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Subject: Re: [PATCH 03/10] x86/cet: Signal handling for shadow stack
To:     Andy Lutomirski <luto@amacapital.net>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        Cyrill Gorcunov <gorcunov@openvz.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. J. Lu" <hjl.tools@gmail.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Oleg Nesterov <oleg@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        mike.kravetz@oracle.com
References: <20180607143807.3611-1-yu-cheng.yu@intel.com>
 <20180607143807.3611-4-yu-cheng.yu@intel.com>
 <CALCETrWo77RS_wOzskw5OG-LdC1S-b_NY=uPWUmPbQEnNwANgQ@mail.gmail.com>
From:   Florian Weimer <fweimer@redhat.com>
Message-ID: <2b920019-cf03-334c-3b6a-b2c6b7f4dfa3@redhat.com>
Date:   Thu, 7 Jun 2018 20:58:56 +0200
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On 06/07/2018 08:30 PM, Andy Lutomirski wrote:
> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>>
>> Set and restore shadow stack pointer for signals.
> 
> How does this interact with siglongjmp()?

We plan to use some unused signal mask bits in the jump buffer (we have 
a lot of those in glibc for some reason) to store the shadow stack pointer.

> This patch makes me extremely nervous due to the possibility of ABI
> issues and CRIU breakage.
> 
>> diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
>> index 844d60eb1882..6c8997a0156a 100644
>> --- a/arch/x86/include/uapi/asm/sigcontext.h
>> +++ b/arch/x86/include/uapi/asm/sigcontext.h
>> @@ -230,6 +230,7 @@ struct sigcontext_32 {
>>          __u32                           fpstate; /* Zero when no FPU/extended context */
>>          __u32                           oldmask;
>>          __u32                           cr2;
>> +       __u32                           ssp;
>>   };
>>
>>   /*
>> @@ -262,6 +263,7 @@ struct sigcontext_64 {
>>          __u64                           trapno;
>>          __u64                           oldmask;
>>          __u64                           cr2;
>> +       __u64                           ssp;
>>
>>          /*
>>           * fpstate is really (struct _fpstate *) or (struct _xstate *)
>> @@ -320,6 +322,7 @@ struct sigcontext {
>>          struct _fpstate __user          *fpstate;
>>          __u32                           oldmask;
>>          __u32                           cr2;
>> +       __u32                           ssp;
> 
> Is it actually okay to modify these structures like this?  They're
> part of the user ABI, and I don't know whether any user code relies on
> the size being constant.

Probably not.  Historically, these things have been tacked at the end of 
the floating point state, see struct _xstate:

         /* New processor state extensions go here: */

However, I'm not sure if this is really ideal because I doubt that 
everyone who needs the shadow stack pointer also wants to sacrifice 
space for the AVX-512 save area (which is already a backwards 
compatibility hazard).  Other architectures have variable offsets and 
some TLV-style setup here.

Thanks,
Florian