On Tue, Mar 08, 2022 at 10:57:42AM +0100, Greg KH wrote:
On Tue, Mar 08, 2022 at 09:15:27AM +0000, Lee Jones wrote:
On Tue, 08 Mar 2022, Greg KH wrote:
On Tue, Mar 08, 2022 at 08:10:06AM +0000, Lee Jones wrote:
On Mon, 07 Mar 2022, Greg KH wrote:
On Mon, Mar 07, 2022 at 07:17:57PM +0000, Lee Jones wrote:
vhost_vsock_handle_tx_kick() already holds the mutex during its call to vhost_get_vq_desc(). All we have to do here is take the same lock during virtqueue clean-up and we mitigate the reported issues.
Also WARN() as a precautionary measure. The purpose of this is to capture possible future race conditions which may pop up over time.
Link: https://syzkaller.appspot.com/bug?extid=279432d30d825e63ba00
Cc: stable@vger.kernel.org Reported-by: syzbot+adc3cb32385586bec859@syzkaller.appspotmail.com Signed-off-by: Lee Jones lee.jones@linaro.org
drivers/vhost/vhost.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 59edb5a1ffe28..ef7e371e3e649 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -693,6 +693,15 @@ void vhost_dev_cleanup(struct vhost_dev *dev) int i; for (i = 0; i < dev->nvqs; ++i) {
/* No workers should run here by design. However, races have* previously occurred where drivers have been unable to flush* all work properly prior to clean-up. Without a successful* flush the guest will malfunction, but avoiding host memory* corruption in those cases does seem preferable.*/WARN_ON(mutex_is_locked(&dev->vqs[i]->mutex));So you are trading one syzbot triggered issue for another one in the future? :)
If this ever can happen, handle it, but don't log it with a WARN_ON() as that will trigger the panic-on-warn boxes, as well as syzbot. Unless you want that to happen?
No, Syzbot doesn't report warnings, only BUGs and memory corruption.
Has it changed? Last I looked, it did trigger on WARN_* calls, which has resulted in a huge number of kernel fixes because of that.
Everything is customisable in syzkaller, so maybe there are specific builds which panic_on_warn enabled, but none that I'm involved with do.
Many systems run with panic-on-warn (i.e. the cloud), as they want to drop a box and restart it if anything goes wrong.
That's why syzbot reports on WARN_* calls. They should never be reachable by userspace actions.
Here follows a topical example. The report above in the Link: tag comes with a crashlog [0]. In there you can see the WARN() at the bottom of vhost_dev_cleanup() trigger many times due to a populated (non-flushed) worker list, before finally tripping the BUG() which triggers the report:
[0] https://syzkaller.appspot.com/text?tag=CrashLog&x=16a61fce700000
Ok, so both happens here. But don't add a warning for something that can't happen. Just handle it and move on. It looks like you are handling it in this code, so please drop the WARN_ON().
thanks,
greg k-h
Hmm. Well this will mean if we ever reintroduce the bug then syzkaller will not catch it for us :( And the bug is there, it just results in a hard to reproduce error for userspace.
Not sure what to do here. Export panic_on_warn flag to modules and check it here?