Comment 0 for bug 1987287

We recently got a bug report for systems running Ubuntu 20.04 that were
crashing with backtraces pointing at the mlx5 driver's handling of mlx5_ethtool_get_link_ksettings()
when this is called through the sysfs (going through ethtool might have different checks).
I managed to find a reliable way to reproduce the issue that I believe isn't tied to IBM Z at all.

The procedure to reproduce is as follows. I created a script to read
the sysfs attributes for the link's speed and duplex mode in a loop:

#!/usr/bin/env bash

if [ $# -lt 1 ]; then
        echo "Usage: $0 <netif>"
        exit 1
fi

while true; do
        cat /sys/class/net/$1/duplex > /dev/null
        cat /sys/class/net/$1/speed > /dev/null
done

Executed with:

# ./script.sh enP10p0s0

I ran this in one bash session and then in another one I triggered a PCI reset with
the follwoing command where one needs to replace <dev> with the PCI address of the NIC:

echo 1 > /sys/bus/pci/devices/<dev>/reset

Then first I got a lot of the following messages:

 mlx5_core 0010:00:00.0 enP16p0s0: mlx5e_ethtool_get_link_ksettings: query port ptys failed: -5

And then as the mlx5 driver's recovery kicks in the oops as below:

[ 659.103947] mlx5_core 0010:00:00.0: wait vital counter value 0x7b399f after 1 iterations
[ 659.103947] mlx5_core 0010:00:00.0: mlx5_pci_resume was called
[ 659.103966] mlx5_core 0010:00:00.0: firmware version: 14.32.1010
[ 659.104169] Unable to handle kernel pointer dereference in virtual kernel address space
[ 659.104171] Failing address: 0000000000000000 TEID: 0000000000000483
[ 659.104172] Fault in home space mode while using kernel ASCE.
[ 659.104173] AS:000000003d29c007 R3:00000000fffd0007 S:00000000fffd5800 P:000000000000003d
[ 659.104200] Oops: 0004 ilc:2 [#1] SMP
[ 659.104202] Modules linked in: s390_trng ism smc pnet chsc_sch eadm_sch vfio_ccw vfio_mdev mdev vfio_iommu_type1 vfio sch_fq_codel drm drm_panel_orientation_quirks i2c_core ip_tables x_tables btrfs zstd_compress zlib_deflate raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 linear mlx5_ib dm_service_time pkey zcrypt crc32_vx_s390 ib_uverbs ghash_s390 ib_core qeth_l2 prng aes_s390 des_s390 nvme libdes sha3_512_s390 sha3_256_s390 sha512_s390 sha256_s390 sha1_s390 sha_common mlx5_core tls mlxfw ptp nvme_core pps_core dasd_eckd_mod dasd_mod zfcp scsi_transport_fc qeth qdio ccwgroup scsi_dh_emc scsi_dh_rdac scsi_dh_alua dm_multipath
[ 659.104232] CPU: 6 PID: 438216 Comm: cat Not tainted 5.4.0-124-generic #140-Ubuntu
[ 659.104233] Hardware name: IBM 3931 XYZ XXXX (LPAR)
[ 659.104234] Krnl PSW : 0404c00180000000 000000003bfa661e (__queue_work+0xfe/0x520)
[ 659.104241] R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
[ 659.104242] Krnl GPRS: 000000003c291570 0000000000000000 0000000000000007 000000007fffffff
[ 659.104243] 00000000e2fe46e0 0000000fffffffe0 0000000000000006 000000003d039588
[ 659.104244] 0000000000000000 0000000000000000 00000000e2fe46e0 00000000bfb3e000
[ 659.104245] 00000000e194c400 000003e007d6fb78 000000003bfa6602 000003e007d6f860
[ 659.104251] Krnl Code: 000000003bfa6612: a77400e5 brc 7,000000003bfa67dc
                          000000003bfa6616: 582003ac l %r2,940
                         #000000003bfa661a: a7180000 lhi %r1,0
                         >000000003bfa661e: ba129000 cs %r1,%r2,0(%r9)
                          000000003bfa6622: a77401a7 brc 7,000000003bfa6970
                          000000003bfa6626: e310b0180012 lt %r1,24(%r11)
                          000000003bfa662c: a78400ff brc 8,000000003bfa682a
                          000000003bfa6630: c0040000004b brcl 0,000000003bfa66c6
[ 659.104261] Call Trace:
[ 659.104263] ([<0000000000000000>] 0x0)
[ 659.104265] [<000000003bfa6aa2>] queue_work_on+0x62/0x70
[ 659.104329] [<000003ff80a2920a>] cmd_exec+0x4ea/0x840 [mlx5_core]
[ 659.104349] [<000003ff80a29680>] mlx5_cmd_exec+0x40/0x70 [mlx5_core]
[ 659.104369] [<000003ff80a334a8>] mlx5_core_access_reg+0x108/0x150 [mlx5_core]
[ 659.104387] [<000003ff80a3354e>] mlx5_query_port_ptys+0x5e/0x70 [mlx5_core]
[ 659.104407] [<000003ff80a5b928>] mlx5e_ethtool_get_link_ksettings+0x58/0x460 [mlx5_core]
[ 659.104410] [<000000003c662a68>] duplex_show+0x78/0xe0
[ 659.104414] [<000000003c538ecc>] dev_attr_show+0x2c/0x70
[ 659.104417] [<000000003c293386>] sysfs_kf_seq_show+0xa6/0x150
[ 659.104420] [<000000003c207470>] seq_read+0xe0/0x4f0
[ 659.104422] [<000000003c1d4cf4>] vfs_read+0x94/0x160
[ 659.104423] [<000000003c1d4ea8>] ksys_read+0x68/0x100
[ 659.104426] [<000000003c7f2034>] system_call+0xd8/0x2c8
[ 659.104427] Last Breaking-Event-Address:
[ 659.104428] [<000000003bfa65d6>] __queue_work+0xb6/0x520
[ 659.104430] ---[ end trace 9fc1a6358b456876 ]---

Digging into the code and git history I found the following upstream commit added in v5.7
which besides being part of the 5.6.x stable patches somehow didn't make it into
the 5.4.x stable queue nor Ubuntu 20.04, possibly because there is a (trivial) merge conflict:

commit f7936ddd35d8b849daf0372770c7c9dbe7910fca
Author: Eran Ben Elisha <email address hidden>
Date: Thu Mar 19 21:43:13 2020 +0200

    net/mlx5: Avoid processing commands before cmdif is ready

    When driver is reloading during recovery flow, it can't get new commands
    till command interface is up again. Otherwise we may get to null pointer
    trying to access non initialized command structures.

    Add cmdif state to avoid processing commands while cmdif is not ready.

    Fixes: e126ba97dba9 ("mlx5: Add driver for Mellanox Connect-IB adapters")
    Signed-off-by: Eran Ben Elisha <email address hidden>
    Signed-off-by: Moshe Shemesh <email address hidden>
    Signed-off-by: Saeed Mahameed <email address hidden>

With a quick backport onto 5.4.0-124.140 (patch attached below) the issue is gone
and the system no longer crashes but instead recovers successfully. I believe the
crash we saw is then exactly the null pointer access mentioned in the commit description.

== Comment: #4 - Niklas Schnelle - 2022-08-22 06:41:41 ==
There was only a trivial merge conflict where the context
of the struct decleration changed.