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

[Daniel P . Berrangé]

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.

Regards,
Daniel
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