RDMA/core: Don't expose hw_counters outside of init net namespace

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

RDMA/core: Don't expose hw_counters outside of init net namespace

Commit 467f432a521a ("RDMA/core: Split port and device counter sysfs
attributes") accidentally almost exposed hw counters to non-init net
namespaces. It didn't expose them fully, as an attempt to read any of
those counters leads to a crash like this one:

[42021.807566] BUG: kernel NULL pointer dereference, address: 0000000000000028
[42021.814463] #PF: supervisor read access in kernel mode
[42021.819549] #PF: error_code(0x0000) - not-present page
[42021.824636] PGD 0 P4D 0
[42021.827145] Oops: 0000 [#1] SMP PTI
[42021.830598] CPU: 82 PID: 2843922 Comm: switchto-defaul Kdump: loaded Tainted: G S W I XXX
[42021.841697] Hardware name: XXX
[42021.849619] RIP: 0010:hw_stat_device_show+0x1e/0x40 [ib_core]
[42021.855362] Code: 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 49 89 d0 4c 8b 5e 20 48 8b 8f b8 04 00 00 48 81 c7 f0 fa ff ff <48> 8b 41 28 48 29 ce 48 83 c6 d0 48 c1 ee 04 69 d6 ab aa aa aa 48
[42021.873931] RSP: 0018:ffff97fe90f03da0 EFLAGS: 00010287
[42021.879108] RAX: ffff9406988a8c60 RBX: ffff940e1072d438 RCX: 0000000000000000
[42021.886169] RDX: ffff94085f1aa000 RSI: ffff93c6cbbdbcb0 RDI: ffff940c7517aef0
[42021.893230] RBP: ffff97fe90f03e70 R08: ffff94085f1aa000 R09: 0000000000000000
[42021.900294] R10: ffff94085f1aa000 R11: ffffffffc0775680 R12: ffffffff87ca2530
[42021.907355] R13: ffff940651602840 R14: ffff93c6cbbdbcb0 R15: ffff94085f1aa000
[42021.914418] FS: 00007fda1a3b9700(0000) GS:ffff94453fb80000(0000) knlGS:0000000000000000
[42021.922423] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[42021.928130] CR2: 0000000000000028 CR3: 00000042dcfb8003 CR4: 00000000003726f0
[42021.935194] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[42021.942257] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[42021.949324] Call Trace:
[42021.951756] <TASK>
[42021.953842] [<ffffffff86c58674>] ? show_regs+0x64/0x70
[42021.959030] [<ffffffff86c58468>] ? __die+0x78/0xc0
[42021.963874] [<ffffffff86c9ef75>] ? page_fault_oops+0x2b5/0x3b0
[42021.969749] [<ffffffff87674b92>] ? exc_page_fault+0x1a2/0x3c0
[42021.975549] [<ffffffff87801326>] ? asm_exc_page_fault+0x26/0x30
[42021.981517] [<ffffffffc0775680>] ? __pfx_show_hw_stats+0x10/0x10 [ib_core]
[42021.988482] [<ffffffffc077564e>] ? hw_stat_device_show+0x1e/0x40 [ib_core]
[42021.995438] [<ffffffff86ac7f8e>] dev_attr_show+0x1e/0x50
[42022.000803] [<ffffffff86a3eeb1>] sysfs_kf_seq_show+0x81/0xe0
[42022.006508] [<ffffffff86a11134>] seq_read_iter+0xf4/0x410
[42022.011954] [<ffffffff869f4b2e>] vfs_read+0x16e/0x2f0
[42022.017058] [<ffffffff869f50ee>] ksys_read+0x6e/0xe0
[42022.022073] [<ffffffff8766f1ca>] do_syscall_64+0x6a/0xa0
[42022.027441] [<ffffffff8780013b>] entry_SYSCALL_64_after_hwframe+0x78/0xe2

The problem can be reproduced using the following steps:
ip netns add foo
ip netns exec foo bash
cat /sys/class/infiniband/mlx4_0/hw_counters/*

The panic occurs because of casting the device pointer into an
ib_device pointer using container_of() in hw_stat_device_show() is
wrong and leads to a memory corruption.

However the real problem is that hw counters should never been exposed
outside of the non-init net namespace.

Fix this by saving the index of the corresponding attribute group
(it might be 1 or 2 depending on the presence of driver-specific
attributes) and zeroing the pointer to hw_counters group for compat
devices during the initialization.

With this fix applied hw_counters are not available in a non-init
net namespace:
find /sys/class/infiniband/mlx4_0/ -name hw_counters
/sys/class/infiniband/mlx4_0/ports/1/hw_counters
/sys/class/infiniband/mlx4_0/ports/2/hw_counters
/sys/class/infiniband/mlx4_0/hw_counters

ip netns add foo
ip netns exec foo bash
find /sys/class/infiniband/mlx4_0/ -name hw_counters