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In the Linux kernel, the following vulnerability has been resolved: Bluetooth: L2CAP: refuse __l2cap_chan_add if chan already has conn l2cap_chan may be linked to l2cap_conn at most once. This is assumed in several places, eg l2cap_chan_del cleanup. There is a TOCTOU race where the invariant is violated: [Task 1] [Task 2] l2cap_chan_connect l2cap_sock_bind l2cap_chan_lock lock_sock l2cap_state_change if (sk->sk_state != BT_OPEN) chan->state = BT_CONNECT l2cap_sock_state_change_cb chan->state = BT_BOUND sk->sk_state = BT_BOUND lock_sock <------------------ release_sock sk->sk_state = BT_CONNECT l2cap_sock_connect() does not check sk->sk_state, so since chan->state is now BT_BOUND, subsequent connect() ends up with second __l2cap_chan_add. Explicitly document and check the invariant in __l2cap_chan_add with WARN_ON_ONCE. The only callsite where it could be hit is l2cap_chan_connect, so add pre-check there to avoid relying on chan->state. chan->state read/write is not properly guarded currently so there can be other TOCTOUC problems. Add l2cap_lock_chan in l2cap_sock_bind() to guard chan->state write.

• https://git.kernel.org/stable/c/85cc289eb08afa975375f838187369e68b429ee0
• https://git.kernel.org/stable/c/820579652b24ce277e0097cd65b42bd9b7b8bf07
• https://git.kernel.org/stable/c/e618d928ec7f88cbeadfa3542adcac40f474e629
• https://git.kernel.org/stable/c/fabaff1c59ff2cc89cb7e5bd0186a5e15f0c73b2
• https://git.kernel.org/stable/c/52fe88e4ef87311eee32b45c65603e7c69e75e7d
• https://git.kernel.org/stable/c/10c705dd706aa93164ba41f2566f15206d6c178e
• https://git.kernel.org/stable/c/73820561d8cbfd3364a29b8fa77f1707f26029da
• https://git.kernel.org/stable/c/d3afd7803f9bc2a8c4270199497b60158b2fc647
• https://git.kernel.org/stable/c/86b55991d3cd690e73c5ef8d20b75ae4905b357a