Describe CVE-2021-47127.
What is CVE-2021-47127?
In the Linux kernel, the following vulnerability has been resolved:

ice: track AF_XDP ZC enabled queues in bitmap

Commit c7a219048e45 ("ice: Remove xsk_buff_pool from VSI structure")
silently introduced a regression and broke the Tx side of AF_XDP in copy
mode. xsk_pool on ice_ring is set only based on the existence of the XDP
prog on the VSI which in turn picks ice_clean_tx_irq_zc to be executed.
That is not something that should happen for copy mode as it should use
the regular data path ice_clean_tx_irq.

This results in a following splat when xdpsock is run in txonly or l2fwd
scenarios in copy mode:

<snip>
[  106.050195] BUG: kernel NULL pointer dereference, address: 0000000000000030
[  106.057269] #PF: supervisor read access in kernel mode
[  106.062493] #PF: error_code(0x0000) - not-present page
[  106.067709] PGD 0 P4D 0
[  106.070293] Oops: 0000 [#1] PREEMPT SMP NOPTI
[  106.074721] CPU: 61 PID: 0 Comm: swapper/61 Not tainted 5.12.0-rc2+ #45
[  106.081436] Hardware name: Intel Corporation S2600WFT/S2600WFT, BIOS SE5C620.86B.02.01.0008.031920191559 03/19/2019
[  106.092027] RIP: 0010:xp_raw_get_dma+0x36/0x50
[  106.096551] Code: 74 14 48 b8 ff ff ff ff ff ff 00 00 48 21 f0 48 c1 ee 30 48 01 c6 48 8b 87 90 00 00 00 48 89 f2 81 e6 ff 0f 00 00 48 c1 ea 0c <48> 8b 04 d0 48 83 e0 fe 48 01 f0 c3 66 66 2e 0f 1f 84 00 00 00 00
[  106.115588] RSP: 0018:ffffc9000d694e50 EFLAGS: 00010206
[  106.120893] RAX: 0000000000000000 RBX: ffff88984b8c8a00 RCX: ffff889852581800
[  106.128137] RDX: 0000000000000006 RSI: 0000000000000000 RDI: ffff88984cd8b800
[  106.135383] RBP: ffff888123b50001 R08: ffff889896800000 R09: 0000000000000800
[  106.142628] R10: 0000000000000000 R11: ffffffff826060c0 R12: 00000000000000ff
[  106.149872] R13: 0000000000000000 R14: 0000000000000040 R15: ffff888123b50018
[  106.157117] FS:  0000000000000000(0000) GS:ffff8897e0f40000(0000) knlGS:0000000000000000
[  106.165332] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  106.171163] CR2: 0000000000000030 CR3: 000000000560a004 CR4: 00000000007706e0
[  106.178408] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  106.185653] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  106.192898] PKRU: 55555554
[  106.195653] Call Trace:
[  106.198143]  <IRQ>
[  106.200196]  ice_clean_tx_irq_zc+0x183/0x2a0 [ice]
[  106.205087]  ice_napi_poll+0x3e/0x590 [ice]
[  106.209356]  __napi_poll+0x2a/0x160
[  106.212911]  net_rx_action+0xd6/0x200
[  106.216634]  __do_softirq+0xbf/0x29b
[  106.220274]  irq_exit_rcu+0x88/0xc0
[  106.223819]  common_interrupt+0x7b/0xa0
[  106.227719]  </IRQ>
[  106.229857]  asm_common_interrupt+0x1e/0x40
</snip>

Fix this by introducing the bitmap of queues that are zero-copy enabled,
where each bit, corresponding to a queue id that xsk pool is being
configured on, will be set/cleared within ice_xsk_pool_{en,dis}able and
checked within ice_xsk_pool(). The latter is a function used for
deciding which napi poll routine is executed.
Idea is being taken from our other drivers such as i40e and ixgbe..