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Date:   Thu, 03 Feb 2022 21:20:05 -0800
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Rick P Edgecombe" <rick.p.edgecombe@intel.com>,
        "Balbir Singh" <bsingharora@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
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        "Dave Hansen" <dave.hansen@linux.intel.com>,
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Subject: Re: [PATCH 00/35] Shadow stacks for userspace
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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.