Re: [Qemu-devel] [PATCH] migration: fix crash in when incoming client channel setup fails

[Daniel P . Berrangé]

On Tue, Jun 19, 2018 at 05:00:34PM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrangé (address@hidden) wrote:
> > On Tue, Jun 19, 2018 at 04:45:05PM +0100, Dr. David Alan Gilbert wrote:
> > > * Daniel P. Berrangé (address@hidden) wrote:
> > > > The way we determine if we can start the incoming migration was
> > > > changed to use migration_has_all_channels() in:
> > > > 
> > > >   commit 428d89084c709e568f9cd301c2f6416a54c53d6d
> > > >   Author: Juan Quintela <address@hidden>
> > > >   Date:   Mon Jul 24 13:06:25 2017 +0200
> > > > 
> > > >     migration: Create migration_has_all_channels
> > > > 
> > > > This method in turn calls multifd_recv_all_channels_created()
> > > > which is hardcoded to always return 'true' when multifd is
> > > > not in use.
> > > > 
> > > > This means that if channel initialization fails with normal
> > > > migration, it'll never notice and attempt to start the
> > > > incoming migration regardless.
> > > 
> > > I'm not sure if that was actually the cause, because older versions
> > > had no check at all in socket_accept_incoming_migration.
> > 
> > Hmm, actually your write - it was a complicated series of refactorings
> > for multifd, and I think I mis-identified. I'll do a proper bisect so
> > we can have an accurate record.
> > 
> > > 
> > > > This can be seen, for example, if a client connects to a server
> > > > requiring TLS, but has an invalid x509 certificate:
> > > > 
> > > > qemu-system-x86_64: The certificate hasn't got a known issuer
> > > > qemu-system-x86_64: migration/migration.c:386: 
> > > > process_incoming_migration_co: Assertion `mis->from_src_file' failed.
> > > > 
> > > >  #0  0x00007fffebd24f2b in raise () at /lib64/libc.so.6
> > > >  #1  0x00007fffebd0f561 in abort () at /lib64/libc.so.6
> > > >  #2  0x00007fffebd0f431 in _nl_load_domain.cold.0 () at /lib64/libc.so.6
> > > >  #3  0x00007fffebd1d692 in  () at /lib64/libc.so.6
> > > >  #4  0x0000555555ad027e in process_incoming_migration_co 
> > > > (opaque=<optimized out>) at migration/migration.c:386
> > > >  #5  0x0000555555c45e8b in coroutine_trampoline (i0=<optimized out>, 
> > > > i1=<optimized out>) at util/coroutine-ucontext.c:116
> > > >  #6  0x00007fffebd3a6a0 in __start_context () at /lib64/libc.so.6
> > > >  #7  0x0000000000000000 in  ()
> > > > 
> > > > To handle the non-multifd case, we check whether mis->from_src_file
> > > > is non-NULL.
> > > 
> > > So, what happens with this fix, does the destination exit cleanly, or
> > > stay to accept another connection or what?
> > 
> > It stays around and can accept another connection, which is actually
> > quite desirable, as it limits impact of a malicious client from DOS'ing
> > the genuine client by racing to connect first.
> 
> OK, that's fine, although I suspect in most other cases QEMU exits on
> the destination with an error (e.g. if you aren't using TLS and just
> connect to the port and squirt garbage down it, I think it'll exit
> telling you that it's not a migration header).

That's ok as non-TLS has zero security anyway, so improving resilience
only really matters for TLS setups, prior to authorizing the client.

Regards,
Daniel
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