touch: drop O_TRUNC on create to close TOCTOU race (#10019)

src/uu/touch/src/touch.rs

@@ -4,7 +4,7 @@
 // file that was distributed with this source code.
 
 // spell-checker:ignore (ToDO) datelike datetime filetime lpszfilepath mktime strtime timelike utime DATETIME UTIME futimens
-// spell-checker:ignore (FORMATS) MMDDhhmm YYYYMMDDHHMM YYMMDDHHMM YYYYMMDDHHMMS
+// spell-checker:ignore (FORMATS) MMDDhhmm YYYYMMDDHHMM YYMMDDHHMM YYYYMMDDHHMMS CREAT
 
 pub mod error;
 
@@ -23,9 +23,8 @@ use rustix::fs::Timestamps;
 use rustix::fs::futimens;
 use std::borrow::Cow;
 use std::ffi::{OsStr, OsString};
-#[cfg(unix)]
+use std::fs;
 use std::fs::OpenOptions;
-use std::fs::{self, File};
 use std::io::{Error, ErrorKind};
 #[cfg(unix)]
 use std::os::unix::fs::OpenOptionsExt;
@@ -436,6 +435,20 @@ pub fn touch(files: &[InputFile], opts: &Options) -> Result<(), TouchError> {
     Ok(())
 }
 
+/// Create `path` if it does not exist, without ever truncating it.
+///
+/// Uses `O_CREAT` but deliberately not `O_TRUNC`: if an attacker plants a
+/// symlink at `path` in the window between the metadata check in
+/// [`touch_file`] and this open, the open follows it but must not zero the
+/// symlink's target. Matches GNU touch (issue #10019).
+fn create_without_truncate(path: &Path) -> std::io::Result<fs::File> {
+    OpenOptions::new()
+        .write(true)
+        .create(true)
+        .truncate(false)
+        .open(path)
+}
+
 /// Create or update the timestamp for a single file.
 ///
 /// # Arguments
@@ -486,7 +499,7 @@ fn touch_file(
             return Ok(());
         }
 
-        if let Err(e) = File::create(path) {
+        if let Err(e) = create_without_truncate(path) {
             // we need to check if the path is the path to a directory (ends with a separator)
             // we can't use File::create to create a directory
             // we cannot use path.is_dir() because it calls fs::metadata which we already called
@@ -981,4 +994,34 @@ mod tests {
         assert_eq!(actual_atime, atime);
         assert_eq!(actual_mtime, mtime);
     }
+
+    // The #10019 fix: the create-open must use O_CREAT without O_TRUNC. During
+    // the TOCTOU race the open lands on an *existing* file (the symlink's
+    // target), so opening an existing file must leave its contents intact. This
+    // deterministically distinguishes the fix from the old File::create, which
+    // used O_TRUNC and would zero the file here.
+    #[cfg(unix)]
+    #[test]
+    fn create_without_truncate_does_not_truncate_existing_file() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("victim");
+        std::fs::write(&path, b"do not truncate me").unwrap();
+
+        super::create_without_truncate(&path).unwrap();
+
+        assert_eq!(std::fs::read(&path).unwrap(), b"do not truncate me");
+    }
+
+    // The other half of the contract: when the path is missing it must be
+    // created (as an empty file), matching the old File::create behavior.
+    #[cfg(unix)]
+    #[test]
+    fn create_without_truncate_creates_missing_file() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("new");
+
+        super::create_without_truncate(&path).unwrap();
+
+        assert_eq!(std::fs::read(&path).unwrap(), b"");
+    }
 }

tests/by-util/test_touch.rs

@@ -1074,3 +1074,38 @@ fn test_touch_device_files() {
         .succeeds()
         .no_output();
 }
+
+// Touching a symlink to an existing file must not truncate the target, like
+// GNU touch. The target exists, so this exercises the update_times path, not
+// the create path changed for #10019 — it guards the general "touch never
+// truncates" contract. The create-path fix itself is covered by the
+// create_without_truncate unit tests in src/touch.rs and the syscall-flag
+// check in util/check-safe-traversal.sh.
+#[test]
+#[cfg(unix)]
+fn test_touch_does_not_truncate_symlink_target() {
+    use std::os::unix::fs::symlink;
+
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.write("victim", "do not truncate me");
+    symlink(at.plus("victim"), at.plus("link")).unwrap();
+
+    ucmd.arg("link").succeeds();
+
+    assert_eq!(at.read("victim"), "do not truncate me");
+}
+
+// Touching a dangling symlink creates its target as an empty file, like GNU.
+#[test]
+#[cfg(unix)]
+fn test_touch_through_dangling_symlink_creates_target() {
+    use std::os::unix::fs::symlink;
+
+    let (at, mut ucmd) = at_and_ucmd!();
+    symlink(at.plus("missing"), at.plus("link")).unwrap();
+
+    ucmd.arg("link").succeeds();
+
+    assert!(at.file_exists("missing"));
+    assert_eq!(at.read("missing"), "");
+}

util/check-safe-traversal.sh

@@ -191,7 +191,7 @@ if [ "$USE_MULTICALL" -eq 1 ]; then
     AVAILABLE_UTILS=$($COREUTILS_BIN --list)
 else
     AVAILABLE_UTILS=""
-    for util in rm chmod chown chgrp du mv cp; do
+    for util in rm chmod chown chgrp du mv cp touch; do
         if [ -f "$PROJECT_ROOT/target/${PROFILE}/$util" ]; then
             AVAILABLE_UTILS="$AVAILABLE_UTILS $util"
         fi
@@ -387,6 +387,28 @@ if echo "$AVAILABLE_UTILS" | grep -q "mv" && [ -d /dev/shm ]; then
     fi
 fi
 
+# Test touch - creating a file must use O_CREAT but never O_TRUNC, so that a
+# symlink planted in the metadata-check/open race window (#10019) is not
+# truncated. This observes the flags directly, which integration tests cannot.
+if echo "$AVAILABLE_UTILS" | grep -q "touch"; then
+    echo ""
+    echo "Testing touch (create_no_truncate)..."
+    if [ "$USE_MULTICALL" -eq 1 ]; then
+        touch_cmd="$COREUTILS_BIN touch"
+    else
+        touch_cmd="$PROJECT_ROOT/target/${PROFILE}/touch"
+    fi
+    strace -f -e trace=openat -o strace_touch_create.log $touch_cmd test_touch_new 2>/dev/null || true
+    cat strace_touch_create.log
+    if ! grep -q 'openat(.*test_touch_new.*O_CREAT' strace_touch_create.log; then
+        fail_immediately "touch did not create test_touch_new via openat(O_CREAT)"
+    fi
+    if grep 'test_touch_new' strace_touch_create.log | grep -q 'O_TRUNC'; then
+        fail_immediately "touch opened the target with O_TRUNC - vulnerable to truncating a symlink target (#10019)"
+    fi
+    echo "✓ touch creates with O_CREAT and without O_TRUNC"
+fi
+
 echo ""
 echo "✓ Basic safe traversal verification completed"
 echo ""