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Re: GRUB and the risk of block list corruption in extX

From: Dr. Tilmann Bubeck
Subject: Re: GRUB and the risk of block list corruption in extX
Date: Fri, 03 May 2013 21:21:47 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130402 Thunderbird/17.0.5

There is a solution under way. Linux 3.10 will include code written by me to secure core.img of grub when running from ext4. This means, that ext4 will be as safe to use for grub chainloading as btrfs or any other filesystem offering "embedding".

I am currently extending grub-setup.c to use this new functionality. I will send a patch to this list in a few days. Hopefully you can apply this patch, so that this issue will be fixed.

Kind regards,

Am 03.05.2013 10:21, schrieb Martin Wilck:

Here is example how using filesystem blocklists may lead to unbootable
system without any extX corruption involved.

- user sets up multiboot system with Windows as primary bootloader
- standard technique to add Linux loaders has always been - copy
   partition boot sector and "launch" it from Windows loader
- user copies Linux partition boot sector which points to core.imng
   absolute disk position
- user updates grub in Linux. core.img is rewritten and its position
- next time user tries to boot Linux (s)he gets blinking cursor

So *any* third party bootloader that relies on being able to
"chainload" *copy* of boot sector will give you the same issue.

I understand. It's generally understood that updating core.img without
updating the boot sector is a bad idea. In this particular case updating
the boot sector is not enough because the copy needs to be updated, too.

The background for my question was a different scenario, with a
chainload-capable boot loader in the MBR and secondary boot loaders in
partition boot sectors. It is that scenario that the new anaconda
installer doesn't support any more, and the major argument from the
Fedora devs for this (apart from sparing dev and QA resources) was the
warning emitted by GRUB when users try to install using block lists.

I am still convinced that the risk of boot loader corruption in that
scenario is extremely low.


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