RDMA/srpt: Add a check for valid 'mad_agent' pointer

Summary

In the Linux kernel, the following vulnerability has been resolved: RDMA/srpt: Add a check for valid 'mad_agent' pointer When unregistering MAD agent, srpt module has a non-null check for 'mad_agent' pointer before invoking ib_unregister_mad_agent(). This check can pass if 'mad_agent' variable holds an error value. The 'mad_agent' can have an error value for a short window when srpt_add_one() and srpt_remove_one() is executed simultaneously. In srpt module, added a valid pointer check for 'sport->mad_agent' before unregistering MAD agent. This issue can hit when RoCE driver unregisters ib_device Stack Trace: ------------ BUG: kernel NULL pointer dereference, address: 000000000000004d PGD 145003067 P4D 145003067 PUD 2324fe067 PMD 0 Oops: 0002 [#1] PREEMPT SMP NOPTI CPU: 10 PID: 4459 Comm: kworker/u80:0 Kdump: loaded Tainted: P Hardware name: Dell Inc. PowerEdge R640/06NR82, BIOS 2.5.4 01/13/2020 Workqueue: bnxt_re bnxt_re_task [bnxt_re] RIP: 0010:_raw_spin_lock_irqsave+0x19/0x40 Call Trace: ib_unregister_mad_agent+0x46/0x2f0 [ib_core] IPv6: ADDRCONF(NETDEV_CHANGE): bond0: link becomes ready ? __schedule+0x20b/0x560 srpt_unregister_mad_agent+0x93/0xd0 [ib_srpt] srpt_remove_one+0x20/0x150 [ib_srpt] remove_client_context+0x88/0xd0 [ib_core] bond0: (slave p2p1): link status definitely up, 100000 Mbps full duplex disable_device+0x8a/0x160 [ib_core] bond0: active interface up! ? kernfs_name_hash+0x12/0x80 (NULL device *): Bonding Info Received: rdev: 000000006c0b8247 __ib_unregister_device+0x42/0xb0 [ib_core] (NULL device *): Master: mode: 4 num_slaves:2 ib_unregister_device+0x22/0x30 [ib_core] (NULL device *): Slave: id: 105069936 name:p2p1 link:0 state:0 bnxt_re_stopqps_and_ib_uninit+0x83/0x90 [bnxt_re] bnxt_re_alloc_lag+0x12e/0x4e0 [bnxt_re]