From 3bb59da6fdb3dcc6aa1634786488250312a99cd2 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Tue, 4 Oct 2022 14:24:27 -0400 Subject: [PATCH] migration: Fix race on qemu_file_shutdown() 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 4f400c2e52653..2d5f74ffc2269 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; }