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Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or
From: |
Wen Congyang |
Subject: |
Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it? |
Date: |
Tue, 18 Sep 2012 09:52:49 +0800 |
User-agent: |
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100413 Fedora/3.0.4-2.fc13 Thunderbird/3.0.4 |
At 09/18/2012 01:56 AM, Luiz Capitulino Wrote:
> Hi Wen,
>
> We've re-reviewed the dump-guest-memory command and found some
> possible issues with the -p option.
>
> The main issue is that it seems possible for a malicious guest to set
> page tables in a way that we allocate a MemoryMapping structure for
> each possible PTE. If IA-32e paging is used, this could lead to the
> allocation of dozens of gigabytes by qemu.
>
> Of course that this is not expected for the regular case, where a
> MemoryMapping allocation can be skipped for several reasons (I/O memory,
> page not present, contiguous/in same range addresses etc), but the
> point is what a malicious guest can do.
>
> Another problem is that the -p option seems to be broken for SMP guests.
> The problem is in qemu_get_guest_memory_mapping():
>
> first_paging_enabled_cpu = find_paging_enabled_cpu(first_cpu);
> if (first_paging_enabled_cpu) {
> for (env = first_paging_enabled_cpu; env != NULL; env =
> env->next_cpu) {
> ret = cpu_get_memory_mapping(list, env);
> if (ret < 0) {
> return -1;
> }
> }
> return 0;
> }
>
> This looks for the first vCPU with paging enabled, and then assumes
> that all the following vCPUs also have paging enabled. How does this
> hold?
>
> Assuming that this last issue is fixable (ie. we can make the -p
> option work well with SMP guests), we should at least document that
> -p can make QEMU allocates lots of memory and end up being killed
> by the OS.
>
> However, I also think that we should consider if having the -p
> feature is really worth it. It's a complex feature and has a number
> of limitations*. If libvirt doesn't use this, dropping it shouldn't
> be a big deal (we can return an error when -p is used).
>
> * The issues discussed in this email plus the fact that the guest
> memory may be corrupted, and the guest may be in real-mode even
> when paging is enabled
>
Yes, there are some limitations with this option. Jan said that he
always use gdb to deal with vmcore, so he needs such information.
Thanks
Wen Congyang
- [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Luiz Capitulino, 2012/09/17
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Eric Blake, 2012/09/17
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?,
Wen Congyang <=
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Jan Kiszka, 2012/09/18
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Markus Armbruster, 2012/09/18
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Jan Kiszka, 2012/09/18
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Luiz Capitulino, 2012/09/18
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Jan Kiszka, 2012/09/18
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Markus Armbruster, 2012/09/18
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Anthony Liguori, 2012/09/18
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Luiz Capitulino, 2012/09/18
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Anthony Liguori, 2012/09/18
- Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?, Wen Congyang, 2012/09/18