Re: [PATCH v3 1/2] wifi: ath9k: fix races between ath9k_wmi_cmd and ath9k_wmi_ctrl_rx

["Toke Høiland-Jørgensen" <toke@toke.dk>]

Fedor Pchelkin <pchelkin@ispras.ru> writes:

> Currently, the synchronization between ath9k_wmi_cmd() and
> ath9k_wmi_ctrl_rx() is exposed to a race condition which, although being
> rather unlikely, can lead to invalid behaviour of ath9k_wmi_cmd().
>
> Consider the following scenario:
>
> CPU0					CPU1
>
> ath9k_wmi_cmd(...)
>   mutex_lock(&wmi->op_mutex)
>   ath9k_wmi_cmd_issue(...)
>   wait_for_completion_timeout(...)
>   ---
>   timeout
>   ---
> 					/* the callback is being processed
> 					 * before last_seq_id became zero
> 					 */
> 					ath9k_wmi_ctrl_rx(...)
> 					  spin_lock_irqsave(...)
> 					  /* wmi->last_seq_id check here
> 					   * doesn't detect timeout yet
> 					   */
> 					  spin_unlock_irqrestore(...)
>   /* last_seq_id is zeroed to
>    * indicate there was a timeout
>    */
>   wmi->last_seq_id = 0
>   mutex_unlock(&wmi->op_mutex)
>   return -ETIMEDOUT
>
> ath9k_wmi_cmd(...)
>   mutex_lock(&wmi->op_mutex)
>   /* the buffer is replaced with
>    * another one
>    */
>   wmi->cmd_rsp_buf = rsp_buf
>   wmi->cmd_rsp_len = rsp_len
>   ath9k_wmi_cmd_issue(...)
>     spin_lock_irqsave(...)
>     spin_unlock_irqrestore(...)
>   wait_for_completion_timeout(...)
> 					/* the continuation of the
> 					 * callback left after the first
> 					 * ath9k_wmi_cmd call
> 					 */
> 					  ath9k_wmi_rsp_callback(...)
> 					    /* copying data designated
> 					     * to already timeouted
> 					     * WMI command into an
> 					     * inappropriate wmi_cmd_buf
> 					     */
> 					    memcpy(...)
> 					    complete(&wmi->cmd_wait)
>   /* awakened by the bogus callback
>    * => invalid return result
>    */
>   mutex_unlock(&wmi->op_mutex)
>   return 0
>
> To fix this, update last_seq_id on timeout path inside ath9k_wmi_cmd()
> under the wmi_lock. Move ath9k_wmi_rsp_callback() under wmi_lock inside
> ath9k_wmi_ctrl_rx() so that the wmi->cmd_wait can be completed only for
> initially designated wmi_cmd call, otherwise the path would be rejected
> with last_seq_id check.
>
> Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
>
> Fixes: fb9987d0f748 ("ath9k_htc: Support for AR9271 chipset.")
> Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>

Alright, finally took the time to dig into this and convince myself that
the fix if correct. Sorry for taking so long!

Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>