[Security] Stop the FIDO2 setup staging its authfile at a predictable /tmp path (backport of #7904)

Backport of the FIDO2 authfile fix (PR #7904 by @mdisec, merged to quattro as
23dab9ec) onto the v4-0-1 release branch.

pamu2fcfg wrote to /tmp/fido2 and the registration was then moved into place
with sudo mv. Any other local user can pre-create /tmp/fido2, and rename(2) does
not dereference the final component, so the privileged move installed the
attacker's symlink itself as pam_u2f's global authfile -- a file consulted by
sufficient lines in /etc/pam.d/sudo and /etc/pam.d/polkit-1.

The same move also carried the staged file's ownership into /etc, so on every
install to date /etc/fido2/fido2 is owned by the invoking user at mode 0644.
That needs no attacker, no race and no second account: anything running as that
uid can append its own credential and satisfy the machine's sudo prompt without
knowing the password.

The setup now creates a unique staging file as root beside the final authfile
and pipes pamu2fcfg into it, so root never reopens a caller-owned pathname,
rejects failed or empty enrollment output, publishes with an atomic mv -Tf,
cleans the exact staging file after every failure, refuses non-regular authfile
states, and installs root:root 0644. A migration repairs machines set up by
earlier versions by replacing the inode rather than chowning in place: a process
that already holds a writable descriptor on the legacy user-owned mapping keeps
it, so the repair has to leave that inode behind where PAM no longer reads it.

Clean cherry-pick: all six files are byte-identical to quattro, so merging
v4-0-1 into quattro resolves without a conflict. The migration's timestamp
(1787494718) is older than others already on this branch, which is harmless:
omarchy-migrate marks each migration by filename rather than tracking a
watermark, so this one runs on every machine that has not run this exact file.

test/shell passes: 195 files, including the three this adds -- the setup's
staging and failure paths, the migration's repair and unrepairable states, and
the removal's symlink handling. test/cli passes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018DEMYa9UWtroz93DhMTtcV

bin/omarchy-remove-security-fido2

@@ -24,9 +24,14 @@ echo -e "\e[32mRemoving FIDO2 device from authentication.\n\e[0m"
 
 remove_pam_config
 
-if [[ -d /etc/fido2 ]]; then
+authdir=/etc/fido2
+
+# -d follows symlinks, so a dangling link at /etc/fido2 would survive this and
+# a later setup would install the authfile through it. rm -rf on a symlink
+# removes the link itself, never the directory it points at.
+if [[ -e $authdir || -L $authdir ]]; then
   echo "Removing FIDO2 configuration..."
-  sudo rm -rf /etc/fido2
+  sudo rm -rf "$authdir"
 fi
 
 echo "Removing FIDO2 packages..."

bin/omarchy-setup-security-fido2

@@ -4,6 +4,7 @@
 # omarchy:requires-sudo=true
 
 set -e
+set -o pipefail
 
 
 check_fido2_hardware() {
@@ -50,13 +51,79 @@ if ! check_fido2_hardware; then
 fi
 
 # Create the pamu2fcfg file
-if [[ ! -f /etc/fido2/fido2 ]]; then
-  sudo mkdir -p /etc/fido2
+authdir=/etc/fido2
+authfile=/etc/fido2/fido2
+
+# install -d follows a symlink here and applies the mode and ownership to
+# whatever it points at, so the credential would be staged and published inside
+# the link target and that directory reopened to root:root 755. This is the
+# threat omarchy-remove-security-fido2 already names on its side.
+if [[ -L $authdir || ( -e $authdir && ! -d $authdir ) ]]; then
+  echo -e "\e[31m\n$authdir is not a FIDO2 configuration directory.\e[0m"
+  echo "Run omarchy-remove-security-fido2 first, then set FIDO2 up again."
+  exit 1
+fi
+
+# -f follows symlinks, so the already-registered check below reads a symlinked
+# authfile as a registration and leaves it in place, and is false for a
+# directory, so it tries to register over one. Only a regular file is a valid
+# pam_u2f authfile.
+if [[ -L $authfile || ( -e $authfile && ! -f $authfile ) ]]; then
+  echo -e "\e[31m\n$authfile is not a FIDO2 registration file.\e[0m"
+  echo "Run omarchy-remove-security-fido2 first, then set FIDO2 up again."
+  exit 1
+fi
+
+if [[ ! -f $authfile ]]; then
+  sudo install -d -m 755 -o root -g root "$authdir"
   echo -e "\e[32m\nLet's setup your device by confirming on the device now.\e[0m"
   echo -e "Touch your FIDO2 key when it lights up...\n"
 
-  if pamu2fcfg >/tmp/fido2; then
-    sudo mv /tmp/fido2 /etc/fido2/fido2
+  # A unique sibling created by root cannot be replaced by another process
+  # running as this user. Stream pamu2fcfg into it instead of asking root to
+  # reopen a caller-owned path: an observed temporary name could otherwise be
+  # replaced with a symlink before the privileged copy. The final rename is
+  # atomic, and -T refuses a directory at the destination. Mode 644 keeps the
+  # root-owned global authfile readable when pam_u2f uses openasuser; only root
+  # can still rewrite it.
+  stage=""
+
+  # mktemp's output is an operand for four privileged commands below, one of
+  # them an rm. Take only the name this script asked for rather than whatever
+  # came back on stdout.
+  safe_stage_path() {
+    local candidate=$1
+    local prefix="$authfile.new."
+    local suffix
+
+    [[ $candidate == "$prefix"* ]] || return 1
+    suffix=${candidate#"$prefix"}
+    [[ $suffix =~ ^[[:alnum:]]{6}$ ]]
+  }
+
+  cleanup_stage() {
+    local status=$?
+
+    if safe_stage_path "$stage"; then
+      sudo rm -f -- "$stage" || true
+    fi
+
+    return "$status"
+  }
+
+  trap cleanup_stage EXIT
+  stage=$(sudo mktemp "$authfile.new.XXXXXX")
+
+  if ! safe_stage_path "$stage" || [[ ! -f $stage || -L $stage ]]; then
+    echo -e "\e[31m\nCould not create a safe staging file beside $authfile.\e[0m"
+    exit 1
+  fi
+
+  if pamu2fcfg | sudo tee "$stage" >/dev/null && [[ -s $stage ]]; then
+    sudo chmod 644 "$stage"
+    sudo mv -Tf "$stage" "$authfile"
+    stage=""
+    trap - EXIT
     echo -e "\e[32mFIDO2 device registered successfully!\e[0m"
   else
     echo -e "\e[31m\nFIDO2 registration failed. Please try again.\e[0m"