linux/dim: Fix divide by 0 in RDMA DIM

JIRA: https://issues.redhat.com/browse/RHEL-80826
CVE: CVE-2022-49670

commit 0fe3dbb
Author: Tao Liu <thomas.liu@ucloud.cn>
Date:   Mon Jun 27 22:00:04 2022 +0800

    linux/dim: Fix divide by 0 in RDMA DIM

    Fix a divide 0 error in rdma_dim_stats_compare() when prev->cpe_ratio ==
    0.

    CallTrace:
      Hardware name: H3C R4900 G3/RS33M2C9S, BIOS 2.00.37P21 03/12/2020
      task: ffff880194b78000 task.stack: ffffc90006714000
      RIP: 0010:backport_rdma_dim+0x10e/0x240 [mlx_compat]
      RSP: 0018:ffff880c10e83ec0 EFLAGS: 00010202
      RAX: 0000000000002710 RBX: ffff88096cd7f780 RCX: 0000000000000064
      RDX: 0000000000000000 RSI: 0000000000000002 RDI: 0000000000000001
      RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
      R10: 0000000000000000 R11: 0000000000000000 R12: 000000001d7c6c09
      R13: ffff88096cd7f780 R14: ffff880b174fe800 R15: 0000000000000000
      FS:  0000000000000000(0000) GS:ffff880c10e80000(0000)
      knlGS:0000000000000000
      CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
      CR2: 00000000a0965b00 CR3: 000000000200a003 CR4: 00000000007606e0
      DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
      DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
      PKRU: 55555554
      Call Trace:
       <IRQ>
       ib_poll_handler+0x43/0x80 [ib_core]
       irq_poll_softirq+0xae/0x110
       __do_softirq+0xd1/0x28c
       irq_exit+0xde/0xf0
       do_IRQ+0x54/0xe0
       common_interrupt+0x8f/0x8f
       </IRQ>
       ? cpuidle_enter_state+0xd9/0x2a0
       ? cpuidle_enter_state+0xc7/0x2a0
       ? do_idle+0x170/0x1d0
       ? cpu_startup_entry+0x6f/0x80
       ? start_secondary+0x1b9/0x210
       ? secondary_startup_64+0xa5/0xb0
      Code: 0f 87 e1 00 00 00 8b 4c 24 14 44 8b 43 14 89 c8 4d 63 c8 44 29 c0 99 31 d0 29 d0 31 d2 48 98 48 8d 04 80 48 8d 04 80 48 c1 e0 02 <49> f7 f1 48 83 f8 0a 0f 86 c1 00 00 00 44 39 c1 7f 10 48 89 df
      RIP: backport_rdma_dim+0x10e/0x240 [mlx_compat] RSP: ffff880c10e83ec0

    Fixes: f491545 ("linux/dim: Implement RDMA adaptive moderation (DIM)")
    Link: https://lore.kernel.org/r/20220627140004.3099-1-thomas.liu@ucloud.cn
    Signed-off-by: Tao Liu <thomas.liu@ucloud.cn>
    Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
    Acked-by: Leon Romanovsky <leonro@nvidia.com>
    Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

include/linux/dim.h

@@ -30,7 +30,7 @@ struct net_device;
  * We consider 10% difference as significant.
  */
 #define IS_SIGNIFICANT_DIFF(val, ref) \
-	(((100UL * abs((val) - (ref))) / (ref)) > 10)
+	((ref) && (((100UL * abs((val) - (ref))) / (ref)) > 10))
 
 /*
  * Calculate the gap between two values.