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migration: Fix race on qemu_file_shutdown()

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In qemu_file_shutdown(), there's a possible race if with current order of
operation.  There're two major things to do:

  (1) Do real shutdown() (e.g. shutdown() syscall on socket)
  (2) Update qemufile's last_error

We must do (2) before (1) otherwise there can be a race condition like:

      page receiver                     other thread
      -------------                     ------------
      qemu_get_buffer()
                                        do shutdown()
        returns 0 (buffer all zero)
        (meanwhile we didn't check this retcode)
      try to detect IO error
        last_error==NULL, IO okay
      install ALL-ZERO page
                                        set last_error
      --> guest crash!

To fix this, we can also check retval of qemu_get_buffer(), but not all
APIs can be properly checked and ultimately we still need to go back to
qemu_file_get_error().  E.g. qemu_get_byte() doesn't return error.

Maybe some day a rework of qemufile API is really needed, but for now keep
using qemu_file_get_error() and fix it by not allowing that race condition
to happen.  Here shutdown() is indeed special because the last_error was
emulated.  For real -EIO errors it'll always be set when e.g. sendmsg()
error triggers so we won't miss those ones, only shutdown() is a bit tricky
here.

Cc: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
This commit is contained in:
Peter Xu 2022-10-04 14:24:27 -04:00 committed by Juan Quintela
parent 4934a5dd7c
commit f5816b5c86
1 changed files with 24 additions and 3 deletions
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27
migration/qemu-file.c
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@ -79,6 +79,30 @@ int qemu_file_shutdown(QEMUFile *f)
int ret = 0;
f->shutdown = true;
/*
* We must set qemufile error before the real shutdown(), otherwise
* there can be a race window where we thought IO all went though
* (because last_error==NULL) but actually IO has already stopped.
*
* If without correct ordering, the race can happen like this:
*
* page receiver other thread
* ------------- ------------
* qemu_get_buffer()
* do shutdown()
* returns 0 (buffer all zero)
* (we didn't check this retcode)
* try to detect IO error
* last_error==NULL, IO okay
* install ALL-ZERO page
* set last_error
* --> guest crash!
*/
if (!f->last_error) {
qemu_file_set_error(f, -EIO);
}
if (!qio_channel_has_feature(f->ioc,
QIO_CHANNEL_FEATURE_SHUTDOWN)) {
return -ENOSYS;
@ -88,9 +112,6 @@ int qemu_file_shutdown(QEMUFile *f)
ret = -EIO;
}
if (!f->last_error) {
qemu_file_set_error(f, -EIO);
}
return ret;
}