Re: hvm_emulate_one() usage

Andrew Cooper
2012-12-28T22:27:49Z

On 28/12/2012 14:34, Razvan Cojocaru wrote:
> Hello,
>
> I have a dom0 userspace application that receives mem_events. Mem_events
> are being received if a page fault occurs, and until I clear the page
> access rights I keep receiving the event in a loop. If I do clear the
> page access rights, I will no longer receive mem_events for said page.
>
> What I thought I''d do was to add a new flag to the mem_event
response
> (MEM_EVENT_FLAG_EMULATE_WRITE), and have this code execute in
> p2m_mem_access_resume() in xen/arch/x86/mm/p2m.c:
>
> mem_event_get_response(d, &rsp);
>
> if ( rsp.flags & MEM_EVENT_FLAG_EMULATE_WRITE )
> {
>      struct hvm_emulate_ctxt ctx[1] = {};
>      struct vcpu *current_vcpu = current;
>
>      set_current(d->vcpu[rsp.vcpu_id]);

Not that I can help you with your problem specifically, but
set_current() here ...

>
>      hvm_emulate_prepare(ctx, guest_cpu_user_regs());
>      hvm_emulate_one(ctx);
>
>      set_current(current_vcpu);

and here are absolutely wrong and will cause bad things to happen. (As
demonstrated by the crash below)

set_current() is only for use with scheduling, and sets which vcpu is
"current" on this pcpu.  As the code currently stands, there is a
thundering great race condition where this particular vcpu might be
current on 2 pcpus at once.

Other than above, which will certainly break the scheduling code,
"current" is used everywhere in the Xen code, so your call to
hvm_emulate_prepare is using the real "current" vcpus registers, with
information from the wrong "current" cpu, including cs and ss segment
registers, which is then going to be interpreted incorrectly as they
will being used in the wrong vcms/gdt.

By this point, bets are certainly on that stuff will break.


Can you describe exactly what behaviour you are attempting to achieve
with this?  It seems to me that you are wanting to step a paused HVM
vcpu on by one instruction based off a hypercall from dom0 ?

~Andrew

> }
>
> The code is supposed to go past the write instruction (without lifting
> the page access restrictions). What it does seem to achieve is this:
>
> (XEN) ----[ Xen-4.1.2  x86_64  debug=n  Not tainted ]----
> (XEN) CPU:    6
> (XEN) RIP:    e008:[<ffff82c4801bf4ea>]
vmx_get_interrupt_shadow+0xa/0x10
> (XEN) RFLAGS: 0000000000010203   CONTEXT: hypervisor
> (XEN) rax: 0000000000004824   rbx: ffff83013c0c7ba0   rcx: 0000000000000008
> (XEN) rdx: 0000000000000005   rsi: ffff83013c0c7f18   rdi: ffff8300bfca8000
> (XEN) rbp: ffff83013c0c7f18   rsp: ffff83013c0c7b50   r8:  0000000000000002
> (XEN) r9:  0000000000000002   r10: ffff82c48020af40   r11: 0000000000000282
> (XEN) r12: ffff8300bfff2000   r13: ffff88012b478b18   r14: 00007fffd669c4c0
> (XEN) r15: ffff83013c0c7e48   cr0: 0000000080050033   cr4: 00000000000026f0
> (XEN) cr3: 000000005d6c4000   cr2: 000000000221e538
> (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e010   cs: e008
> (XEN) Xen stack trace from rsp=ffff83013c0c7b50:
> (XEN)    ffff82c4801a1a91 ffff83013f986000 ffff83013f986000
ffff83013c0c7f18
> (XEN)    ffff82c4801ce0e1 0000000500050000 000000000003f31a
0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000
0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000
0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000
0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000
0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000
0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000
0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000
0000000000000000
> (XEN)    0000000000000000 0000000000000000 0000000000000000
0000000000000000
> (XEN)    0000000000000000 ffff83013f986000 ffff8300bfff2000
ffff83013c0c7e48
> (XEN)    ffff82c4801d1d81 ffff8300bfcac000 ffff82c4801d05c5
ffff83013c0c7f18
> (XEN)    ffff82c4801a1447 ffff8300bfcac000 0000000000d7e004
0000000000d7e004
> (XEN)    ffff83013c0c7e48 ffff88012b478b18 00007fffd669c4c0
ffff83013c0c7e48
> (XEN)    ffff82c48014eb79 0000000000000000 000000000005d6f9
ffff82f600badf20
> (XEN)    0000000000000000 4000000000000000 ffff82f600badf20
0000000000000000
> (XEN)    ffff88012fc0b928 0000000000000001 ffff82c48016bc4b
ffff82f600badf20
> (XEN)    ffff82c48016c0b8 ffff83013c0ac000 ffff83013c0ac000
ffff82f600bb1940
> (XEN)    000000000000000f ffff83013c0c7f18 ffff83013c0ac000
ffff82f600bb1940
> (XEN)    fffffffffffffff3 0000000000d7e004 ffff83013c0c7e48
ffff88012b478b18
> (XEN) Xen call trace:
> (XEN)    [<ffff82c4801bf4ea>] vmx_get_interrupt_shadow+0xa/0x10
> (XEN)    [<ffff82c4801a1a91>] hvm_emulate_prepare+0x31/0x80
> (XEN)    [<ffff82c4801ce0e1>] p2m_mem_access_resume+0xe1/0x120
> (XEN)    [<ffff82c4801d1d81>] mem_access_domctl+0x21/0x30
> (XEN)    [<ffff82c4801d05c5>] mem_event_domctl+0x295/0x3b0
> (XEN)    [<ffff82c4801a1447>] hvmemul_do_pio+0x27/0x30
> (XEN)    [<ffff82c48014eb79>] arch_do_domctl+0x2e9/0x28a0
> (XEN)    [<ffff82c48016bc4b>] get_page_type+0xb/0x20
> (XEN)    [<ffff82c48016c0b8>] get_page_and_type_from_pagenr+0x78/0xe0
> (XEN)    [<ffff82c4801025bb>] do_domctl+0xfb/0x10b0
> (XEN)    [<ffff82c4801f2fa6>] ept_get_entry+0x136/0x250
> (XEN)    [<ffff82c480180965>] copy_to_user+0x25/0x70
> (XEN)    [<ffff82c4801f8778>] syscall_enter+0x88/0x8d
> (XEN)
> (XEN)
> (XEN) ****************************************
> (XEN) Panic on CPU 6:
> (XEN) FATAL TRAP: vector = 6 (invalid opcode)
> (XEN) ****************************************
>
> I could find no documentation on either the hvm_*(), or the cpu-related
> functions. Obviously the hvm_emulate_prepare() call crashes the
> hypervisor, most likely because of the guest_cpu_user_regs() parameter,
> but "regs" is not being passed to p2m_mem_access_resume() (like
it is
> being passed to p2m_mem_access_check()). I would appreciate your help in
> figuring out how to implement this.
>
> Thanks, and happy holidays,
> Razvan Cojocaru
>
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