How to access Macintosh HD from live cd environment

Hello, I am working on someone's 2011 iMac with intel processor. Their iMac boots with a keel panic and it will not proceed further. I am now trying to recover data from the hard drive labelled, "Macintosh HD" but, I don't have permissions to access it. I don't want to write to the hard drive, only read and copy from.

How do I go about doing this?

iMac specifications:

2011 iMac
Mac OS X 10.8
3.9GB RAM
Intel Core 2 Duo CPU E7600 @ 3.06GHz x2
Graphics: Gallium 0.4 on AMD RV730 (DRM 2.43.0, LLVM 3.8.0)

Live CD is Ubuntu 16.04 32bit i386

I'm comfortable using a terminal. Please, help would be appreciated. Thank you in advance.

#Urgent