November « 2012 « codeblog

November 28, 2012

clean module disabling

Filed under: Blogging,Chrome OS,Security,Ubuntu,Ubuntu-Server — kees @ 3:55 pm

I think I found a way to make disabling kernel module loading (via /proc/sys/kernel/modules_disabled) easier for server admins. Right now there’s kind of a weird problem on some distros where reading /etc/modules races with reading /etc/sysctl.{conf,d}. In these cases, you can’t just put “kernel.modules_disabled=1” in the latter since you might not have finished loading modules from /etc/modules.

Before now, on my own systems, I’d added the sysctl call to my /etc/rc.local, which seems like a hack — that file is related to neither sysctl nor modules and both subsystems have their own configuration files, but it does happen absolutely last.

Instead, I’ve now defined “disable” as a modprobe alias via /etc/modprobe.d/disable.conf:

# To disable module loading after boot, "modprobe disable" can be used to
# set the sysctl that controls module loading.
install disable /sbin/sysctl kernel.modules_disabled=1

And then in /etc/modules I can list all the modules I actually need, and then put “disable” on the last line. (Or, if I want to not remember the sysctl path, I can manually run “modprobe disable” to turn off modules at some later point.)

I think it’d be cool this this become an internal alias in upstream kmod.

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November 9, 2012

product search in Ubuntu 12.10

Filed under: Blogging,Security,Ubuntu,Web — kees @ 3:18 pm

The EFF has already discussed the product search “feature” in Ubuntu 12.10’s Unity UI. Ways for disabling it are covered:

sudo apt-get remove unity-lens-shopping – it isn’t easy to generally blacklist a package, it might end up getting re-installed later, etc.
System Settings / Privacy / Search Results – the naming says nothing about it disabling product search results.
use a UI other than Unity – this is what I do.

Here’s another way, that overrides the URL used for the product searching (restart your session after making this change):

$ sudo -s
# echo 'OFFERS_URI="https://localhost:0/"' >> /etc/environment

Or, if you run an organization where you build devices that run Ubuntu, and want to snoop on all the things people type into their Unity search bar, just change that to a URL you control.

I’m astonished by Canonical’s blatant disregard for providing a way to opt-in to this gaping privacy hole. This is a dramatic case of “calling home”, and provides a large amount of information about the user, in real-time. Besides sending the content of their searches and the version of the software installed, it also sends every keystroke, which means in some weird cases, even passive observers can examine keystroke timing which has been shown to potentially leak what is being typed:

127.0.0.1 - - [09/Nov/2012:14:29:41 -0800] "GET //v1/search?q=p HTTP/1.1" 404 522 "-" "Unity Shopping Lens 6.8.0"
127.0.0.1 - - [09/Nov/2012:14:29:41 -0800] "GET //v1/search?q=pw HTTP/1.1" 404 521 "-" "Unity Shopping Lens 6.8.0"
127.0.0.1 - - [09/Nov/2012:14:29:41 -0800] "GET //v1/search?q=pwn HTTP/1.1" 404 521 "-" "Unity Shopping Lens 6.8.0"

Ubuntu is a general-purpose OS, with Unity as its default interface. It is not a vendor-tied appliance nor a telephone company device, and Unity is not a browser (in fact, even in a browser there are visual indicators of where what you have typed will go).

Even if the default for this is enabled, there needs to be (likely at install-time) a page describing what to expect, and the system owner can choose “yes, search online” or “no thanks”. This behavior needs to be fixed in 13.04 and SRUed into 12.10. If there is no fast solution, then it just needs to be disabled by default until it has a sane notification flow.

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November 2, 2012

ARM assembly

Filed under: Blogging,Chrome OS,Embedded,Ubuntu,Ubuntu-Server — kees @ 10:01 am

While I’ve been skimming ARM assembly here and there, yesterday I actually had to write some from scratch to hook up seccomp on ARM. I got stumped for a while, and ended up using two references frequently:

The suffix one is pretty interesting because ARM allows for instructions to be conditional, rather than being required to rely on branching, like x86. For example, if you wanted something like this in C:

    if (i == 0)
        i = 1;
    i = i + 1;

In x86 assembly, you’d have a compare followed by a jump to skip the moving of the “1” value:

    cmp %ecx, $0
    jne 2
    mov %ecx, $1
2:  inc %ecx

In ARM assembly, you can make the move conditional with a suffix (“mov if equal”):

    cmp   r2, #0
    moveq r2, #1
    add   r2, r2, #1

The real thing that stumped me yesterday, though, was the “!” suffix on load/store. Mainly, I didn’t notice it was there until I’d stared at the objdump output and systematically trimmed away all other other code that wasn’t changing the behavior:

    ldr r0, [sp, #OFFSET]
    str r0, [sp, #OFFSET]!

I was reading this as “variable = variable;” and I thought I was going crazy; how could a self-assignment change the code at all? In the second reference above, I found the that the trailing “!” means “(pre)increment the base by the offset”. I was doing a meaningless assignment, but it had the side-effect of pushing the “sp” register forward, and suddenly it all made sense (I needed to unwind the stack). The actual solution I needed was:

    add sp, sp, #S_OFF

Yay for a crash-course in actual ARM assembly. :)

(And yes, I’m aware of x86’s “cmov”, but I just wanted to do a simple illustration. ARM can do conditional calls!)

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