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

[Luiz Capitulino]

On Tue, 18 Sep 2012 16:13:30 -0500
Anthony Liguori <address@hidden> wrote:

> Markus Armbruster <address@hidden> writes:
> 
> > Jan Kiszka <address@hidden> writes:
> >
> >>>>>>  * 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.
> >>>>
> >>>> The point is to overcome the focus on Linux-only dump processing tools.
> >>> 
> >>> While I don't care for supporting alternate dump processing tools
> >>> myself, I certainly don't mind supporting them, as long as the code
> >>> satisfies basic safety and reliability requirements.
> >>> 
> >>> This code doesn't, as far as I can tell.
> >>
> >> It works, thought not under all circumstances.
> >
> > I don't doubt it works often enough to be useful to somebody.  But basic
> > safety and reliability requirements are a bit more than that.  They
> > include "don't explode in ways a reasonable user can't be expected to
> > foresee".  I don't think a reasonable user can be expected to see that
> > -p is safe only for trusted guests.
> 
> We shipped the API, we're not removing it.  Our compatibility isn't
> "whatever libvirt is currently using".
> 
> It's perfectly reasonable to ask to document the behavior of the
> method.  It's also a trivial patch to qapi-schema.json.

I feel that documenting it is not enough. It would be fine to do that
if the worst case was a bad dump file, but the worst case as the
code stands right now will affect the host.

> It's unreasonable to ask for an interface to be removed just because it
> could be misused when it has a legimitate use-case.

The point is not that it can be misused. The issue we're concerned about
is that a malicious guest could cause qemu to allocate dozens of
gigabytes of RAM.

Jan suggested a fix that could make it less worse, which is to avoid
allocating any memory while walking the guest page tables. However,
it's not clear if this is hard to do and, more importantly, if it's
backportable to -stable.