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

Andrew Lutomirski
2014-04-29T00:21:14Z

On Mon, Apr 28, 2014 at 5:02 PM, Andrew Lutomirski <amluto@gmail.com> wrote:
> On Mon, Apr 28, 2014 at 4:08 PM, H. Peter Anvin <hpa@linux.intel.com> wrote:
>> On 04/28/2014 04:05 PM, H. Peter Anvin wrote:
>>>
>>> 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.
>>>
>>
>> Current status of the above code: if I remove the randomization in
>> espfix_64.c then the first test passes; the second generally crashes the
>> machine. With the randomization there, both generally crash the machine.
>>
>> All my testing so far has been under KVM or Qemu, so there is always the
>> possibility that I'm chasing a KVM/Qemu bug, but I suspect it is
>> something simpler than that.
>
> I'm compiling your branch. In the mean time, two possibly stupid questions:
>
> What's the assembly code in the double-fault entry for?
>
> Have you tried hbreak in qemu? I've had better luck with hbreak than
> regular break in the past.
>

ldttest segfaults on 3.13 and 3.14 for me. It reboots (triple fault?)
on your branch. It even said this:

qemu-system-x86_64: 9pfs:virtfs_reset: One or more uncluncked fids
found during reset

I have no idea what an uncluncked fd is :)

hello.elf fails to sigbus. weird. gdb says:

1: x/i $pc
=> 0xffffffff8170559c <irq_return_ldt+90>:
jmp 0xffffffff81705537 <irq_return_iret>
(gdb) si
<signal handler called>
1: x/i $pc
=> 0xffffffff81705537 <irq_return_iret>: iretq
(gdb) si
Cannot access memory at address 0xf0000000f
(gdb) info registers
rax 0xffe4000f00001000 -7881234923384832
rbx 0x1000000010 68719476752
rcx 0xffe4f5580000f000 -7611541041909760
rdx 0x805d000 134598656
rsi 0x102170000ffe3 283772784279523
rdi 0xf00000007 64424509447
rbp 0xf0000000f 0xf0000000f
rsp 0xf0000000f 0xf0000000f
r8 0x0 0
r9 0x0 0
r10 0x0 0
r11 0x0 0
r12 0x0 0
r13 0x0 0
r14 0x0 0
r15 0x0 0
rip 0x0 0x0 <irq_stack_union>
eflags 0x0 [ ]
cs 0x0 0
ss 0x37f 895
ds 0x0 0
es 0x0 0
fs 0x0 0
---Type <return> to continue, or q <return> to quit---
gs 0x0 0

I got this with 'hbreak irq_return_ldt' using 'target remote :1234'
and virtme-run --console --kimg
~/apps/linux-devel/arch/x86/boot/bzImage --qemu-opts -s

This set of registers looks thoroughly bogus. I don't trust it. I'm
now stuck -- single-stepping stays exactly where it started.
Something is rather screwed up here. Telling gdb to continue causes
gdb to explode and 'Hello, Afterworld!' to be displayed.

I was not able to get a breakpoint on __do_double_fault to hit.

FWIW, I think that gdb is known to have issues debugging a guest that
switches bitness.

--Andy