[PATCH net-next v4 0/2] nfc: Fix UAF during datagram sending caused by missing refcounting

[Siddh Raman Pant <code@siddh.me>]

(Very sorry for the delay in sending this.)

For connectionless transmission, llcp_sock_sendmsg() codepath will
eventually call nfc_alloc_send_skb() which takes in an nfc_dev as
an argument for calculating the total size for skb allocation.

virtual_ncidev_close() codepath eventually releases socket by calling
nfc_llcp_socket_release() (which sets the sk->sk_state to LLCP_CLOSED)
and afterwards the nfc_dev will be eventually freed.

When an ndev gets freed, llcp_sock_sendmsg() will result in an
use-after-free as it

(1) doesn't have any checks in place for avoiding the datagram sending.

(2) calls nfc_llcp_send_ui_frame(), which also has a do-while loop
    which can race with freeing. This loop contains the call to
    nfc_alloc_send_skb() where we dereference the nfc_dev pointer.

nfc_dev is being freed because we do not hold a reference to it when
we hold a reference to llcp_local. Thus, virtual_ncidev_close()
eventually calls nfc_release() due to refcount going to 0.

Since state has to be LLCP_BOUND for datagram sending, we can bail out
early in llcp_sock_sendmsg().

Please review and let me know if any errors are there, and hopefully
this gets accepted.

Thanks,
Siddh

Changes in v4:
- Fix put ordering and comments.
- Separate freeing in recv() into end labels.
- Remove obvious comment and add reasoning.
- Picked up r-bs by Suman.

Changes in v3:
- Fix missing freeing statements.

Changes in v2:
- Add net-next in patch subject.
- Removed unnecessary extra lock and hold nfc_dev ref when holding llcp_sock.
- Remove last formatting patch.
- Picked up r-b from Krzysztof for LLCP_BOUND patch.

Siddh Raman Pant (2):
  nfc: llcp_core: Hold a ref to llcp_local->dev when holding a ref to
    llcp_local
  nfc: Do not send datagram if socket state isn't LLCP_BOUND

 net/nfc/llcp_core.c | 55 ++++++++++++++++++++++++++++++++++-----------
 net/nfc/llcp_sock.c |  5 +++++
 2 files changed, 47 insertions(+), 13 deletions(-)

-- 
2.42.0