From: Sungho Bae <[email protected]> With no_console_suspend enabled, hvc console output can continue while virtio_console is freezing. In that window, put_chars can still enqueue buffers to the output virtqueue while virtcons_freeze is tearing queues down.
This races with virtqueue_detach_unused_buf_split, which expects all descriptors to be reclaimed during teardown and can hit: BUG_ON(vq->vq.num_free != vq->split.vring.num) Root cause is concurrent virtqueue_add_outbuf activity from the console TX path during freeze. Add a pm_freezing flag in ports_device and set it at the beginning of virtcons_freeze. Check the flag in put_chars and __send_to_port to drop output while freeze is in progress, preventing new buffer enqueue before vq teardown. Clear the flag on restore only after all port->out_vq pointers have been reassigned to the newly allocated virtqueues, which prevents a use-after-free window where TX paths could dereference already-freed virtqueues. Place the pm_freezing check inside outvq_lock in __send_to_port so that it serializes with remove_port_data, which also takes outvq_lock during freeze. This provides a formal guarantee: once remove_port_data returns for a given port, no concurrent __send_to_port call can still be adding buffers to the vq, eliminating the race window with remove_vqs. Use smp_store_release/smp_load_acquire for the flag to guarantee correct cross-CPU visibility on weakly ordered architectures. Signed-off-by: Sungho Bae <[email protected]> --- drivers/char/virtio_console.c | 46 +++++++++++++++++++++++++++++++++++ 1 file changed, 46 insertions(+) diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c index 9a33217c68d9..0fbe40a7f1e7 100644 --- a/drivers/char/virtio_console.c +++ b/drivers/char/virtio_console.c @@ -157,6 +157,12 @@ struct ports_device { /* Major number for this device. Ports will be created as minors. */ int chr_major; + + /* + * Set to true during PM freeze to block TX paths that may race + * with virtqueue teardown (e.g. hvc put_chars with no_console_suspend). + */ + bool pm_freezing; }; struct port_stats { @@ -606,6 +612,20 @@ static ssize_t __send_to_port(struct port *port, struct scatterlist *sg, spin_lock_irqsave(&port->outvq_lock, flags); + /* + * Check freeze flag under the lock so that the flag check and + * virtqueue_add_outbuf() are atomic with respect to + * remove_port_data() which also takes outvq_lock. This + * guarantees that once remove_port_data() returns, no new + * buffers can be added before remove_vqs() tears down the vq. + * Pairs with smp_store_release() in virtcons_freeze/restore. + */ + if (!port->portdev || + smp_load_acquire(&port->portdev->pm_freezing)) { /* pairs with freeze/restore */ + in_count = 0; + goto done; + } + reclaim_consumed_buffers(port); err = virtqueue_add_outbuf(out_vq, sg, nents, data, GFP_ATOMIC); @@ -1115,6 +1135,17 @@ static ssize_t put_chars(u32 vtermno, const u8 *buf, size_t count) if (!port) return -EPIPE; + /* + * Silently drop output during freeze: the console is still active + * with no_console_suspend but the virtqueues are being torn down. + * This early check avoids a pointless GFP_ATOMIC allocation; + * __send_to_port() rechecks under outvq_lock for correctness. + * Pairs with smp_store_release() in virtcons_freeze/restore. + */ + if (!port->portdev || + smp_load_acquire(&port->portdev->pm_freezing)) /* pairs with freeze/restore */ + return count; + data = kmemdup(buf, count, GFP_ATOMIC); if (!data) return -ENOMEM; @@ -1972,6 +2003,7 @@ static int virtcons_probe(struct virtio_device *vdev) /* Attach this portdev to this virtio_device, and vice-versa. */ portdev->vdev = vdev; vdev->priv = portdev; + portdev->pm_freezing = false; portdev->chr_major = register_chrdev(0, "virtio-portsdev", &portdev_fops); @@ -2092,6 +2124,13 @@ static int virtcons_freeze(struct virtio_device *vdev) portdev = vdev->priv; + /* + * Block TX paths (put_chars, __send_to_port) before resetting the + * device and tearing down virtqueues. This prevents races with + * hvc console writes that remain active under no_console_suspend. + */ + smp_store_release(&portdev->pm_freezing, true); + virtio_reset_device(vdev); if (use_multiport(portdev)) @@ -2153,6 +2192,13 @@ static int virtcons_restore(struct virtio_device *vdev) if (port->guest_connected) send_control_msg(port, VIRTIO_CONSOLE_PORT_OPEN, 1); } + + /* + * Allow TX paths only after all port->out_vq pointers have + * been reassigned to the newly allocated virtqueues. + */ + smp_store_release(&portdev->pm_freezing, false); + return 0; } #endif -- 2.34.1
