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Ship a 64-bit (x86_64) instead of 32-bit userspace

Originally created by @Dr_Whax on #8183 (Redmine)

Currently, Tails ship an x86 (32-bit) userland but will load an x86_64 (64-bit) kernel if the system you’re using has 64-bit support.

Supporting the “32-bit userspace on 64-bit hardware” combination has historically caused lots of trouble, both for developers and for users (e.g. #11518 (closed), #9969 (closed), #5606 (closed)). Also, software built for 64-bit processors is more interesting from a security standpoint (e.g. it’s harder to bruteforce offsets/addresses, ASLR becomes stronger in that sense as is PIE support).

So, we have a few good reasons to consider switching our userspace to 64-bit. This implies to drop support for 32-bit hardware. Is it acceptable to do that in Tails 3.0, that we will release at some point between 2017Q2 and 2018Q1?

32-bit vs. 64-bit kernel stats among WhisperBack bug reports:
32-bit % 64-bit %
2014Q2 31 15 171 85
2014Q3 53 18 244 82
2014Q4 34 13 226 86
2015Q1 30 10 243 89
2015Q2 27 15 155 85
2015Q3 36 14 213 86
2015Q4 17 7 210 92
2016Q1 32 8 349 91
2016Q2 14 6 201 93
2016Q3 18 7 215 92

Note that a good share of the 32-bit systems are virtual machines: e.g. in 2016Q1, 11 of the 32 32-bit systems were VirtualBox and VMware. It seems safe to assume that the hardware able to run Tails in a VM is most likely 64-bit, and is running a 64-bit host OS (this seems plausible given our current hardware requirements, and e.g. the VirtualBox ones are probably due to https://www.virtualbox.org/ticket/11037 that forces us to tell users to set up a 32-bit VM). So we should just ignore the 32-bit VMs when looking at these stats.

Other than those, we have (32 - 11) / (32 + 349) = 5.5% of bare metal 32-bit systems. On #8183 (comment 95275) we have analyzed these systems, and to sum up, among these 21 bare metal systems:

  • 4 supports only 64-bit CPU so will still work once we switch to full 64-bit (let’s blame syslinux CPU auto-detection) => no regression
  • 1 supports max. 512MB of RAM => is not supported currently
  • 3 unknown
  • 10 will be 10+ years old when we release Tails 3.x, and support max. 2GB of RAM => we can be that this hardware won’t last much longer
  • the 3 remaining systems are from 2009 or 2012, and support max. 2GB of RAM

=> even including the 10+ years old systems in the equation, we’re talking of dropping support for 16 (4.2%) of systems that currently report bugs about Tails.

Feature Branch: feature/8183-64bit-userspace

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Edited by Dr_Whax
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