Re: [Libguestfs] [libnbd PATCH] copy: Fail nbdcopy if NBD read or write fails

FIXME: This is CVE-2022-XXXXX (still awaiting assignment of the CVE number). nbdcopy has a nasty bug when performing multi-threaded copies using asynchronous nbd calls - it was blindly treating the completion of an asynchronous command as successful, rather than checking the *error parameter. This can result in the silent creation of a corrupted image in two different ways: when a read fails, we blindly wrote garbage to the destination; when a write fails, we did not flag that the destination was not written.

Since nbdcopy already calls exit() on a synchronous read or write failure to a file, doing the same for an asynchronous op to an NBD server is the simplest solution. A nicer solution, but more invasive to write and thus not done here, might be to allow up to N retries of the transaction (in case the read or write failure was transient), or even having a mode where as much data is copied as possible (portions of the copy that failed would be logged on stderr, and nbdcopy would still fail with a non-zero exit status, but this would copy more than just stopping at the first error).

Note that since we rely on auto-retiring and do NOT call nbd_aio_command_completed, our completion callbacks must always return 1 (if they do not exit() first), even when acting on *error. As such, there is no sane way to return an error to a manual caller of the callback, and therefore we can drop dead code that exit()s if the callback "fails". It is also worth documenting the contract on when we must manually call the callback during the asynch_zero callback, so that we do not leak or double-free the command.

And since we are now exit()ing in the callback if *error is set, we must be careful that we do not leak an unknown value of errno on paths where we did not encounter a failure. Reported-by: Nir Soffer <nsoffer(a)redhat.com&gt; Fixes: bc896eec4d ("copy: Implement multi-conn, multiple threads, multiple requests in flight.", v1.5.6) Fixes: https://bugzilla.redhat.com/2046194 --- Once the CVE is assigned, I will also update docs/libnbd-security.pod, and backport this to affected stable branches. I'm not sure if I like passing &errno in the manual calls to cb.callback(); better might be 'int dummy = 0; cb.callback(..., &dummy)', but changing that could be a separate patch.

I verified that the copy-nbd-error.sh test fails (in both the read and write directions, by modifying the filter arguments) without this patch. TODO | 1 + copy/Makefile.am | 4 ++- copy/copy-nbd-error.sh | 54 +++++++++++++++++++++++++++++++++++++ copy/file-ops.c | 21 +++++---------- copy/multi-thread-copying.c | 18 ++++++++++++- copy/nbdcopy.h | 7 ++--- copy/null-ops.c | 12 +++------ 7 files changed, 88 insertions(+), 29 deletions(-) create mode 100755 copy/copy-nbd-error.sh diff --git a/TODO b/TODO index da157942..7c9c15e2 100644 --- a/TODO +++ b/TODO @@ -33,6 +33,7 @@ nbdcopy: - Better page cache usage, see nbdkit-file-plugin options fadvise=sequential cache=none. - Consider io_uring if there are performance bottlenecks. + - Configurable retries in response to read or write failures. nbdfuse: - If you write beyond the end of the virtual file, it returns EIO. diff --git a/copy/Makefile.am b/copy/Makefile.am index f2100853..85989798 100644 --- a/copy/Makefile.am +++ b/copy/Makefile.am @@ -1,5 +1,5 @@ # nbd client library in userspace -# Copyright (C) 2020 Red Hat Inc. +# Copyright (C) 2020-2022 Red Hat Inc. # # This library is free software; you can redistribute it and/or # modify it under the terms of the GNU Lesser General Public @@ -33,6 +33,7 @@ EXTRA_DIST = \ copy-nbd-to-small-nbd-error.sh \ copy-nbd-to-sparse-file.sh \ copy-nbd-to-stdout.sh \ + copy-nbd-error.sh \ copy-progress-bar.sh \ copy-sparse.sh \ copy-sparse-allocated.sh \ @@ -124,6 +125,7 @@ TESTS += \ copy-stdin-to-nbd.sh \ copy-stdin-to-null.sh \ copy-nbd-to-stdout.sh \ + copy-nbd-error.sh \ copy-progress-bar.sh \ copy-sparse.sh \ copy-sparse-allocated.sh \ diff --git a/copy/copy-nbd-error.sh b/copy/copy-nbd-error.sh new file mode 100755 index 00000000..50723195 --- /dev/null +++ b/copy/copy-nbd-error.sh @@ -0,0 +1,54 @@ +#!/usr/bin/env bash +# nbd client library in userspace +# Copyright (C) 2022 Red Hat Inc. +# +# This library is free software; you can redistribute it and/or +# modify it under the terms of the GNU Lesser General Public +# License as published by the Free Software Foundation; either +# version 2 of the License, or (at your option) any later version. +# +# This library is distributed in the hope that it will be useful, +# but WITHOUT ANY WARRANTY; without even the implied warranty of +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU +# Lesser General Public License for more details. +# +# You should have received a copy of the GNU Lesser General Public +# License along with this library; if not, write to the Free Software +# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA + +. ../tests/functions.sh + +set -e +set -x + +requires nbdkit --exit-with-parent --version + +pidfile=copy-nbd-error.pid +sock=$(mktemp -u /tmp/libnbd-test-copy.XXXXXX) +cleanup_fn rm -f $pidfile $sock + +# Run an nbdkit server that randomly fails. +nbdkit --exit-with-parent -f -v -P $pidfile -U $sock \ + --filter=error --filter=noextents \ + memory size=5M error-pread-rate=0.5 error-pwrite-rate=0.5 &

+# Wait for the pidfile to appear. +for i in {1..60}; do + if test -f $pidfile; then + break + fi + sleep 1 +done +if ! test -f $pidfile; then + echo "$0: nbdkit did not start up" + exit 1 +fi + +fail=0 + +# Failure to read should be fatal +$VG nbdcopy -- "nbd+unix:///?socket=$sock" null: && fail=1 + +# Failure to write should be fatal +$VG nbdcopy -- [ nbdkit --exit-with-parent -v pattern 5M ] "nbd+unix:///?socket=$sock" && fail=1 + +exit $fail diff --git a/copy/file-ops.c b/copy/file-ops.c index 84704341..eb30f924 100644 --- a/copy/file-ops.c +++ b/copy/file-ops.c @@ -1,5 +1,5 @@ /* NBD client library in userspace. - * Copyright (C) 2020 Red Hat Inc. + * Copyright (C) 2020-2022 Red Hat Inc. * * This library is free software; you can redistribute it and/or * modify it under the terms of the GNU Lesser General Public @@ -589,11 +589,8 @@ file_asynch_read (struct rw *rw, { file_synch_read (rw, slice_ptr (command->slice), command->slice.len, command->offset); - errno = 0; - if (cb.callback (cb.user_data, &errno) == -1) { - perror (rw->name); - exit (EXIT_FAILURE); - } + errno = 0; /* safe, since file_synch_read exits on error */ + cb.callback (cb.user_data, &errno);

} static void @@ -603,11 +600,8 @@ file_asynch_write (struct rw *rw, { file_synch_write (rw, slice_ptr (command->slice), command->slice.len, command->offset); - errno = 0; - if (cb.callback (cb.user_data, &errno) == -1) { - perror (rw->name); - exit (EXIT_FAILURE); - } + errno = 0; /* safe, since file_synch_write exits on error */ + cb.callback (cb.user_data, &errno); } static bool @@ -617,10 +611,7 @@ file_asynch_zero (struct rw *rw, struct command *command, if (!file_synch_zero (rw, command->offset, command->slice.len, allocate)) return false; errno = 0; - if (cb.callback (cb.user_data, &errno) == -1) { - perror (rw->name); - exit (EXIT_FAILURE); - } + cb.callback (cb.user_data, &errno); return true; } diff --git a/copy/multi-thread-copying.c b/copy/multi-thread-copying.c index b17ca598..c578c4bc 100644 --- a/copy/multi-thread-copying.c +++ b/copy/multi-thread-copying.c @@ -1,5 +1,5 @@ /* NBD client library in userspace. - * Copyright (C) 2020 Red Hat Inc. + * Copyright (C) 2020-2022 Red Hat Inc. * * This library is free software; you can redistribute it and/or * modify it under the terms of the GNU Lesser General Public @@ -376,6 +376,13 @@ finished_read (void *vp, int *error) { struct command *command = vp; + /* XXX - is it worth retrying a failed command? */ + if (*error) { + errno = *error; + perror("read"); + exit (EXIT_FAILURE); + }

+ if (allocated || sparse_size == 0) { /* If sparseness detection (see below) is turned off then we write * the whole command. @@ -475,6 +482,7 @@ finished_read (void *vp, int *error) /* Free the original command since it has been split into * subcommands and the original is no longer needed. */ + errno = 0; free_command (command, &errno);

} @@ -537,6 +545,7 @@ fill_dst_range_with_zeroes (struct command *command) free (data); free_and_return: + errno = 0; free_command (command, &errno); } @@ -546,6 +555,13 @@ free_command (void *vp, int *error) struct command *command = vp; struct buffer *buffer = command->slice.buffer; + /* XXX - is it worth retrying a failed command? */ + if (*error) { + errno = *error; + perror("write"); + exit (EXIT_FAILURE); + } + if (buffer != NULL) { if (--buffer->refs == 0) { free (buffer->data); diff --git a/copy/nbdcopy.h b/copy/nbdcopy.h index e7fe1eab..c070f8d7 100644 --- a/copy/nbdcopy.h +++ b/copy/nbdcopy.h @@ -1,5 +1,5 @@ /* NBD client library in userspace. - * Copyright (C) 2020-2021 Red Hat Inc. + * Copyright (C) 2020-2022 Red Hat Inc. * * This library is free software; you can redistribute it and/or * modify it under the terms of the GNU Lesser General Public @@ -157,7 +157,8 @@ struct rw_ops { bool allocate); /* Asynchronous I/O operations. These start the operation and call - * 'cb' on completion. + * 'cb' on completion. 'cb' will return 1, for auto-retiring with + * asynchronous libnbd calls. * * The file_ops versions are actually implemented synchronously, but * still call 'cb'. @@ -173,7 +174,7 @@ struct rw_ops { nbd_completion_callback cb); /* Asynchronously zero. command->slice.buffer is not used. If not possible, - * returns false. + * returns false. 'cb' must be called only if returning true. */ bool (*asynch_zero) (struct rw *rw, struct command *command, nbd_completion_callback cb, bool allocate); diff --git a/copy/null-ops.c b/copy/null-ops.c index a38666d6..924eaf1e 100644 --- a/copy/null-ops.c +++ b/copy/null-ops.c @@ -1,5 +1,5 @@ /* NBD client library in userspace. - * Copyright (C) 2020-2021 Red Hat Inc. + * Copyright (C) 2020-2022 Red Hat Inc. * * This library is free software; you can redistribute it and/or * modify it under the terms of the GNU Lesser General Public @@ -118,10 +118,7 @@ null_asynch_write (struct rw *rw, nbd_completion_callback cb) { errno = 0; - if (cb.callback (cb.user_data, &errno) == -1) { - perror (rw->name); - exit (EXIT_FAILURE); - } + cb.callback (cb.user_data, &errno); } static bool @@ -129,10 +126,7 @@ null_asynch_zero (struct rw *rw, struct command *command, nbd_completion_callback cb, bool allocate) { errno = 0; - if (cb.callback (cb.user_data, &errno) == -1) { - perror (rw->name); - exit (EXIT_FAILURE); - } + cb.callback (cb.user_data, &errno); return true; } -- 2.34.1