[PATCH 2/2] media: pci: saa7164: fix slab-use-after-free in saa7164_dvb_register

[xiaopeitux@foxmail.com]

From: Pei Xiao <xiaopei01@kylinos.cn>

A use-after-free (UAF) issue similar to the one in au0828 was found in
the saa7164 DVB driver. When dvb_register_frontend() fails before the
frontend's kref is initialized (e.g., due to -ENOMEM in kzalloc of
fe->frontend_priv), the error path still calls dvb_frontend_detach(),
which eventually invokes dvb_frontend_put(), immediately frees the frontend
via dvb_frontend_free().

Later, the outer error path in saa7164_dvb_register() again calls
dvb->frontend->ops.release() on the already freed frontend, leading
to a double free and a KASAN slab-use-after-free report.

The call chain is:

  saa7164_dvb_register()
    -> dvb_register(port)
      -> dvb_register_frontend()
          (fails before kref_init, e.g., -ENOMEM)
      -> fail_frontend:
        -> dvb_frontend_detach()
          -> dvb_frontend_put()
                -> __dvb_frontend_free()
                  -> dvb_frontend_invoke_release(fe, fe->ops.release)
                    -> (frontend-specific release, frees the object)
    (back to saa7164_dvb_register)
    if (ret < 0) {
        if (dvb->frontend->ops.release)
            dvb->frontend->ops.release(dvb->frontend);  // USE-AFTER-FREE
    }

The second release is redundant and unsafe because the frontend has
already been freed by the first release via dvb_frontend_put.

Fix this by removing the redundant manual release call in the outer
error path. The frontend is already destroyed by dvb_frontend_put()
in the inner error path, so we only need to set dvb->frontend to NULL
to avoid any further misuse.

Modified by referring to au0828 UAF.

Fixes: ead666000a5f ("media: dvb_frontend: only use kref after initialized")
Reported-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
Closes: https://lore.kernel.org/lkml/AAC17221-93BF-436F-92AC-906A18F882F0@gmail.com/
Cc: stable@vger.kernel.org
Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
---
 drivers/media/pci/saa7164/saa7164-dvb.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/media/pci/saa7164/saa7164-dvb.c b/drivers/media/pci/saa7164/saa7164-dvb.c
index 3eb749db1ca7..ba6e83b360ac 100644
--- a/drivers/media/pci/saa7164/saa7164-dvb.c
+++ b/drivers/media/pci/saa7164/saa7164-dvb.c
@@ -727,8 +727,10 @@ int saa7164_dvb_register(struct saa7164_port *port)
 	/* register everything */
 	ret = dvb_register(port);
 	if (ret < 0) {
-		if (dvb->frontend->ops.release)
-			dvb->frontend->ops.release(dvb->frontend);
+		/* dvb->frontend freed in dvb_frontend_put, now we can set
+		 * frontend to NULL
+		 */
+		dvb->frontend = NULL;
 		return ret;
 	}
 
-- 
2.25.1