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Recently, I've replaced my old desktop (MB, CPU, RAM, chasis; I've kept the old disks) and upgraded to Fedora 29 (it was a fresh install) and now my machine occasionally crashes (sometimes a few days apart, but more often a few weeks apart).

The crash results in a completely frozen, unresponsive machine, with DE (xfce, if that matters) left at untouched (with nothing, not even the mouse pointer, moving or responding). The SSH access also stops working, so the machine is properly dead; it's not just the DE.

My first impulse was to check /var/log/messages and that's where I got this surprise:

Sep 21 04:03:23 machinename.localdomain audit[693]: USER_LOGIN pid=693 uid=0 auid=4294967295 ses=4294967295 msg='op
=login acct="(unknown)" exe="/usr/sbin/sshd" hostname=? addr=153.36.242.143 terminal=ssh res=failed'
Sep 21 04:05:09 machinename.localdomain NetworkManager[946]: <info>  [1569035109.9317] policy: set 'enp0s31f6' (enp
0s31f6) as default for IPv6 routing and DNS
^@^@^@^@... (very long, without newline at the end)Sep 24 09:42:35 machinename syslog-ng[829]: syslog-ng starting up; version='3.17.2'

So, the usual message of "someone's trying to get your ssh" and likes, nothing weird, and then suddenly a very weird string of ^@ character (I think it's a special char, not two characters). I remember it being exactly the same character on previous crashes too.

The hardware is pretty standard:

# lspci
00:00.0 Host bridge: Intel Corporation Xeon E3-1200 v6/7th Gen Core Processor Host Bridge/DRAM Registers (rev 05)
00:02.0 VGA compatible controller: Intel Corporation HD Graphics 630 (rev 04)
00:14.0 USB controller: Intel Corporation 200 Series/Z370 Chipset Family USB 3.0 xHCI Controller
00:16.0 Communication controller: Intel Corporation 200 Series PCH CSME HECI #1
00:17.0 SATA controller: Intel Corporation 200 Series PCH SATA controller [AHCI mode]
00:1b.0 PCI bridge: Intel Corporation 200 Series PCH PCI Express Root Port #17 (rev f0)
00:1c.0 PCI bridge: Intel Corporation 200 Series PCH PCI Express Root Port #1 (rev f0)
00:1c.4 PCI bridge: Intel Corporation 200 Series PCH PCI Express Root Port #5 (rev f0)
00:1d.0 PCI bridge: Intel Corporation 200 Series PCH PCI Express Root Port #9 (rev f0)
00:1f.0 ISA bridge: Intel Corporation 200 Series PCH LPC Controller (Z270)
00:1f.2 Memory controller: Intel Corporation 200 Series/Z370 Chipset Family Power Management Controller
00:1f.3 Audio device: Intel Corporation 200 Series PCH HD Audio
00:1f.4 SMBus: Intel Corporation 200 Series/Z370 Chipset Family SMBus Controller
00:1f.6 Ethernet controller: Intel Corporation Ethernet Connection (2) I219-V
03:00.0 USB controller: ASMedia Technology Inc. ASM2142 USB 3.1 Host Controller
# uname -a
Linux machinename.localdomain 5.1.18-200.fc29.x86_64 #1 SMP Mon Jul 15 16:09:08 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

It seems to be more likely to crash if I use an external 3.5" HDD (the same disks and caddy I used with the old system, which was perfectly stable), but not immediately. Usually, it's hours or even a day or two after I unmount and unplug the disks. I'm not sure, but the crash might also not always happen in this situation.

Also, it doesn't have to be related to that external USB storage. For starters, it doesn't seem to be triggered by a SD card reader. Also, for example, I was just away for over a week and it crashed on day 5, even though no one was using it. I don't exactly remember when was the last crash before that, but I think it was just a few weeks before.

I have no idea what to make of this log output, nor how to even Google it. I'm a bit afraid it might be some hard to trace HW failure, but I really don't know where to start.

Upgrading to a newer Fedora is not an option, as F30 has libc incompatible with Chrome's Flash, which I sadly still need, by no choice of my own.

So, how can I determine what's the problem here?

Update

Memtest86+ v4.3.7 didn't find any problems with RAM, and I've upgraded kernel to 5.3.1-150.vanilla.knurd.1.fc29.x86_64. I'm open to any other suggestions.

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    The ^@ character is a NUL character (i.e. 0x00), and is unrelated to the cause of the crash. It's often seen in log files after a crash (see Server crash with ascii NUL characters in syslog ( ^@^@^@… ). BTW, my guess is that your hardware is just unreliable....and memory is often the most likely culprit. Have you tried running memtest86+ or similar? – cas Sep 25 at 8:13
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    Otherwise, try a different kernel version. 5.2 has been out for a while now, and 5.3 was released recently. – cas Sep 25 at 8:17
  • Thank you, @cas. Regarding memtest, I did not test with it. It says "memtest86+ does not support EFI platforms" and I don't know how to go around that. As for a newer kernel, do you know if it's possible to do it via dnf without migrating to newer Fedora? My days of manually compiling kernels are looong gone (the last time I did it was some 15-20 years ago) and I don't feel willing nor capable to go back to that. – Vedran Šego Sep 25 at 10:17
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See IP Address Lookup for Vedran Sego. I'm assuming you aren't from China, so my first step would be to disable SSH

  1. sudo -i
  2. systmctl disable ssh
  3. systemctl stop ssh

After this Test the uptime without having to worry about a root takeover. If the uptime improves, and you require SSH, perform the opposite of the steps above after disabling root access over SSH

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    I need SSH. Also, I always disable the root's SSH before enabling the SSH daemon, so I don't care about these attempts, and I don't think that this is the cause of the issue. – Vedran Šego Sep 25 at 10:12

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