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

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]