ssh_filter_btrbk.sh is mainly intended for security purposes and should
therefore itself be written with that in mind.
It is written for bash, which greatly extends the POSIX Shell Command Language
and is incompatible with it in some niche cases.
For several reasons, it seems a good idea to convert the program to (mostly)
pure POSIX Shell Command Language:
• People may try to use the program with other shells (for example when bash is
not available) and make errors.
More pure `sh` implementations like dash …
• … have far less code and also less dependencies, which possibly also reduces
the chance for bugs or exploits,
• … are less dynamic in development (and have thus possibly a lower chance of
incompatible changes) …
• … and may run faster.
This commit replaces any unnecessary “bashishms” with purely POSIX compatible
code, with the exception of the `local`-built-in, which is however supported by
most POSIX compatible shells (including dash, klibc-utils’s `sh` and BusyBox’
`sh`) in some way.
Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Double quote any variable expansions that might ever contain field separators.
Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
In strings that don’t contain `'` nor do any expansions, use single quotes to
avoid any future unintended expansions or escapes.
Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
This disallows newline (that is: LF characters) in the SSH command, which could
have been exploited for arbitrary code execution, since commit
77a39282de.
Example:
# export SSH_ORIGINAL_COMMAND=$'readlink /dev/stdout\ncat /etc/shadow'
# ssh_filter_btrbk.sh
Since `readlink` is a generally allowed command, this works with any of
ssh_filter_btrbk.sh’s options.
But most likely, other commands that are “added” via `allow_cmd()` can be used,
too.
Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
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# Date: Wed Nov 30 04:29:53 2022 +0100
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# On branch fix-remote-code-execution
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This adds support for bzip3 [1].
[1] https://github.com/kspalaiologos/bzip3
Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Cosmetics: swap order pbzip2 / bzip3
Signed-off-by: Axel Burri <axel@tty0.ch>
Security vulnerability fixed in alternation regex. Specialy crafted
commands may be executed without being propely checked.
Affects all versions >= btrbk-v0.23.0
Regression from:
ccb5ed5e71 ssh_filter_btrbk: allow "realpath" and "cat /proc/self/mounts" on targets
Reported by: @protree (responsible disclosure)
Add compat, compat_local, compat_remote configuration options.
Used for busybox: instead of running `readlink -e` (which is not
available on busybox), run `readlink -f` followed by `test -d`.
btrbk requires "btrfs subvolume list|show" queries from the mount
point in order to build btrfs trees. This conflicts with tightly set
--restrict-path.
It is possible that the subvolume path is not accessible by the user
calling btrbk. When resolving mount points, "readlink" is used on the
path, which also needs to be wrapped with "sudo".
btrbk now runs "btrfs subvolume list" from the mountpoint instead of
the volume path, which for some users is not below --restrict-path. As
the output of "btrfs subvolume list" is the same (complete btrfs tree
for the filesystem), it is ok to ignore the restrict-path here.
Dropped readin of subvolid and realpath by btrfs_subvolume_show(), we
now always read /proc/self/mounts (and call readlink).
When picking the best common parent in get_best_parent(), we want to
list as many snapshots as possible. For now, we list all from the
mountpoint of snaproot ($sroot/<snapshot_dir>), due to a bug in
btrfs-progs [1]. Also added code (commented out) to list snapshots
from all known mountpoints.
[1] https://github.com/kdave/btrfs-progs/issues/96
bugfix for: 796b6bd9bf
Replace realpath with readlink in allowed commands. Commit 796b6bd substituted readlink for realpath in file "btrbk"; this commit propagates the change to ssh_filter_btrbk.sh.
Christoph Anton Mitterer