[PATCH] media: vidtv: fix nfeeds state corruption on start_streaming failure

[Ruslan Valiyev <linuxoid@gmail.com>]

syzbot reported a memory leak in vidtv_psi_service_desc_init [1].

When vidtv_start_streaming() fails inside vidtv_start_feed(), the
nfeeds counter is left incremented even though no feed was actually
started. This corrupts the driver state: subsequent start_feed calls
see nfeeds > 1 and skip starting the mux, while stop_feed calls
eventually try to stop a non-existent stream.

This state corruption can also lead to memory leaks, since the mux
and channel resources may be partially allocated during a failed
start_streaming but never cleaned up, as the stop path finds
dvb->streaming == false and returns early.

Fix by decrementing nfeeds back when start_streaming fails, keeping
the counter in sync with the actual number of active feeds.

[1]
BUG: memory leak
unreferenced object 0xffff888145b50820 (size 32):
 comm "syz.0.17", pid 6068, jiffies 4294944486
 backtrace (crc 90a0c7d4):
  vidtv_psi_service_desc_init+0x74/0x1b0 drivers/media/test-drivers/vidtv/vidtv_psi.c:288
  vidtv_channel_s302m_init+0xb1/0x2a0 drivers/media/test-drivers/vidtv/vidtv_channel.c:83
  vidtv_channels_init+0x1b/0x40 drivers/media/test-drivers/vidtv/vidtv_channel.c:524
  vidtv_mux_init+0x516/0xbe0 drivers/media/test-drivers/vidtv/vidtv_mux.c:518
  vidtv_start_streaming drivers/media/test-drivers/vidtv/vidtv_bridge.c:194 [inline]
  vidtv_start_feed+0x33e/0x4d0 drivers/media/test-drivers/vidtv/vidtv_bridge.c:239

Fixes: f90cf6079bf67 ("media: vidtv: add a bridge driver")
Cc: stable@vger.kernel.org
Reported-by: syzbot+639ebc6ec75e96674741@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=639ebc6ec75e96674741
Signed-off-by: Ruslan Valiyev <linuxoid@gmail.com>
---
 drivers/media/test-drivers/vidtv/vidtv_bridge.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/media/test-drivers/vidtv/vidtv_bridge.c b/drivers/media/test-drivers/vidtv/vidtv_bridge.c
index b6203e10e37aa..a8a76434989c2 100644
--- a/drivers/media/test-drivers/vidtv/vidtv_bridge.c
+++ b/drivers/media/test-drivers/vidtv/vidtv_bridge.c
@@ -237,8 +237,10 @@ static int vidtv_start_feed(struct dvb_demux_feed *feed)
 
 	if (dvb->nfeeds == 1) {
 		ret = vidtv_start_streaming(dvb);
-		if (ret < 0)
+		if (ret < 0) {
+			dvb->nfeeds--;
 			rc = ret;
+		}
 	}
 
 	mutex_unlock(&dvb->feed_lock);
-- 
2.43.0