> Why isn't it the job of the distros to keep up on upstream security disclosures?
They can't, because (responsible) security disclosures are private, _not public_. That's the whole point of the system: notify the developers in private ahead of time (usually 30, 60 or 90 days) so they can write, test and roll-out the fixes before you release the info to the whole world. This is to minimize the time between when bad actors gain access to the exploits vs. when users install the patch. So "keeping up on security disclosures" cannot ever be a 'pull' process.
Usually the maintainers of the big distros are part of (private) security mailinglists and receive such info. Just not in this case it seems.
It would be best if distros kept tap on kernel changes and update as soon as possible when they see a security issue fixed.
Sending emails to some big distros would still result with e.g. Gentoo not getting that info because they are not a big distro.
The problem is that the kernel devs (correctly imo) consider all bugfixes security fixes. So the distros need to decide for themselves which ones are important enough to warrant an update. Apparently this one had a quite unclear commit message, so it importance was missed.
Not ideal, but also: shit happens? It's always a balancing act choosing the lesser of multiple evils and most of the time it seems to work ok-ish, which is probably the best we can hope for ;-P
The kernel maintainers don't flag "security fixes" as special, and they have a well-thought-out reason for that, see many other comments in this thread.
That, and they flag pretty much any random patch with a CVE these days, making it harder for distro maintainers to keep up.
For this specific "bug" they took care to not mention any security angle in the commit message, making it extremely hard for an outsider to even realize this was a critical patch. I assume this was because they wanted to push the fix without breaking embargo.