linux-kernel - Re: [PATCH 00/35] Shadow stacks for userspace

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Date:   Thu, 03 Feb 2022 21:20:05 -0800
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Rick P Edgecombe" <rick.p.edgecombe@...el.com>,
        "Balbir Singh" <bsingharora@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Eugene Syromiatnikov" <esyr@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Randy Dunlap" <rdunlap@...radead.org>,
        "Kees Cook" <keescook@...omium.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Florian Weimer" <fweimer@...hat.com>,
        "Nadav Amit" <nadav.amit@...il.com>,
        "Jann Horn" <jannh@...gle.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "Borislav Petkov" <bp@...en8.de>,
        "Oleg Nesterov" <oleg@...hat.com>, "H.J. Lu" <hjl.tools@...il.com>,
        "Weijiang Yang" <weijiang.yang@...el.com>,
        "Pavel Machek" <pavel@....cz>, "Arnd Bergmann" <arnd@...db.de>,
        "Moreira, Joao" <joao.moreira@...el.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Mike Kravetz" <mike.kravetz@...cle.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "Dave Martin" <Dave.Martin@....com>,
        "john.allen@....com" <john.allen@....com>,
        "Ingo Molnar" <mingo@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Jonathan Corbet" <corbet@....net>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Linux API" <linux-api@...r.kernel.org>,
        "Cyrill Gorcunov" <gorcunov@...il.com>
Subject: Re: [PATCH 00/35] Shadow stacks for userspace



On Thu, Feb 3, 2022, at 5:08 PM, Edgecombe, Rick P wrote:
> Hi Thomas,

>> >        Signals
>> >        -------
>> >        Originally signals placed the location of the shadow stack
>> > restore 
>> >        token inside the saved state on the stack. This was
>> > problematic from a 
>> >        past ABI promises perspective.

What was the actual problem?

>> > So the restore location was
>> > instead just 
>> >        assumed from the shadow stack pointer. This works because in
>> > normal 
>> >        allowed cases of calling sigreturn, the shadow stack pointer
>> > should be 
>> >        right at the restore token at that time. There is no
>> > alternate shadow 
>> >        stack support. If an alt shadow stack is added later we
>> > would
>> >        need to
>> 
>> So how is that going to work? altstack is not an esoteric corner
>> case.
>
> My understanding is that the main usages for the signal stack were
> handling stack overflows and corruption. Since the shadow stack only
> contains return addresses rather than large stack allocations, and is
> not generally writable or pivotable, I thought there was a good
> possibility an alt shadow stack would not end up being especially
> useful. Does it seem like reasonable guesswork?

It's also used for things like DOSEMU that execute in a weird context and then trap back out to the outer program using a signal handler and an altstack.  Also, imagine someone writing a SIGSEGV handler specifically intended to handle shadow stack overflow.

The shadow stack can be pivoted using RSTORSSP.