Fix potential null pointer dereference on kernfs

[Kai-Heng Feng]

[Impact]
We are seeing a null kernel dereference on kernfs during system sleep
stress.

[Fix]
Make sure kernfs root is not null.

[Test]
Run sleep stress. Not seeing the issue after the fix is applied.

[Regression Potential]
Low. Fix limits to kernfs, also the it has been in upstream kernel for
quite a while.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1874221

and then change the status of the bug to 'Confirmed'.

If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.

This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-eoan' to 'verification-done-eoan'. If the problem still exists, change the tag 'verification-needed-eoan' to 'verification-failed-eoan'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[Launchpad Janitor (janitor)]

This bug was fixed in the package linux - 5.3.0-53.47

---------------
linux (5.3.0-53.47) eoan; urgency=medium

  * eoan/linux: 5.3.0-53.47 -proposed tracker (LP: #1877257)

  * Intermittent display blackouts on event (LP: #1875254)
    - drm/i915: Limit audio CDCLK>=2*BCLK constraint back to GLK only

  * Unable to handle kernel pointer dereference in virtual kernel address space
    on Eoan (LP: #1876645)
    - SAUCE: overlayfs: fix shitfs special-casing

linux (5.3.0-52.46) eoan; urgency=medium

  * eoan/linux: 5.3.0-52.46 -proposed tracker (LP: #1874752)

  * alsa: make the dmic detection align to the mainline kernel-5.6
    (LP: #1871284)
    - ALSA: hda: add Intel DSP configuration / probe code
    - ALSA: hda: fix intel DSP config
    - ALSA: hda: Allow non-Intel device probe gracefully
    - ALSA: hda: More constifications
    - ALSA: hda: Rename back to dmic_detect option
    - [Config] SND_INTEL_DSP_CONFIG=m
    - [packaging] Remove snd-intel-nhlt from modules

  * built-using constraints preventing uploads (LP: #1875601)
    - temporarily drop Built-Using data

  * ubuntu/focal64 fails to mount Vagrant shared folders (LP: #1873506)
    - [Packaging] Move virtualbox modules to linux-modules
    - [Packaging] Remove vbox and zfs modules from generic.inclusion-list

  * linux-image-5.0.0-35-generic breaks checkpointing of container
    (LP: #1857257)
    - SAUCE: overlayfs: use shiftfs hacks only with shiftfs as underlay

  * shiftfs: broken shiftfs nesting (LP: #1872094)
    - SAUCE: shiftfs: record correct creator credentials

  * Add debian/rules targets to compile/run kernel selftests (LP: #1874286)
    - [Packaging] add support to compile/run selftests

  * shiftfs: O_TMPFILE reports ESTALE (LP: #1872757)
    - SAUCE: shiftfs: fix dentry revalidation

  * getitimer returns it_value=0 erroneously (LP: #1349028)
    - [Config] CONTEXT_TRACKING_FORCE policy should be unset

  * 5.3.0-46-generic - i915 - frequent GPU hangs / resets rcs0 (LP: #1872001)
    - drm/i915/execlists: Preempt-to-busy
    - drm/i915/gt: Detect if we miss WaIdleLiteRestore
    - drm/i915/execlists: Always force a context reload when rewinding RING_TAIL

  * alsa/sof: external mic can't be deteced on Lenovo and HP laptops
    (LP: #1872569)
    - SAUCE: ASoC: intel/skl/hda - set autosuspend timeout for hda codecs

  * Eoan update: upstream stable patchset 2020-04-22 (LP: #1874325)
    - ARM: dts: sun8i-a83t-tbs-a711: HM5065 doesn't like such a high voltage
    - bus: sunxi-rsb: Return correct data when mixing 16-bit and 8-bit reads
    - net: vxge: fix wrong __VA_ARGS__ usage
    - hinic: fix a bug of waitting for IO stopped
    - hinic: fix wrong para of wait_for_completion_timeout
    - cxgb4/ptp: pass the sign of offset delta in FW CMD
    - qlcnic: Fix bad kzalloc null test
    - i2c: st: fix missing struct parameter description
    - cpufreq: imx6q: Fixes unwanted cpu overclocking on i.MX6ULL
    - media: venus: hfi_parser: Ignore HEVC encoding for V1
    - firmware: arm_sdei: fix double-lock on hibernate with shared events
    - null_blk: Fix the null_add_dev() error path
    - null_blk: Handle null_add_dev() failures properly
    - null_blk: fix spuri...