could you please share
>> your logic on why you want a prctl() as opposed to a whole new syscall?
>>
>
> A new syscall is more intrusive, I think. When creating a new shadow
> stack, the kernel also installs a restore token on the top of the new
> shadow stack, and it is somewhat x86-specific. So far no other arch's
> need this.
>
> Yes, madvise is better if the kernel only needs to change the mapping.
> The application itself can create the restore token before calling
> madvise().
After looking at this more, I found the changes are more similar to
mprotect() than madvise(). We are going to change an anonymous mapping
to a read-only mapping, and add the VM_SHSTK flag to it. Would an
x86-specific mprotect(PROT_SHSTK) make more sense?
One alternative would be requiring a read-only mapping for
madvise(MADV_SHSTK). But that is inconvenient for the application.
Yu-cheng
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