Re: [PATCH v3] btrfs: new test for devt change between mounts

From: Filipe Manana <fdmanana@kernel.org>
To: Boris Burkov <boris@bur.io>
Cc: linux-btrfs@vger.kernel.org, kernel-team@fb.com, fstests@vger.kernel.org
Subject: Re: [PATCH v3] btrfs: new test for devt change between mounts
Date: Sun, 10 Mar 2024 17:16:26 +0000	[thread overview]
Message-ID: <CAL3q7H5FPa8336s3ZXtScvjTnUHjBaFETBa3eUDAtxzb46jBcw@mail.gmail.com> (raw)
In-Reply-To: <9dde3b18f00a30cae78197c9069db503f720fe71.1709844612.git.boris@bur.io>

On Fri, Mar 8, 2024 at 5:42 PM Boris Burkov <boris@bur.io> wrote:
>
> It is possible to confuse the btrfs device cache (fs_devices) by
> starting with a multi-device filesystem, then removing and re-adding a
> device in a way which changes its dev_t while the filesystem is
> unmounted. After this procedure, if we remount, then we are in a funny
> state where struct btrfs_device's "devt" field does not match the bd_dev
> of the "bdev" field. I would say this is bad enough, as we have violated
> a pretty clear invariant.
>
> But for style points, we can then remove the extra device from the fs,
> making it a single device fs, which enables the "temp_fsid" feature,
> which permits multiple separate mounts of different devices with the
> same fsid. Since btrfs is confused and *thinks* there are different
> devices (based on device->devt), it allows a second redundant mount of
> the same device (not a bind mount!). This then allows us to corrupt the
> original mount by doing stuff to the one that should be a bind mount.
>
> Signed-off-by: Boris Burkov <boris@bur.io>

It fails on for-next (as expected) and it passes with the following
patch applied:

    "btrfs: validate device maj:min during open"

If the consensus is to use that patch as a fix, we can later add the
_fixed_by_kernel_commit call to the test.
Also, the test number 311 has been taken for a week now.

Anyway:

Reviewed-by: Filipe Manana <fdmanana@suse.com>

Thanks.

> ---
> Changelog:
> v3:
> - fstests convention improvements (helpers, output, comments, etc...)
> v2:
> - fix numerous fundamental issues, v1 wasn't really ready
>
>  common/config       |   1 +
>  tests/btrfs/311     | 105 ++++++++++++++++++++++++++++++++++++++++++++
>  tests/btrfs/311.out |   2 +
>  3 files changed, 108 insertions(+)
>  create mode 100755 tests/btrfs/311
>  create mode 100644 tests/btrfs/311.out
>
> diff --git a/common/config b/common/config
> index a3b15b96f..43b517fda 100644
> --- a/common/config
> +++ b/common/config
> @@ -235,6 +235,7 @@ export BLKZONE_PROG="$(type -P blkzone)"
>  export GZIP_PROG="$(type -P gzip)"
>  export BTRFS_IMAGE_PROG="$(type -P btrfs-image)"
>  export BTRFS_MAP_LOGICAL_PROG=$(type -P btrfs-map-logical)
> +export PARTED_PROG="$(type -P parted)"
>
>  # use 'udevadm settle' or 'udevsettle' to wait for lv to be settled.
>  # newer systems have udevadm command but older systems like RHEL5 don't.
> diff --git a/tests/btrfs/311 b/tests/btrfs/311
> new file mode 100755
> index 000000000..a7fa541c4
> --- /dev/null
> +++ b/tests/btrfs/311
> @@ -0,0 +1,105 @@
> +#! /bin/bash
> +# SPDX-License-Identifier: GPL-2.0
> +# Copyright (C) 2024 Meta, Inc. All Rights Reserved.
> +#
> +# FS QA Test 311
> +#
> +# Test an edge case of multi device volume management in btrfs.
> +# If a device changes devt between mounts of a multi device fs, we can trick
> +# btrfs into mounting the same device twice fully (not as a bind mount). From
> +# there, it is trivial to induce corruption.
> +#
> +. ./common/preamble
> +_begin_fstest auto quick volume scrub
> +
> +# real QA test starts here
> +_supported_fs btrfs
> +_require_test
> +_require_command "$PARTED_PROG" parted
> +_require_batched_discard "$TEST_DIR"
> +
> +_cleanup() {
> +       cd /
> +       $UMOUNT_PROG $MNT
> +       $UMOUNT_PROG $BIND
> +       losetup -d $DEV0
> +       losetup -d $DEV1
> +       losetup -d $DEV2
> +       rm $IMG0
> +       rm $IMG1
> +       rm $IMG2
> +}
> +
> +IMG0=$TEST_DIR/$$.img0
> +IMG1=$TEST_DIR/$$.img1
> +IMG2=$TEST_DIR/$$.img2
> +truncate -s 1G $IMG0
> +truncate -s 1G $IMG1
> +truncate -s 1G $IMG2
> +DEV0=$(losetup -f $IMG0 --show)
> +DEV1=$(losetup -f $IMG1 --show)
> +DEV2=$(losetup -f $IMG2 --show)
> +D0P1=$DEV0"p1"
> +D1P1=$DEV1"p1"
> +MNT=$TEST_DIR/mnt
> +BIND=$TEST_DIR/bind
> +
> +# Setup partition table with one partition on each device.
> +$PARTED_PROG $DEV0 'mktable gpt' --script
> +$PARTED_PROG $DEV1 'mktable gpt' --script
> +$PARTED_PROG $DEV0 'mkpart mypart 1M 100%' --script
> +$PARTED_PROG $DEV1 'mkpart mypart 1M 100%' --script
> +
> +# mkfs with two devices to avoid clearing devices on close
> +# single raid to allow removing DEV2.
> +$MKFS_BTRFS_PROG -f -msingle -dsingle $D0P1 $DEV2 >>$seqres.full 2>&1 || _fail "failed to mkfs.btrfs"
> +
> +# Cycle mount the two device fs to populate both devices into the
> +# stale device cache.
> +mkdir -p $MNT
> +_mount $D0P1 $MNT
> +$UMOUNT_PROG $MNT
> +
> +# Swap the partition dev_ts. This leaves the dev_t in the cache out of date.
> +$PARTED_PROG $DEV0 'rm 1' --script
> +$PARTED_PROG $DEV1 'rm 1' --script
> +$PARTED_PROG $DEV1 'mkpart mypart 1M 100%' --script
> +$PARTED_PROG $DEV0 'mkpart mypart 1M 100%' --script
> +
> +# Mount with mismatched dev_t!
> +_mount $D0P1 $MNT || _fail "failed to remount; don't proceed and do dangerous stuff on raw mount point"
> +
> +# Remove the extra device to bring temp-fsid back in the fray.
> +$BTRFS_UTIL_PROG device remove $DEV2 $MNT
> +
> +# Create the would be bind mount.
> +mkdir -p $BIND
> +_mount $D0P1 $BIND
> +mount_show=$($BTRFS_UTIL_PROG filesystem show $MNT)
> +bind_show=$($BTRFS_UTIL_PROG filesystem show $BIND)
> +# If they're different, we are in trouble.
> +[ "$mount_show" = "$bind_show" ] || echo "$mount_show != $bind_show"
> +
> +# Now really prove it by corrupting the first mount with the second.
> +for i in $(seq 20); do
> +       $XFS_IO_PROG -f -c "pwrite 0 50M" $MNT/foo.$i >>$seqres.full 2>&1
> +done
> +for i in $(seq 20); do
> +       $XFS_IO_PROG -f -c "pwrite 0 50M" $BIND/foo.$i >>$seqres.full 2>&1
> +done
> +
> +# sync so that we really write the large file data out to the shared device
> +sync
> +
> +# now delete from one view of the shared device
> +find $BIND -type f -delete
> +# sync so that fstrim definitely has deleted data to trim
> +sync
> +# This should blow up both mounts, if the writes somehow didn't overlap at all.
> +$FSTRIM_PROG $BIND
> +# drop caches to improve the odds we read from the corrupted device while scrubbing.
> +echo 3 > /proc/sys/vm/drop_caches
> +$BTRFS_UTIL_PROG scrub start -B $MNT | grep "Error summary:"
> +
> +status=0
> +exit
> diff --git a/tests/btrfs/311.out b/tests/btrfs/311.out
> new file mode 100644
> index 000000000..70a6db809
> --- /dev/null
> +++ b/tests/btrfs/311.out
> @@ -0,0 +1,2 @@
> +QA output created by 311
> +Error summary:    no errors found
> --
> 2.43.0
>
>