apt: keep the keyring cache private to the invoking user

The shared world-writable /tmp/pkh-keyrings directory, combined with the
skip-if-exists logic, let any local user pre-plant keyrings that pkh
then trusts into the chroot's trusted.gpg.d. Use a per-uid 0700
directory instead, refuse to reuse a pre-existing directory that is not
owned by the current user or is group/other-writable, and drop the now
unnecessary world-accessibility chmods (mmdebstrap in unshare mode runs
with the same real uid).

src/apt/keyring.rs

@@ -7,6 +7,7 @@ use crate::context;
 use crate::distro_info;
 use serde::Deserialize;
 use std::error::Error;
+use std::os::unix::fs::MetadataExt;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
@@ -47,18 +48,47 @@ pub async fn download_cache_keyrings(
     // Use system temp directory for keyrings since it's accessible from unshare mode
     // The home directory may not be accessible from mmdebstrap's unshare namespace
     let temp_dir = std::env::temp_dir();
-    let keyring_dir = temp_dir.join("pkh-keyrings");
+    // Name the cache directory per-uid: a single shared /tmp directory would
+    // be writable by any local user, and the skip-if-exists logic below
+    // trusts pre-existing keyrings, so it must never be shared.
+    let euid = current_euid();
+    let keyring_dir = temp_dir.join(format!("pkh-keyrings-{euid}"));
 
-    // Create keyring directory if it doesn't exist
-    if !ctx.exists(&keyring_dir)? {
-        ctx.command("mkdir").arg("-p").arg(&keyring_dir).status()?;
+    if ctx.exists(&keyring_dir)? {
+        if let context::ContextConfig::Local = ctx.config {
+            // Cached keyrings are trusted as-is whenever they already exist,
+            // so refuse to reuse a directory that is not owned by the current
+            // user or is writable by group/others (it could have been planted
+            // by another local user).
+            let metadata = std::fs::symlink_metadata(&keyring_dir)?;
+            validate_keyring_dir(metadata.uid(), metadata.mode(), euid).map_err(|reason| {
+                format!(
+                    "Refusing to use keyring cache directory {}: {reason}; \
+                     remove the directory and re-run pkh",
+                    keyring_dir.display()
+                )
+            })?;
+        } else {
+            // Remote contexts (e.g. ssh) have no stat/metadata access through
+            // the context API, so the ownership guard cannot be performed;
+            // keep the previous best-effort behavior of tightening the
+            // directory permissions instead (0700 instead of the former
+            // world-writable a+rwx).
+            ctx.command("chmod").arg("700").arg(&keyring_dir).status()?;
         }
-
-    // Make keyring directory world-accessible so mmdebstrap in unshare mode can access it
-    ctx.command("chmod")
-        .arg("a+rwx")
+    } else {
+        // Create the directory private to the invoking user (0700). This is
+        // sufficient for mmdebstrap in unshare mode: it runs with the same
+        // real uid (the user namespace only maps that uid to root, file
+        // access still happens as the real uid), so no world-accessible
+        // permissions are needed.
+        ctx.command("mkdir")
+            .arg("-p")
+            .arg("-m")
+            .arg("700")
             .arg(&keyring_dir)
             .status()?;
+    }
 
     for keyring_url in keyring_urls {
         // Extract the original filename from the keyring URL
@@ -116,9 +146,6 @@ pub async fn download_cache_keyrings(
                 let _ = ctx.command("rm").arg("-f").arg(&download_path).status();
             }
 
-            // Make the keyring file world-readable so mmdebstrap in unshare mode can access it
-            ctx.command("chmod").arg("a+r").arg(&binary_path).status()?;
-
             log::info!(
                 "Successfully downloaded keyring for {} to {}",
                 series,
@@ -129,8 +156,6 @@ pub async fn download_cache_keyrings(
                 "Keyring already exists at {}, skipping download",
                 binary_path.display()
             );
-            // Ensure existing keyring is world-readable
-            ctx.command("chmod").arg("a+r").arg(&binary_path).status()?;
         }
     }
 
@@ -143,6 +168,67 @@ pub async fn download_cache_keyrings(
     Ok(keyring_dir)
 }
 
+/// Effective uid of the current process
+fn current_euid() -> u32 {
+    unsafe { libc::geteuid() }
+}
+
+/// Check that an existing keyring cache directory is safe to reuse
+///
+/// Cached keyrings are trusted whenever the files already exist (see the
+/// skip-if-exists logic in [`download_cache_keyrings`]), so the directory
+/// must be owned by the current user and must not be writable by group or
+/// others, otherwise another local user could plant a malicious keyring.
+///
+/// Takes the directory's owner uid and permission mode (e.g. from
+/// `std::fs::symlink_metadata`) so it can be unit tested without touching
+/// the filesystem.
+fn validate_keyring_dir(dir_uid: u32, mode: u32, euid: u32) -> Result<(), String> {
+    if dir_uid != euid {
+        return Err(format!(
+            "owned by uid {dir_uid}, not by the current user (uid {euid})"
+        ));
+    }
+    if mode & 0o022 != 0 {
+        return Err(format!(
+            "writable by group or others (permissions {:04o})",
+            mode & 0o7777
+        ));
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_validate_keyring_dir_accepts_private_dir_owned_by_current_user() {
+        assert!(validate_keyring_dir(1000, 0o700, 1000).is_ok());
+        assert!(validate_keyring_dir(1000, 0o750, 1000).is_ok());
+        assert!(validate_keyring_dir(1000, 0o1744, 1000).is_ok());
+        assert!(validate_keyring_dir(0, 0o700, 0).is_ok());
+    }
+
+    #[test]
+    fn test_validate_keyring_dir_rejects_foreign_owner() {
+        let err = validate_keyring_dir(1000, 0o700, 1001).unwrap_err();
+        assert!(err.contains("owned by uid 1000"));
+        let err = validate_keyring_dir(1001, 0o700, 1000).unwrap_err();
+        assert!(err.contains("owned by uid 1001"));
+    }
+
+    #[test]
+    fn test_validate_keyring_dir_rejects_group_or_other_writable() {
+        assert!(validate_keyring_dir(1000, 0o770, 1000).is_err());
+        assert!(validate_keyring_dir(1000, 0o706, 1000).is_err());
+        assert!(validate_keyring_dir(1000, 0o707, 1000).is_err());
+        assert!(validate_keyring_dir(1000, 0o777, 1000).is_err());
+        // Sticky bit does not neutralize the group/other write bits.
+        assert!(validate_keyring_dir(1000, 0o1777, 1000).is_err());
+    }
+}
+
 /// Download and import a PPA key using Launchpad API
 ///
 /// # Arguments

src/prune.rs

@@ -11,7 +11,8 @@
 //!   unmounting before they can be removed.
 //! - **Cached chroot tarballs** (`~/.cache/pkh/*-buildd.tar.xz`) and their
 //!   **stale download lockfiles** (`~/.cache/pkh/*.lock`).
-//! - **The shared apt keyring directory** (`/tmp/pkh-keyrings`), used by
+//! - **The apt keyring cache directories** (`pkh-keyrings` and the per-uid
+//!   `pkh-keyrings-<uid>` under the system temp directory), used by
 //!   mmdebstrap runs.
 //! - **Build logs** (`~/.cache/pkh/logs/deb-*.log`) written by `pkh deb`. By
 //!   default only logs beyond a small retention window (the newest
@@ -67,7 +68,8 @@ pub(crate) const KEEP_LOGS: usize = 10;
 enum Artifact {
     /// A residual build/chroot directory under the system temp dir.
     TempDir(PathBuf),
-    /// The shared apt keyring directory (`/tmp/pkh-keyrings`).
+    /// The apt keyring cache directory (`pkh-keyrings*` under the system
+    /// temp dir).
     KeyringDir(PathBuf),
     /// A stale chroot tarball download lockfile.
     LockFile(PathBuf),
@@ -105,11 +107,24 @@ pub fn cache_dir() -> Option<PathBuf> {
     ProjectDirs::from("com", "pkh", "pkh").map(|d| d.cache_dir().to_path_buf())
 }
 
+/// Check whether a directory entry name is the apt keyring cache directory,
+/// i.e. the legacy shared `pkh-keyrings` name or the per-uid
+/// `pkh-keyrings-<uid>` used by current pkh versions.
+fn is_keyring_dir_name(name: &str) -> bool {
+    if name == "pkh-keyrings" {
+        return true;
+    }
+    match name.strip_prefix("pkh-keyrings-") {
+        Some(uid) => !uid.is_empty() && uid.bytes().all(|b| b.is_ascii_digit()),
+        None => false,
+    }
+}
+
 /// Check whether a directory entry name is a residual pkh build directory,
 /// i.e. it matches `pkh-<digits>` or `pkh-<digits>-<digits>`.
 ///
-/// This deliberately rejects the `pkh-keyrings` and `pkh-build-*` names so they
-/// are not mistaken for residual build chroots.
+/// This deliberately rejects the `pkh-keyrings*` and `pkh-build-*` names so
+/// they are not mistaken for residual build chroots.
 fn is_residual_temp_dir(name: &str) -> bool {
     let Some(rest) = name.strip_prefix("pkh-") else {
         return false;
@@ -189,7 +204,7 @@ fn discover_artifacts(temp_dir: &Path, cache_dir: Option<&Path>) -> Vec<Artifact
             }
             if is_residual_temp_dir(&name) {
                 artifacts.push(Artifact::TempDir(path));
-            } else if name == "pkh-keyrings" {
+            } else if is_keyring_dir_name(&name) {
                 artifacts.push(Artifact::KeyringDir(path));
             }
         }
@@ -404,6 +419,7 @@ mod tests {
 
         // Keyrings and unshare work dirs must NOT match.
         assert!(!is_residual_temp_dir("pkh-keyrings"));
+        assert!(!is_residual_temp_dir("pkh-keyrings-1000"));
         assert!(!is_residual_temp_dir("pkh-build-1700000000"));
 
         // Non-numeric / malformed names.
@@ -415,6 +431,21 @@ mod tests {
         assert!(!is_residual_temp_dir("other-123"));
     }
 
+    #[test]
+    fn test_is_keyring_dir_name() {
+        // Legacy shared name and current per-uid names are matched.
+        assert!(is_keyring_dir_name("pkh-keyrings"));
+        assert!(is_keyring_dir_name("pkh-keyrings-0"));
+        assert!(is_keyring_dir_name("pkh-keyrings-1000"));
+
+        // Everything else is not.
+        assert!(!is_keyring_dir_name("pkh-keyrings-"));
+        assert!(!is_keyring_dir_name("pkh-keyrings-abc"));
+        assert!(!is_keyring_dir_name("pkh-keyrings-1000-2"));
+        assert!(!is_keyring_dir_name("keyrings"));
+        assert!(!is_keyring_dir_name("pkh-1000"));
+    }
+
     #[test]
     fn test_unescape_mountpath() {
         assert_eq!(unescape_mountpath("/tmp/simple"), "/tmp/simple");
@@ -455,8 +486,9 @@ none /tmp/other proc rw 0 0
         // Residual build dirs.
         fs::create_dir_all(temp_path.join("pkh-1700000000")).unwrap();
         fs::create_dir_all(temp_path.join("pkh-1700000001-2")).unwrap();
-        // Keyring dir.
+        // Keyring dirs (legacy shared name and per-uid name).
         fs::create_dir_all(temp_path.join("pkh-keyrings")).unwrap();
+        fs::create_dir_all(temp_path.join("pkh-keyrings-1000")).unwrap();
         // Non-matching entries that should be ignored.
         fs::create_dir_all(temp_path.join("pkh-build-1700000000")).unwrap();
         fs::create_dir_all(temp_path.join("other-dir")).unwrap();
@@ -477,21 +509,21 @@ none /tmp/other proc rw 0 0
         let artifacts = discover_artifacts(temp_path, Some(cache_path));
 
         let mut temp_dirs = 0;
-        let mut keyring = false;
+        let mut keyring_dirs = 0;
         let mut locks = 0;
         let mut tarballs = 0;
         let mut logs = 0;
         for a in &artifacts {
             match a {
                 Artifact::TempDir(_) => temp_dirs += 1,
-                Artifact::KeyringDir(_) => keyring = true,
+                Artifact::KeyringDir(_) => keyring_dirs += 1,
                 Artifact::LockFile(_) => locks += 1,
                 Artifact::Tarball(_) => tarballs += 1,
                 Artifact::LogFile(_) => logs += 1,
             }
         }
         assert_eq!(temp_dirs, 2);
-        assert!(keyring);
+        assert_eq!(keyring_dirs, 2);
         assert_eq!(locks, 1);
         assert_eq!(tarballs, 2);
         assert_eq!(logs, 1);
@@ -564,6 +596,7 @@ none /tmp/other proc rw 0 0
         let dir = temp_path.join("pkh-1700000000");
         fs::create_dir_all(&dir).unwrap();
         fs::create_dir_all(temp_path.join("pkh-keyrings")).unwrap();
+        fs::create_dir_all(temp_path.join("pkh-keyrings-1000")).unwrap();
         fs::write(cache_path.join("noble-buildd.tar.xz"), "tarball").unwrap();
         fs::write(cache_path.join("noble-buildd.tar.lock"), "lock").unwrap();
 
@@ -587,6 +620,12 @@ none /tmp/other proc rw 0 0
                 .iter()
                 .any(|p| p == &temp_path.join("pkh-keyrings"))
         );
+        assert!(
+            report
+                .removed
+                .iter()
+                .any(|p| p == &temp_path.join("pkh-keyrings-1000"))
+        );
         assert!(
             report
                 .removed
@@ -604,6 +643,7 @@ none /tmp/other proc rw 0 0
         // Nothing was actually removed.
         assert!(dir.exists());
         assert!(temp_path.join("pkh-keyrings").exists());
+        assert!(temp_path.join("pkh-keyrings-1000").exists());
         assert!(cache_path.join("noble-buildd.tar.xz").exists());
         assert!(cache_path.join("noble-buildd.tar.lock").exists());
     }
@@ -619,6 +659,7 @@ none /tmp/other proc rw 0 0
         let cache_path = cache.path();
 
         fs::create_dir_all(temp_path.join("pkh-keyrings")).unwrap();
+        fs::create_dir_all(temp_path.join("pkh-keyrings-1000")).unwrap();
         fs::write(cache_path.join("noble-buildd.tar.lock"), "lock").unwrap();
         fs::write(cache_path.join("noble-buildd.tar.xz"), "tarball").unwrap();
 
@@ -639,6 +680,12 @@ none /tmp/other proc rw 0 0
                 .iter()
                 .any(|p| p == &temp_path.join("pkh-keyrings"))
         );
+        assert!(
+            report
+                .removed
+                .iter()
+                .any(|p| p == &temp_path.join("pkh-keyrings-1000"))
+        );
         assert!(
             report
                 .removed
@@ -647,6 +694,7 @@ none /tmp/other proc rw 0 0
         );
 
         assert!(!temp_path.join("pkh-keyrings").exists());
+        assert!(!temp_path.join("pkh-keyrings-1000").exists());
         assert!(!cache_path.join("noble-buildd.tar.lock").exists());
         // Tarball is preserved when --all is not set.
         assert!(cache_path.join("noble-buildd.tar.xz").exists());