Re: [PATCH] migration: introduce decompress-error-check

[Eric Blake <eblake@redhat.com>]

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On 04/26/2018 04:15 AM, guangrong.xiao@gmail.com wrote:
> From: Xiao Guangrong <xiaoguangrong@tencent.com>
> 
> QEMU 2.13 enables strict check for compression & decompression to
> make the migration more robuster, that depends on the source to fix

s/robuster/robust/

> the internal design which triggers the unexpected error conditions

2.13 hasn't been released yet.  Why do we need a knob to explicitly turn
off strict checking?  Can we not instead make 2.13 automatically smart
enough to tell if the incoming stream is coming from an older qemu
(which might fail if the strict checks are enabled) vs. a newer qemu
(the sender gave us what we need to ensure the strict checks are
worthwhile)?

> 
> To make it work for migrating old version QEMU to 2.13 QEMU, we
> introduce this parameter to disable the error check on the
> destination
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
> ---

> +++ b/qapi/migration.json
> @@ -455,6 +455,17 @@
>  #          compression, so set the decompress-threads to the number about 1/4
>  #          of compress-threads is adequate.
>  #
> +# @decompress-error-check: check decompression errors. When false, the errors
> +#                          triggered by memory decompression are ignored.

What are the consequences of such an error?  Is it a security hole to
leave this at false, when a malicious migration stream can cause us to
misbehave by ignoring the errors?

> +#                          When true, migration is aborted if the errors are
> +#                          detected. For the old QEMU versions (< 2.13) the
> +#                          internal design will cause decompression to fail
> +#                          so the destination should completely ignore the
> +#                          error conditions, i.e, make it be false if these
> +#                          QEMUs are going to be migrated. Since 2.13, this
> +#                          design is fixed, make it be true to avoid corrupting
> +#                          the VM silently (Since 2.13)

Rather wordy; I'd suggest:

@decompress-error-check: Set to true to abort the migration if
        decompression errors are detected at the destination. Should be
        left at false (default) for qemu older than 2.13, since only
        newer qemu sends streams that do not trigger spurious
        decompression errors. (Since 2.13)

But that's if we even need it (it SHOULD be possible to design something
into the migration stream so that you can detect this property
automatically instead of relying on the user to set the property).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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