Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?

[Wen Congyang]

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