From 720754a50446df9c158c14919ea2959d70c9c9fe Mon Sep 17 00:00:00 2001
From: Kevin McCormick <github@mccormicklabs.com>
Date: Thu, 9 Mar 2017 11:55:57 -0800
Subject: [PATCH] ssh mode: use forced backup-zfs-shell

Ensure the backup connection to the SSH server can't do anything other
than designed. Uses ssh's authorized_keys command= statement to launch
backup-zfs-shell, which then supports only the handful of operations
required to perform backups.

$destpath is no longer used, but still included in the documentation
until it can be entirely written out.
---
 backup-zfs       | 20 ++++++++++----------
 backup-zfs-shell | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 10 deletions(-)
 create mode 100755 backup-zfs-shell

diff --git a/backup-zfs b/backup-zfs
index b37f1fc..ea33b21 100755
--- a/backup-zfs
+++ b/backup-zfs
@@ -148,7 +148,7 @@ if $tossh ; then
 	### get newest snapshot on dest - it must exist on src
 	###
 	#last="$(ZFS "$desthost" list -d 1 -t snapshot -H -S creation -o name $destfs/$srcbase | head -n1 | cut -f2 -d@)"
-	last="$(ssh "$desthost" cat "$destpath/.last")"
+	last="$(ssh "$desthost" zfslast)"
 
 	###
 	### send
@@ -166,14 +166,14 @@ if $tossh ; then
 		if [[ -n $gpgid ]] ; then
 			ZFS "$srchost" send $send_opts -R -I "$last" "$cur" \
 				| gpg --trust-model always --encrypt --recipient "$gpgid" \
-				| ssh "$desthost" "cat > \"$destpath/${tag}_$date.zfssnap.gpg\"" \
+				| ssh "$desthost" zfswrite "${tag}_$date.zfssnap.gpg" \
 				|| die $? "zfs incremental send failed"
-			ssh "$desthost" "echo \"$snap\" > \"$destpath/.last\""
+			ssh "$desthost" zfslast "$snap"
 		else
 			ZFS "$srchost" send $send_opts -R -I "$last" "$cur" \
-				| ssh "$desthost" "cat > \"$destpath/${tag}_$date.zfssnap\"" \
+				| ssh "$desthost" zfswrite "${tag}_$date.zfssnap" \
 				|| die $? "zfs incremental send failed"
-			ssh "$desthost" "echo \"$snap\" > \"$destpath/.last\""
+			ssh "$desthost" zfslast "$snap"
 		fi
 	fi
 
@@ -201,13 +201,13 @@ elif $fromssh ; then
 	###
 	log "receiving incremental snapshot from $src to $dest"
 	#ZFS "$srchost" send $send_opts -R -I "$last" "$cur" | ZFS "$desthost" receive $recv_opts -Fue "$destfs" || die $? "zfs incremental send failed"
-	for file in $(ssh "$srchost" "find \"$srcpath\" -name \"*.zfssnap\" -o -name \"*.zfssnap.gpg\"") ; do
+	for file in $(ssh "$srchost" zfsfind "$srcpath" | sort) ; do
 		if [[ $file =~ \.gpg$ ]] ; then
-			ssh "$srchost" "cat \"$file\"" | gpg | ZFS "$desthost" receive $recv_opts -Fue "$dest" \
-				&& ssh "$srchost" "rm \"$file\""
+			ssh "$srchost" zfsget "$file" | gpg | ZFS "$desthost" receive $recv_opts -Fue "$dest" \
+				&& ssh "$srchost" rm "$file"
 		else
-			ssh "$srchost" "cat \"$file\"" | ZFS "$desthost" receive $recv_opts -Fue "$dest" \
-				&& ssh "$srchost" "rm \"$file\""
+			ssh "$srchost" zfsget "$file" | ZFS "$desthost" receive $recv_opts -Fue "$dest" \
+				&& ssh "$srchost" rm "$file"
 		fi
 	done
 
diff --git a/backup-zfs-shell b/backup-zfs-shell
new file mode 100755
index 0000000..e897435
--- /dev/null
+++ b/backup-zfs-shell
@@ -0,0 +1,33 @@
+#!/bin/bash
+
+# A restricted shell for backup-zfs's SSH mode. Must be installed on the
+# SSH server, and then configured with command="/path/to/backup-zfs-shell"
+# in the user's authorized_keys file.
+# TODO: tag & dest should support better customization
+dest=zfssnap.nobackup
+tag=frodo
+
+case "$SSH_ORIGINAL_COMMAND" in
+	zfslast)
+		exec cat $dest/.last ;;
+	zfslast\ ${tag}_20[12][0-9]-[01][0-9]-[0-3][0-9]_[012][0-9]:[0-5][0-9]:[0-5][0-9])
+		exec echo "${SSH_ORIGINAL_COMMAND/zfslast /}" > "$dest/.last" ;;
+	zfswrite\ ${tag}_20[12][0-9]-[01][0-9]-[0-3][0-9]_[012][0-9]:[0-5][0-9]:[0-5][0-9].zfssnap.gpg)
+		exec cat > "$dest/${SSH_ORIGINAL_COMMAND/zfswrite /}" ;;
+	zfswrite\ ${tag}_20[12][0-9]-[01][0-9]-[0-3][0-9]_[012][0-9]:[0-5][0-9]:[0-5][0-9].zfssnap)
+		exec cat > "$dest/${SSH_ORIGINAL_COMMAND/zfswrite /}" ;;
+	zfsget\ $dest/${tag}_20[12][0-9]-[01][0-9]-[0-3][0-9]_[012][0-9]:[0-5][0-9]:[0-5][0-9].zfssnap.gpg)
+		exec cat "${SSH_ORIGINAL_COMMAND/zfsget /}" ;;
+	zfsget\ $dest/${tag}_20[12][0-9]-[01][0-9]-[0-3][0-9]_[012][0-9]:[0-5][0-9]:[0-5][0-9].zfssnap)
+		exec cat "${SSH_ORIGINAL_COMMAND/zfsget /}" ;;
+	zfsfind)
+		exec find "$dest" -name "*.zfssnap" -o -name "*zfssnap.gpg" ;;
+	rm\ $dest/${tag}_20[12][0-9]-[01][0-9]-[0-3][0-9]_[012][0-9]:[0-5][0-9]:[0-5][0-9].zfssnap.gpg)
+		exec rm "${SSH_ORIGINAL_COMMAND/rm /}" ;;
+	rm\ $dest/${tag}_20[12][0-9]-[01][0-9]-[0-3][0-9]_[012][0-9]:[0-5][0-9]:[0-5][0-9].zfssnap)
+		exec rm "${SSH_ORIGINAL_COMMAND/rm /}" ;;
+	*)
+		echo "Command not allowed: $SSH_ORIGINAL_COMMAND" >&2
+		exit 1
+		;;
+esac