[PATCH v2] virtio: console: Fix atomicity violation in fill_readbuf()

[Qiu-ji Chen <chenqiuji666@gmail.com>]

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 added a separate validity check for the variable 
buf after its assignment. This ensures that an invalid buf does not proceed
further into the program, thereby preventing a null pointer dereference 
error.

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>
---
V2: 
The logic of the fix has been modified.
---
 drivers/char/virtio_console.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index c62b208b42f1..54fee192d93c 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -660,6 +660,10 @@ static ssize_t fill_readbuf(struct port *port, u8 __user *out_buf,
 		return 0;
 
 	buf = port->inbuf;
+
+	if (!buf)
+		return 0;
+
 	out_count = min(out_count, buf->len - buf->offset);
 
 	if (to_user) {
-- 
2.34.1