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Re: [Qemu-block] [PATCH v4 17/21] nbd/client: Add nbd_receive_export_lis

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH v4 17/21] nbd/client: Add nbd_receive_export_list()
Date: Fri, 18 Jan 2019 12:07:43 +0000

17.01.2019 22:36, Eric Blake wrote:
> We want to be able to detect whether a given qemu NBD server is
> exposing the right export(s) and dirty bitmaps, at least for
> regression testing.  We could use 'nbd-client -l' from the upstream
> NBD project to list exports, but it's annoying to rely on
> out-of-tree binaries; furthermore, nbd-client doesn't necessarily
> know about all of the qemu NBD extensions.  Thus, we plan on adding
> a new mode to qemu-nbd that merely sniffs all possible information
> from the server during handshake phase, then disconnects and dumps
> the information.
> This patch adds the low-level client code for grabbing the list
> of exports.  It benefits from the recent refactoring patches, in
> order to share as much code as possible when it comes to doing
> validation of server replies.  The resulting information is stored
> in an array of NBDExportInfo which has been expanded to any
> description string, along with a convenience function for freeing
> the list.
> Note: a malicious server could exhaust memory of a client by feeding
> an unending loop of exports; perhaps we should place a limit on how
> many we are willing to receive. But note that a server could
> reasonably be serving an export for every file in a large directory,
> where an arbitrary limit in the client means we can't list anything
> from such a server; the same happens if we just run until the client
> fails to malloc() and thus dies by an abort(), where the limit is
> no longer arbitrary but determined by available memory.  Since the
> client is already planning on being short-lived, it's hard to call
> this a denial of service attack that would starve off other uses,
> so it does not appear to be a security issue.
> Signed-off-by: Eric Blake<address@hidden>
> Reviewed-by: Richard W.M. Jones<address@hidden>

Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

Best regards,

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