flak rss random www

the activity person examined

One of the basic objects in ActivityPub is the actor. Also known as a Person, although there’s no promise it’s a human. If you are building ActivityPub software, or curious how the network works, it’s a good place to start. The ActivityPub spec (and underlying ActivityStreams vocabulary explain what could or should be here, but not necessarily what you’ll see in the wild.


Posted 08 Jul 2019 18:44 by tedu Updated: 06 Aug 2019 15:24
Tagged: activitypub web

404 Found

When a web page (or other resource) cannot be found, a web server is supposed to return code 404, Not Found. Additionally, it can return some other content for a human viewer. And so, if you visit https://mastodon.social/honktime](https://mastodon.social/honktime) with a browser, you can watch a tooter tantrum, but requesting the same URL with curl displays < HTTP/2 404.


Posted 04 Jul 2019 18:55 by tedu Updated: 04 Jul 2019 23:21
Tagged: web

what happens when you activity post

You’re out there posting on your federated status federator, and people are reading your posts, and you’re reading their posts, but how exactly does it happen? What’s talking to what? (Equally applicable to tooting, but we don’t use that word in my house.)


Posted 02 Jul 2019 21:24 by tedu Updated: 02 Jul 2019 21:24
Tagged: activitypub web

random ip id comments

There’s a new paper, From IP ID to Device ID and KASLR Bypass, which I liked. It’s at the intersection of networking, old but not obsolete standards, random, security, and implementation defined behavior. By all means, read the paper, but the really short version is they accomplished two things. They reverse engineered a per host random seed from network traffic on Windows and Linux, allowing fingerprinting, and more surprising, turned this into a KASLR break on Linux. Pretty wild.


Posted 01 Jul 2019 01:23 by tedu Updated: 01 Jul 2019 01:23
Tagged: networking openbsd security

extension injection detection

One of the unexpected but very interesting side effects of the brutalist html quine is that anything added to the page suddenly becomes visible as well. In the common case, this might be browser extensions adding custom stylesheets, but it might also be stylesheet or script injection by a network interloper.

What if every web page had a little something like this embedded in it?

That’s an actual style. It should be the only one visible on this page. If there’s anything else visible, it’s not coming from me.

Posted 05 Jun 2019 10:12 by tedu Updated: 05 Jun 2019 10:12
Tagged: web

honking for fun and profit

It’s been a little while, so a few more notes about ActivityPub implementation, federation, and other odds and ends. There’s no real order to these notes, just things that have come up in the past two months.


Posted 03 Jun 2019 09:03 by tedu Updated: 06 Aug 2019 15:52
Tagged: activitypub project web

hello android

I’ve had an iPhone for many years, and an iPad for not quite as long. People would tell me I should switch to Android. I thought they were crazy. I recently got some Android devices. Now I know they are crazy. Some notes on recent experiences with a Moto G6 and Samsung Tab S5e.


Posted 01 Jun 2019 18:21 by tedu Updated: 05 Sep 2019 11:28
Tagged: gadget review

ssh in https

The wifi network at BSDcan, really the UOttawa network, blocks a bunch of ports. This makes it difficult to connect to outside machines using “exotic” protocols, basically anything except http or https. There are many ways to resolve this, here’s what I did.


Posted 17 May 2019 17:32 by tedu Updated: 15 Jul 2019 21:15
Tagged: openbsd software web

syzkaller found a bug

Common problem for operating system fuzzers is breaking the system they’re running on. Some forms of damage are expected, some are not, and sometimes it’s difficult to tell which is which.

A few days ago, a stack leak bug was fixed in FreeBSD. A similar fix for OpenBSD was committed. And then syzkaller came kalling just a few days later.

panic: bad dir

There’s a few possible causes for this, but inspection revealed that the most likely case might be a directory entry missing the nul terminator. The timing certainly seemed suspicious. Could there be an off by one?

memset(newdirp->d_name + (cnp->cn_namelen & ~(DIR_ROUNDUP-1)), 0, DIR_ROUNDUP);

Actually no. syzkaller had found a way to create filesystem corruption through one of the “expected” damage paths, but the test case was a little obfuscated. More study revealed it was calling mknod to create a new device that happened to be equal to /dev/sd0c and opening it, and then calling pwrite to write some garbage to a random spot.

mknod("banana", 0777, 0x0402);
pwrite(3, "oops", 4, 0x9000);

Not recommended.

Further complicating the matter was that syzkaller didn’t know that pwrite is one of the magic syscalls that takes a padding argument before off_t. This didn’t affect the test, per se, but makes it harder to interpret because syzkaller calls things directly. (The actual syscall in use was the iovec variant, pwritev.)

syscall(SYS_pwritev, r[0], 0x200002c0, 1, 0);

If you read the man page for pwritev that looks correct. But inspecting src/sys/kern/syscalls.master reveals that the fourth argument is actually a pad argument, and the offset is the fifth argument. So the call above was writing to an offset that was not zero.

Not the first fuzzer to encounter this oddity. More details here.

In the end, it was just coincidence that syzkaller found a new way to corrupt its filesystem a few days after a filesystem commit.

Posted 10 May 2019 16:02 by tedu Updated: 10 May 2019 16:02
Tagged: openbsd

viewport and iphone reflow

Something that’s annoyed me for some years is that all the web sites I build don’t work quite right with my iphone. Scroll down a page, visit a link, go back, and safari jumps back to the top of the page. Very annoying. Pretty much no other site I visit seems to have this problem, yet I couldn’t figure out what I was doing wrong since I’m barely doing anything at all. There are some support forum complaints about similar bugs, but mostly from several years ago, and mostly “solved: it works now” without explanation.

Finally, figured out what seems to be the problem. The iphone introduces its own viewport meta tag, to define the screen dimensions, and control whether the user can zoom or not. A lot of sites abuse this to the point of unusability, so I very determinedly stayed clear. But without a viewport tag, safari is really dumb.

Without a viewport setting safari picks some defaults and renders the page to fit. They seem fine to me. The problem is that after leaving a page and coming back, safari has forgotten what size it picked, picks again, and then has to reflow all the page content. Even though it has picked exactly the same dimensions as the previous render. With the result that it forgets its scroll position and resets to the top of the page. Sigh. At least that’s what I’ve determined is going on.

So I finally broke down and added a viewport tag to the header. This required futzing with the CSS some more because now it rendered to a much smaller virtual canvas, but generally solvable.

Anyway, this frustrated me for a long time, I couldn’t find any useful information about it, and now it seems to work.

Posted 19 Apr 2019 17:27 by tedu Updated: 22 Apr 2019 18:55
Tagged: web