Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider: - [Critical] virtio_console: double free and UAF of port->inbuf in fill_readbuf() - [Medium] virtio_console: lock gap in unplug_port() leaks port->inbuf on read race --
commit a643b15852e7b766cdcc04cfb5d89a66c569bc43 Author: Yaohan Li <[email protected]> virtio_console: guard read-side virtqueue access during unplug This commit adds port->portdev checks under port->inbuf_lock in port_has_data() and fill_readbuf(), and clears port->portdev under port->inbuf_lock in unplug_port() to prevent read-side access to input virtqueues during unplug. > diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c > index 019bcae81af5c..e97e0daa34395 100644 > --- a/drivers/char/virtio_console.c > +++ b/drivers/char/virtio_console.c [ ... ] > @@ -725,6 +731,16 @@ static ssize_t fill_readbuf(struct port *port, u8 __user > *out_buf, > spin_lock_irqsave(&port->inbuf_lock, flags); > port->inbuf = NULL; > > + /* > + * The port may have been unplugged while we were copying > + * data out of the buffer: the vqs are then already gone. > + */ > + if (!port->portdev) { > + spin_unlock_irqrestore(&port->inbuf_lock, flags); > + free_buf(buf, true); > + return out_count; > + } [Severity: Critical] Can this free_buf(buf, true) call cause a double-free of buf when fill_readbuf() races with unplug_port()? In fill_readbuf(), port_has_data(port) acquires port->inbuf_lock, sets port->inbuf = get_inbuf(port), and drops port->inbuf_lock while leaving port->inbuf pointing to buf during the unlocked copy_to_user() or memcpy(): drivers/char/virtio_console.c:fill_readbuf() { if (!out_count || !port_has_data(port)) return 0; buf = port->inbuf; out_count = min(out_count, buf->len - buf->offset); if (to_user) { ssize_t ret; ret = copy_to_user(out_buf, buf->buf + buf->offset, out_count); ... } ... buf->offset += out_count; } If unplug_port() runs concurrently while fill_readbuf() is unlocked, it calls remove_port_data(port) -> discard_port_data(port) under port->inbuf_lock before setting port->portdev = NULL: drivers/char/virtio_console.c:discard_port_data() { if (!port->portdev) { /* Device has been unplugged. vqs are already gone. */ return; } buf = get_inbuf(port); err = 0; while (buf) { port->stats.bytes_discarded += buf->len - buf->offset; if (add_inbuf(port->in_vq, buf) < 0) { err++; free_buf(buf, false); } port->inbuf = NULL; buf = get_inbuf(port); } } Because port->inbuf still holds buf during the copy, get_inbuf(port) in discard_port_data() returns that same active buf and either frees it via free_buf(buf, false) (when port->in_vq is broken during device removal) or re-queues it to port->in_vq (where remove_vqs() detaches and frees it), while copy_to_user() is still reading from buf->buf or before fill_readbuf() loads buf = port->inbuf (which is now NULL). When fill_readbuf() finishes copying the buffer, it re-acquires port->inbuf_lock, sees !port->portdev, and calls free_buf(buf, true) on the same buf a second time. > @@ -1568,6 +1584,18 @@ static void unplug_port(struct port *port) > * a close on an open port later from sending a stale control > * message. > */ > + /* > + * Null out portdev under inbuf_lock as well, so the read path > + * (port_has_data(), fill_readbuf()) cannot race with the vq > + * teardown: it checks port->portdev inside the same lock and > + * bails out if NULL, preventing any virtqueue access to an > + * already torn-down virtqueue — mirroring the outvq_lock fence > + * below for the write path. > + */ > + spin_lock_irq(&port->inbuf_lock); > + port->portdev = NULL; > + spin_unlock_irq(&port->inbuf_lock); [Severity: Medium] Does clearing port->portdev in a separate lock acquisition after remove_port_data(port) leave a race window that can leak port->inbuf? In unplug_port(), remove_port_data(port) acquires port->inbuf_lock, runs discard_port_data(port), and drops port->inbuf_lock before unplug_port() re-acquires port->inbuf_lock here to set port->portdev = NULL: drivers/char/virtio_console.c:remove_port_data() { spin_lock_irq(&port->inbuf_lock); /* Remove unused data this port might have received. */ discard_port_data(port); spin_unlock_irq(&port->inbuf_lock); spin_lock_irq(&port->outvq_lock); reclaim_consumed_buffers(port); spin_unlock_irq(&port->outvq_lock); } If a concurrent reader calls port_has_data(port) in the gap after discard_port_data(port) unlocks port->inbuf_lock and before port->portdev is set to NULL, port_has_data(port) sees port->portdev != NULL and detaches a buffer from port->in_vq into port->inbuf. Once port->portdev is set to NULL, if port_fops_read() exits on !port->guest_connected or fill_readbuf() only partially consumes the buffer (or fails with -EFAULT), that detached buffer remains in port->inbuf and discard_port_data() in port_fops_release() will bail out on !port->portdev without freeing it. -- Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1
