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Berrange" Subject: [PULL 4/8] migration: Fix race on qemu_file_shutdown() Date: Mon, 21 Nov 2022 13:59:03 +0100 Message-Id: <20221121125907.62469-5-quintela@redhat.com> In-Reply-To: <20221121125907.62469-1-quintela@redhat.com> References: <20221121125907.62469-1-quintela@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Received-SPF: pass client-ip=170.10.129.124; envelope-from=quintela@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org From: Peter Xu In qemu_file_shutdown(), there's a possible race if with current order of operation. There're two major things to do: (1) Do real shutdown() (e.g. shutdown() syscall on socket) (2) Update qemufile's last_error We must do (2) before (1) otherwise there can be a race condition like: page receiver other thread ------------- ------------ qemu_get_buffer() do shutdown() returns 0 (buffer all zero) (meanwhile we didn't check this retcode) try to detect IO error last_error==NULL, IO okay install ALL-ZERO page set last_error --> guest crash! To fix this, we can also check retval of qemu_get_buffer(), but not all APIs can be properly checked and ultimately we still need to go back to qemu_file_get_error(). E.g. qemu_get_byte() doesn't return error. Maybe some day a rework of qemufile API is really needed, but for now keep using qemu_file_get_error() and fix it by not allowing that race condition to happen. Here shutdown() is indeed special because the last_error was emulated. For real -EIO errors it'll always be set when e.g. sendmsg() error triggers so we won't miss those ones, only shutdown() is a bit tricky here. Cc: Daniel P. Berrange Reviewed-by: Dr. David Alan Gilbert Signed-off-by: Peter Xu Reviewed-by: Juan Quintela Signed-off-by: Juan Quintela --- migration/qemu-file.c | 27 ++++++++++++++++++++++++--- 1 file changed, 24 insertions(+), 3 deletions(-) diff --git a/migration/qemu-file.c b/migration/qemu-file.c index 4f400c2e52..2d5f74ffc2 100644 --- a/migration/qemu-file.c +++ b/migration/qemu-file.c @@ -79,6 +79,30 @@ int qemu_file_shutdown(QEMUFile *f) int ret = 0; f->shutdown = true; + + /* + * We must set qemufile error before the real shutdown(), otherwise + * there can be a race window where we thought IO all went though + * (because last_error==NULL) but actually IO has already stopped. + * + * If without correct ordering, the race can happen like this: + * + * page receiver other thread + * ------------- ------------ + * qemu_get_buffer() + * do shutdown() + * returns 0 (buffer all zero) + * (we didn't check this retcode) + * try to detect IO error + * last_error==NULL, IO okay + * install ALL-ZERO page + * set last_error + * --> guest crash! + */ + if (!f->last_error) { + qemu_file_set_error(f, -EIO); + } + if (!qio_channel_has_feature(f->ioc, QIO_CHANNEL_FEATURE_SHUTDOWN)) { return -ENOSYS; @@ -88,9 +112,6 @@ int qemu_file_shutdown(QEMUFile *f) ret = -EIO; } - if (!f->last_error) { - qemu_file_set_error(f, -EIO); - } return ret; } -- 2.38.1