linux-kernel - Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

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Date:	Tue, 7 Oct 2014 11:16:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	libc-alpha <libc-alpha@...rceware.org>,
	David Daney <ddaney.cavm@...il.com>,
	David Daney <david.daney@...ium.com>,
	Rich Felker <dalias@...c.org>,
	David Daney <david.s.daney@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

On Oct 7, 2014 9:09 AM, "David Daney" <ddaney@...iumnetworks.com> wrote:
>
> On 10/07/2014 04:11 AM, Rich Felker wrote:
>>
>> On Mon, Oct 06, 2014 at 09:50:47PM -0700, David Daney wrote:
>>>>>
>>>>> the out-of-line execution trick, but do it somewhere other than in
>>>>> stack memory.
>>>>
>>>> How do you answer Andy Lutomirski's question about what happens when a
>>>> signal handler interrupts execution while the program counter is
>>>> pointing at this "out-of-line execution" trampoline? This seems like a
>>>> show-stopper for using anything other than the stack.
>>>
>>> It would be nice to support, but not doing so would not be a
>>> regression from current behavior.
>>
>>
>> It's not just "nice" to support, it's mandatory. Otherwise you will
>> execute essentially *random instructions* in this case, providing a
>> very nice attack vector that can almost certainly be elevated to
>> arbitrary code execution via timing of signals during floating point
>> code.
>>
>> The current behavior in regards to this is correct: because you have a
>> *stack*, each trampoline is pushed onto the stack in its own context,
>> and popped when it's no longer needed. You can have arbitrarily many
>> such trampolines up to the stack size. Note that each nested signal
>> handler already requires sizeof(ucontext_t) in stack space, so these
>> trampolines are a negligible additional cost without major effects on
>> the number of signal handlers you can nest without overflowing the
>> stack.
>
>
> Yes, the stack takes care of the allocations, but the current implementation has many problems:
>
> 1) Signals clobber the emulation area.
> 2) Signals caused by the emulation, have incorrect saved machine state.
>
> We have a low bar to pass, any new solution doesn't have to be perfect, it only has to be an improvement.
>
> Keep in mind that we are not starting from a clean slate, there are many years of legacy code that has built up here.

A lesson I learned when doing the x86 vsyscall stuff: Don't waste time
improving legacy crap without a really good reason.  Especially don't
extend the interface.  Deprecate it (without breaking working user
code) and move on.

--Andy

>
> David Daney
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