Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

H. Peter Anvin
2014-04-28T23:06:55Z

On 04/23/2014 09:53 PM, Andrew Lutomirski wrote:
>
> This particular vector hurts: you can safely keep trying until it works.
>
> This just gave me an evil idea: what if we make the whole espfix area
> read-only? This has some weird effects. To switch to the espfix
> stack, you have to write to an alias. That's a little strange but
> harmless and barely complicates the implementation. If the iret
> faults, though, I think the result will be a #DF. This may actually
> be a good thing: if the #DF handler detects that the cause was a bad
> espfix iret, it could just return directly to bad_iret or send the
> signal itself the same way that do_stack_segment does. This could
> even be written in C :)
>
> Peter, is this idea completely nuts? The only exceptions that can
> happen there are NMI, MCE, #DB, #SS, and #GP. The first four use IST,
> so they won't double-fault.
>

So I tried writing this bit up, but it fails in some rather spectacular
ways. Furthermore, I have been unable to debug it under Qemu, because
breakpoints don't work right (common Qemu problem, sadly.)

The kernel code is at:

https://git.kernel.org/cgit/linux/kernel/git/hpa/espfix64.git/

There are two tests:

git://git.zytor.com/users/hpa/test16/test16.git, build it, and run
./run16 test/hello.elf
http://www.zytor.com/~hpa/ldttest.c

The former will exercise the irq_return_ldt path, but not the fault
path; the latter will exercise the fault path, but doesn't actually use
a 16-bit segment.

Under the 3.14 stock kernel, the former should die with SIGBUS and the
latter should pass.

-hpa