After cherry picking patch 1 & 2, I saw the following with 2 UC errors injected
into the user space buffer passed into write(2), as expected:

[  287.994754] Kernel panic - not syncing: Machine checks to different
user pages

Interesting.  What are the offsets of the two injected errors in your test (both
w.r.t. the start of the buffer, and within a page).

The kernel tested with has its x86/mce and mm/memory-failure aligned with
upstream till around 2020/11.

Is there any other patch that I have missed to the write syscall etc?

There is a long series of patches from Al Viro to lib/iov_iter.c that are maybe
also relevent in making the kernel copy from user stop at the first poison
address in the buffer.

-Tony