IB/ipoib: Fix mcast list locking

In the Linux kernel, the following vulnerability has been resolved:

IB/ipoib: Fix mcast list locking

Releasing the `priv->lock` while iterating the `priv->multicast_list` in

`ipoib_mcast_join_task()` opens a window for `ipoib_mcast_dev_flush()` to

remove the items while in the middle of iteration. If the mcast is removed

while the lock was dropped, the for loop spins forever resulting in a hard

lockup (as was reported on RHEL 4.18.0-372.75.1.el8_6 kernel):

Task A (kworker/u72:2 below) | Task B (kworker/u72:0 below)

-----------------------------------+-----------------------------------

ipoib_mcast_join_task(work) | ipoib_ib_dev_flush_light(work)

spin_lock_irq(&priv->lock) | __ipoib_ib_dev_flush(priv, ...)

list_for_each_entry(mcast, | ipoib_mcast_dev_flush(dev = priv->dev)

&priv->multicast_list, list) |

ipoib_mcast_join(dev, mcast) |

spin_unlock_irq(&priv->lock) |

| spin_lock_irqsave(&priv->lock, flags)

| list_for_each_entry_safe(mcast, tmcast,

| &priv->multicast_list, list)

| list_del(&mcast->list);

| list_add_tail(&mcast->list, &remove_list)

| spin_unlock_irqrestore(&priv->lock, flags)

spin_lock_irq(&priv->lock) |

| ipoib_mcast_remove_list(&remove_list)

(Here, `mcast` is no longer on the | list_for_each_entry_safe(mcast, tmcast,

`priv->multicast_list` and we keep | remove_list, list)

spinning on the `remove_list` of | >>> wait_for_completion(&mcast->done)

the other thread which is blocked |

and the list is still valid on |

it's stack.)

Fix this by keeping the lock held and changing to GFP_ATOMIC to prevent

eventual sleeps.

Unfortunately we could not reproduce the lockup and confirm this fix but

based on the code review I think this fix should address such lockups.

crash> bc 31

PID: 747 TASK: ff1c6a1a007e8000 CPU: 31 COMMAND: "kworker/u72:2"

--

[exception RIP: ipoib_mcast_join_task+0x1b1]

RIP: ffffffffc0944ac1 RSP: ff646f199a8c7e00 RFLAGS: 00000002

RAX: 0000000000000000 RBX: ff1c6a1a04dc82f8 RCX: 0000000000000000

work (&priv->mcast_task{,.work})

RDX: ff1c6a192d60ac68 RSI: 0000000000000286 RDI: ff1c6a1a04dc8000

&mcast->list

RBP: ff646f199a8c7e90 R8: ff1c699980019420 R9: ff1c6a1920c9a000

R10: ff646f199a8c7e00 R11: ff1c6a191a7d9800 R12: ff1c6a192d60ac00

mcast

R13: ff1c6a1d82200000 R14: ff1c6a1a04dc8000 R15: ff1c6a1a04dc82d8

dev priv (&priv->lock) &priv->multicast_list (aka head)

ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018

--- <NMI exception stack> ---

#5 [ff646f199a8c7e00] ipoib_mcast_join_task+0x1b1 at ffffffffc0944ac1 [ib_ipoib]

#6 [ff646f199a8c7e98] process_one_work+0x1a7 at ffffffff9bf10967

crash> rx ff646f199a8c7e68

ff646f199a8c7e68: ff1c6a1a04dc82f8 <<< work = &priv->mcast_task.work

crash> list -hO ipoib_dev_priv.multicast_list ff1c6a1a04dc8000

(empty)

crash> ipoib_dev_priv.mcast_task.work.func,mcast_mutex.owner.counter ff1c6a1a04dc8000

mcast_task.work.func = 0xffffffffc0944910 <ipoib_mcast_join_task>,

mcast_mutex.owner.counter = 0xff1c69998efec000

crash> b 8

PID: 8 TASK: ff1c69998efec000 CPU: 33 COMMAND: "kworker/u72:0"

--

#3 [ff646f1980153d50] wait_for_completion+0x96 at ffffffff9c7d7646

#4 [ff646f1980153d90] ipoib_mcast_remove_list+0x56 at ffffffffc0944dc6 [ib_ipoib]

#5 [ff646f1980153de8] ipoib_mcast_dev_flush+0x1a7 at ffffffffc09455a7 [ib_ipoib]

#6 [ff646f1980153e58] __ipoib_ib_dev_flush+0x1a4 at ffffffffc09431a4 [ib_ipoib]

#7 [ff

---truncated---