Comment 7 for bug 1874055

------- Comment From <email address hidden> 2020-05-26 04:04 EDT-------
(In reply to comment #12)
> Hi Niklas, please can you have a look at the SRU Justification that I've now
> added to the bug description, with special focus on Test Case and Regression
> Potential.
> Please let me know if is inaccurate or missing or in case additional
> information should be added (this is mandatory information needed for the
> SRU).
> Thx

Sounds good, maybe to add that this can be triggered with shipping user space tools (including in the Ubuntu repos) e.g.:

1. install the rdma tools:
sudo apt-get install ibverbs-providers ibverbs-utils

2. verify you have some RDMA devices (requires ConnectX adapter)
pcidev@T224LP06:~$ ibv_devices
device node GUID
------ ----------------
mlx5_0 98039b0300c682b4

3. verify MIO instructions are enabled for the device
pcidev@T224LP06:~$ cat /sys/bus/pci/devices/0000\:00\:00.0/mio_enabled
1

4. try to run an RDMA application from user space, e.g. ibv_rc_pingpong

server process:
ibv_rc_pingpong -d mlx5_0 -g 0 &

client process:
ibv_rc_pingpong -d mlx5_0 -g 0 localhost

5. verify that the kernel crashes

[92406.190525] Unable to handle kernel pointer dereference in virtual kernel add
ress space
[92406.190529] Failing address: ed00000000090000 TEID: ed00000000090403
[92406.190529] Fault in home space mode while using kernel ASCE.
[92406.190531] AS:0000000c1c98c007 R3:0000000ff3bd0007 S:0000000ff3bd6000 P:0000
00000009013d

Also this patch made it into v5.7-rc7 and Linus himself commented:

"And none of the fixes look like there's anything particularly scary
going on. Most of it is very small, and the slightly larger patches
aren't huge either and are well-contained (the two slightly larger
patches are to s390 and rxrpc - and even those patches aren't really
all _that_ big)"

(this patch obviously being the bigger s390 change)