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

[David Daney <ddaney@caviumnetworks.com>]

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.

David Daney