[SRU][F][PATCH 0/1] btrfs: trimming a btrfs device which has been shrunk previously fails and fills root disk with garbage data

[Matthew Ruffell]

BugLink: https://bugs.launchpad.net/bugs/1896154

[Impact]

Since 929be17a9b49 ("btrfs: Switch btrfs_trim_free_extents to find_first_clear_extent_bit")
which landed in 5.3, btrfs wont trim a range that has already been trimmed, and
will instead go looking for a range where the CHUNK_TRIMMED and CHUNK_ALLOCATED
bits aren't set.

If a device had been shrunk, the CHUNK_TRIMMED and CHUNK_ALLOCATED bits are
never cleared, which means that btrfs could go looking for a range to trim which
is beyond the new device size. This leads to an underflow in a length calculation
for the range to trim, and we will end up trimming past the device's boundary.

This has an unfortunate side effect of mangling and filling the root disk with
garbage data, and it will not stop until the root disk is totally filled, and
makes the instance unusuable.

[Fix]

The issue was fixed in the following commit, in 5.9-rc1:

commit c57dd1f2f6a7cd1bb61802344f59ccdc5278c983
Author: Qu Wenruo <wqu at suse.com>
Date: Fri Jul 31 19:29:11 2020 +0800
Subject: btrfs: trim: fix underflow in trim length to prevent access beyond device boundary
Link: https://github.com/torvalds/linux/commit/c57dd1f2f6a7cd1bb61802344f59ccdc5278c983

The fix clears the CHUNK_TRIMMED and CHUNK_ALLOCATED bits when a device is
being shrunk, and performs some additional checks to ensure we do not trim
past the device size boundary.

The fix was backported to 5.7.17 and 5.8.3 upstream stable, but it seems 5.4 was
skipped.

The patch required a minor backport to 5.4, with the CHUNK_STATE_MASK #define
moving files back to fs/btrfs/extent_io.h, as the file had been renamed in
later kernels.

[Testcase]

The easiest way to reproduce is to use a cloud instance that supplies a real
NVMe drive, that supports TRIM and block discards. 

Warning, this will fill the root disk with garbage data, ONLY run on a throwaway
instance!

Run the following commands:

$ dev=/dev/nvme0n1
$ mnt=/mnt
$ mkfs.btrfs -f $dev -b 10G
$ mount $dev $mnt
$ fstrim $mnt
$ btrfs filesystem resize 1:-1G $mnt
$ fstrim $mnt 

The last command will appear to hang, while the root filesystem will begin
filling with garbage data. Once the root filesystem fills, you will see the
following error:

fstrim: /mnt: FITRIM ioctl failed: Input/output error 
/dev/sda1 29G 29G 0 100% / 

A test kernel is available from the following PPA:

https://launchpad.net/~mruffell/+archive/ubuntu/sf293389-test

If you install the test kernel, then the final fstrim command completes
successfully in a short amount of time.

[Regression Potential]

If a regression were to occur, it could affect users who are attempting to
shrink or resize their btrfs volume. Most users already understand that changing
the size of a volume is a risky operation, and would have a backup.

If a regression occurs, then there is potential for data loss when users
resize or shrink their btrfs volumes. Standard volume creation would not be
affected.

The patches have been backported to upstream stable, and are trusted by the
community.

Qu Wenruo (1):
  btrfs: trim: fix underflow in trim length to prevent access beyond
    device boundary

 fs/btrfs/extent-tree.c | 14 ++++++++++++++
 fs/btrfs/extent_io.h   |  2 ++
 fs/btrfs/volumes.c     |  4 ++++
 3 files changed, 20 insertions(+)

-- 
2.27.0