Re: [Libguestfs] [nbdkit PATCH] nbd: Opt in to libnbd pread_initialize speedup

Our nbd plugin has always properly checked for asynch errors (and thus has never been at risk of a vulnerability similar to CVE-2022-0485 just fixed in nbdcopy). What's more, the nbdkit core guarantees (since commit b1ce255e in 2019, v1.13.1) that the buffer handed to a .pread callback has been pre-sanitized to not leak heap contents, regardless of how buggy a plugin might be (see server/protocol.c:protocol_recv_request_send_reply(), which uses server/threadlocal.c:threadlocal_buffer() for a safe buffer). Thus, we do not need libnbd to spend time re-initializing the buffer, if libnbd is new enough to give us that knob. --- I know that Laszlo was sceptical whether any real client might want to use the libnbd knob [1], but I'm actually comfortable with this one. [1] https://listman.redhat.com/archives/libguestfs/2022-February/msg00144.html plugins/nbd/nbd.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c index ae595ea7..2e154e8d 100644 --- a/plugins/nbd/nbd.c +++ b/plugins/nbd/nbd.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2017-2020, 2022 Red Hat Inc. + * Copyright (C) 2017-2022 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -637,6 +637,15 @@ nbdplug_open_handle (int readonly, const char *client_export) #if LIBNBD_HAVE_NBD_SET_FULL_INFO if (nbd_set_full_info (h->nbd, 1) == -1) goto errnbd; +#endif +#if LIBNBD_HAVE_NBD_SET_PREAD_INITIALIZE + /* nbdkit guarantees that the buffers passed to our .pread callback + * are pre-initialized; and we in turn ensure that the buffer is not + * dereferenced if the NBD server replied with an error. Thus, we + * are safe opting in to this libnbd speedup. + */ + if (nbd_set_pread_initialize (h->nbd, false) == -1) + goto errnbd; #endif if (dynamic_export && uri) { #if LIBNBD_HAVE_NBD_SET_OPT_MODE