diff --git a/deployment/proxmox/provision.sh b/deployment/proxmox/provision.sh
index 30f5f6f..6daadba 100755
--- a/deployment/proxmox/provision.sh
+++ b/deployment/proxmox/provision.sh
@@ -46,7 +46,8 @@ Others:
 EOF
 }
 
-die () { printf "$@"; printf '\n'; help; exit 2; }
+die () { printf '\033[31m'; printf "$@"; printf '\033[0m\n'; exit 2; }
+die_with_help () { printf '\033[31m'; printf "$@"; printf '\033[0m\n'; help; exit 2; }
 
 while [ $# -gt 0 ]; do
   argument=$1
@@ -61,7 +62,7 @@ while [ $# -gt 0 ]; do
 
     -h|-\?|--help) help; exit 0 ;;
 
-    -*) die 'Unknown argument: `%s`.' "$argument" ;;
+    -*) die_with_help 'Unknown argument: `%s`.' "$argument" ;;
 
     *) vmids="$vmids $argument" ;;
   esac
@@ -71,7 +72,7 @@ if [ -z "$username" ] || [ -z "$password" ]; then
   if [ -f .proxmox ]; then
     { read username; read password; } < .proxmox
   else
-    die 'Required: `--username` and `--password`.\n'
+    die_with_help 'Required: `--username` and `--password`.\n'
   fi
 fi
 
@@ -103,35 +104,48 @@ readonly ticket=$(echo "$response" | jq -r .data.ticket)
 readonly csrfToken=$(echo "$response" | jq -r .data.CSRFPreventionToken)
 printf ' done.\n'
 
+acquire_lock () {
+  until mkdir $tmpdir/lock-$1 2>/dev/null; do sleep 1; done
+}
+release_lock () {
+  rmdir $tmpdir/lock-$1
+}
+
 proxmox () {
+  acquire_lock proxmox
   http \
     --form \
     --verify no \
+    --ignore-stdin \
     "$@" \
     "Cookie:PVEAuthCookie=$ticket" \
     "CSRFPreventionToken:$csrfToken"
+  release_lock proxmox
 }
 
 ## Synchronous variant for when the `proxmox` function would just respond an
 ## UPID in the `data` JSON field.
-proxmox_sync () {
+proxmox_sync () (
   response=$(proxmox "$@")
   upid=$(echo "$response" | jq -r .data)
+
   while :; do
     response=$(proxmox GET $apiurl/nodes/$node/tasks/$upid/status)
     status=$(echo "$response" | jq -r .data.status)
+
     case $status in
       running) sleep 1 ;;
       stopped) break ;;
       *) die 'unexpected status: `%s`' "$status" ;;
     esac
   done
-}
+)
 
 ################################################################################
 ## Build ISO
 
 build_iso () {
+  acquire_lock build
   printf 'Building ISO for VM %d...\n' $1
 
   nix build \
@@ -142,12 +156,14 @@ build_iso () {
   ln -sf $tmpdir/installer-fedi$1/iso/installer.iso $tmpdir/installer-fedi$1.iso
 
   printf 'done building ISO for VM %d.\n' $1
+  release_lock build
 }
 
 ################################################################################
 ## Upload ISO
 
 upload_iso () {
+  acquire_lock upload
   printf 'Uploading ISO for VM %d...\n' $1
 
   proxmox_sync POST $apiurl/nodes/$node/storage/local/upload \
@@ -155,6 +171,7 @@ upload_iso () {
     content==iso
 
   printf 'done uploading ISO for VM %d.\n' $1
+  release_lock upload
 }
 
 ################################################################################
@@ -201,7 +218,7 @@ create_vm () {
 ################################################################################
 ## Install VM
 
-install_vm () {
+install_vm () (
   printf 'Installing VM %d...\n' $1
 
   proxmox_sync POST $apiurl/nodes/$node/qemu/$1/status/start
@@ -217,7 +234,7 @@ install_vm () {
   done
 
   printf 'done installing VM %d.\n' $1
-}
+)
 
 ################################################################################
 ## Start VM
@@ -252,8 +269,9 @@ provision_vm () {
 }
 
 for vmid in $vmids; do
-  provision_vm $vmid
+  provision_vm $vmid &
 done
+wait
 
 printf 'done provisioning VMs%s.\n' "$vmids"