[Libguestfs] [PATCH nbdkit v2 2/7] tls-fallback: Fix filter for new .block_size callback

[Eric Blake]

On Thu, Feb 17, 2022 at 02:36:43PM +0000, Richard W.M. Jones wrote:
> This filter doesn't call the next_open function in the non-TLS case,
> and therefore it never opens the plugin.  This leaves the internal
> state of nbdkit a bit strange.  There is no plugin context allocated,
> and the last filter in the chain has a context c_next pointer of NULL.
> 
> This works, provided we intercept every possible callback, check the
> non-TLS case, and prevent it from calling the next function (because
> it would dereference the NULL c_next).
> 
> To avoid a crash in backend_block_size we must therefore provide a
> .block_size callback in this filter.
> ---
>  filters/tls-fallback/tls-fallback.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)

ACK.

Would you like to squash this in, and/or have me commit this separately?

commit 8c00ca2fe418aeecf0818feed227a72e76d87f18
Author: Eric Blake <eblake at redhat.com>
Date:   Thu Feb 17 10:24:50 2022 -0600

    tls-fallback: Enhance comments about required callbacks
    
    Rich ran into a crash while trying to add a new .block_size callback
    reachable during the client connection handshake (via .prepare), and
    nearly worked around the crash by reintroducing CVE-2019-14850 instead
    of the proper fix of implementing the new callback.  Add some comments
    to help us avoid a similar regression the next time we add a callback.

diff --git a/filters/tls-fallback/tls-fallback.c b/filters/tls-fallback/tls-fallback.c
index fab9e58b..64e84926 100644
--- a/filters/tls-fallback/tls-fallback.c
+++ b/filters/tls-fallback/tls-fallback.c
@@ -107,7 +107,11 @@ tls_fallback_open (nbdkit_next_open *next, nbdkit_context *nxdata,
                    const char *exportname, int is_tls)
 {
   /* We do NOT want to call next() when insecure, because we don't
-   * know how long it will take.
+   * know how long it will take.  See also CVE-2019-14850 in
+   * nbdkit-security.pod.  But that means that this filter must
+   * override every possible callback that can be reached during
+   * handshake, to avoid passing through a non-TLS call to a missing
+   * backend.
    */
   if (!is_tls)
     return &message; /* See NOT_TLS for this choice of handle */


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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