Re: Some linux kernel with KAISER/KPTI patch can't work under qemu + haxm.

[lepton]

I tried some debug, it seems it crashed after switch CR3:


I tried 2 different kernel, so actual crash points are different, but
they have same pattern.  It crashed when trying to pop %rax after
switch CR3:

for 4.10 with kaiser patch: it crashed at this point:

Dump of assembler code for function ret_from_fork:
   0xffffffff8190ad00 <+0>:     push   %rbp
   0xffffffff8190ad01 <+1>:     mov    %rsp,%rbp
   0xffffffff8190ad04 <+4>:     mov    %rax,%rdi
   0xffffffff8190ad07 <+7>:     callq  0xffffffff81083670 <schedule_tail>
   0xffffffff8190ad0c <+12>:    test   %rbx,%rbx
   0xffffffff8190ad0f <+15>:    jne    0xffffffff8190ad32 <ret_from_fork+50>
   0xffffffff8190ad11 <+17>:    lea    0x8(%rsp),%rdi
   0xffffffff8190ad16 <+22>:    callq  0xffffffff810027a0
<syscall_return_slowpath>
   0xffffffff8190ad1b <+27>:    push   %rax
   0xffffffff8190ad1c <+28>:    mov    %cr3,%rax
   0xffffffff8190ad1f <+31>:    or     $0x1000,%rax
   0xffffffff8190ad25 <+37>:    mov    %rax,%cr3
   0xffffffff8190ad28 <+40>:    pop    %rax
<-----------------------------------  crashed here
   0xffffffff8190ad29 <+41>:    swapgs
   0xffffffff8190ad2c <+44>:    pop    %rbp


For 4.4.110:

Dump of assembler code for function opportunistic_sysret_failed:
   0xffffffff81887165 <+0>: jmp    0xffffffff81887181
<opportunistic_sysret_failed+28>
   0xffffffff81887167 <+2>: mov    %cr3,%rax
   0xffffffff8188716a <+5>: or     %gs:0xf0c0,%rax
   0xffffffff81887173 <+14>: js     0xffffffff8188717d
<opportunistic_sysret_failed+24>
   0xffffffff81887175 <+16>: mov    %al,%gs:0xf0c7
   0xffffffff8188717d <+24>: mov    %rax,%cr3
   0xffffffff81887180 <+27>: pop    %rax
<---------------------------------------- crashed here
   0xffffffff81887181 <+28>: swapgs
   0xffffffff81887184 <+31>: jmpq   0xffffffff81887ab0 <restore_c_regs_and_iret>
   0xffffffff81887189 <+36>: nopl   0x0(%rax)

On Thu, Jan 4, 2018 at 2:13 PM, lepton <ytht.net@xxxxxxxxx> wrote:
> It seems for some reason, some linux kernel with KAISER/KPTI patch
> can't work with qemu + haxm.
> The mainline kernel from Linus is fine. But the patch to 4.4/4.10 doesn't work.
>
> I am not familiar with HAXM and KPTI either. so not sure if this is a
> qemu bug or KPTI bug or haxm bug.
>
> The same kernel works fine under qemu + kvm.
>
> This is the way to reproduce it:
>
> 1. Download qemu for windows, follow instructions here:
>
> https://www.qemu.org/2017/11/22/haxm-usage-windows/
>
> 2. Build a kernel with KAISER/KPTI,  I am using kernel here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git/log/?h=linux-4.4.y
> Or follow instruction here to build a 4.10 kernel:
> https://github.com/IAIK/KAISER/tree/master/KAISER
>
> 3. Build an ext2 image which has a simple init:
> dd of=img count=8192 bs=4096 if=/dev/zero
> mke2fs img
> gcc --static -o init init.c
> debugfs -R "write init init" -w img
> cat init.c
> #include <stdio.h>
> #include <time.h>
>
> int main()
> {
>         while(1) {
>                 printf("This is init %d\n", time(NULL));
>                 sleep(3600);
>         }
> }
>
> 4. copy kernel and disk image generated from 3 to windows and run it:
> qemu-system-x86_64.exe -kernel bzImage -hda img -append "init=/init
> root=/dev/sda" -serial stdio -accel hax
>
> You will see kernel panic or "vpu shutdown reqeust" to qemu.