The atomicity violation issue is due to the invalidation of the function port_has_data()'s check caused by concurrency. Imagine a scenario where a port that contains data passes the validity check but is simultaneously assigned a value with no data. This could result in an empty port passing the validity check, potentially leading to a null pointer dereference error later in the program, which is inconsistent.
To address this issue, we believe there is a problem with the original logic. Since the validity check and the read operation were separated, it could lead to inconsistencies between the data that passes the check and the data being read. Therefore, we moved the main logic of the port_has_data function into this function and placed the read operation within a lock, ensuring that the validity check and read operation are not separated, thus resolving the problem.
This possible bug is found by an experimental static analysis tool developed by our team. This tool analyzes the locking APIs to extract function pairs that can be concurrently executed, and then analyzes the instructions in the paired functions to identify possible concurrency bugs including data races and atomicity violations.
Fixes: 203baab8ba31 ("virtio: console: Introduce function to hand off data from host to readers") Cc: stable@vger.kernel.org Signed-off-by: Qiu-ji Chen chenqiuji666@gmail.com --- drivers/char/virtio_console.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c index de7d720d99fa..5aaf07f71a4c 100644 --- a/drivers/char/virtio_console.c +++ b/drivers/char/virtio_console.c @@ -656,10 +656,14 @@ static ssize_t fill_readbuf(struct port *port, u8 __user *out_buf, struct port_buffer *buf; unsigned long flags;
- if (!out_count || !port_has_data(port)) + if (!out_count) return 0;
- buf = port->inbuf; + spin_lock_irqsave(&port->inbuf_lock, flags); + buf = port->inbuf = get_inbuf(port); + spin_unlock_irqrestore(&port->inbuf_lock, flags); + if (!buf) + return 0; out_count = min(out_count, buf->len - buf->offset);
if (to_user) {