[Bug 510067] [NEW] ext3 default compile time option should be changed from writeback to ordered

[Launchpad Bug Tracker]

*** This bug is a security vulnerability ***

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SRU Justification:

Impact: The default writeback mode for ext3 changed implicitly in Karmic
without notice. The new default is supposed to be faster (though it
seems not that much) but also can cause more likely data corruption when
the system crashes.

Fix: Change back the default mode to "ordered" which was used before, is
used for Lucid too and is recommended by fs developers anyway.

Testcase: cat /proc/mounts should show ext3 filesystems with
data=ordered (if no special intervention was done to change that).

---

Imagine A and B to be the steps of a journaling operation.

data = ordered mode
A) Write data to FS on disk
B) Write metadata to journal

For data = writeback mode
the above steps - A) and B) could switch order.
So you could have the following:
A) write meta data to journal first and then
B) write data to FS on disk

OR

A) write data to FS on disk first and then
B) write metadata to journal.

(since the order is not guranteed in data=writeback mode)

Now imagine a reboot/crash/kernel lockup occurring in between steps A
and B

A)
CRASH
B) cannot take place because of the crash.

Now look at data = ordered mode. Since data is always first written to
disk before writing the metadata, you always have the most current data
on disk. Hence data = ordered gurantees that you will never have stale
data on disk.

Whereas for data=writeback mode, when the metadata is written first and  you crash before writing the data to fs, this is what happens:
You have stale data, but updated metadata. So what can now happen is, you still read old data and this could be
a security threat (since the user does not expect this behavior) .

However, such a threat does not exist in data=ordered mode. Hence the
default mount time option should rightly be data=ordered instead of data
= writeback. Change the compile time option accordingly to reflect this
change.

** Affects: linux (Ubuntu)
     Importance: High
     Assignee: Surbhi Palande (csurbhi)
         Status: Fix Committed

** Affects: linux (Ubuntu Lucid)
     Importance: High
     Assignee: Surbhi Palande (csurbhi)
         Status: Fix Committed

** Affects: linux (Ubuntu Karmic)
     Importance: High
     Assignee: Surbhi Palande (csurbhi)
         Status: Fix Committed

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ext3 default compile time option should be changed from writeback to ordered
https://bugs.edge.launchpad.net/bugs/510067
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