dstack-util: bound LUKS2 keyslot area to the metadata region

The in-memory header validator pinned the cipher/KDF shape but never
checked the keyslot area.offset/size — the byte range the encrypted key
material is read from. A host with raw disk access could redirect it
inside an otherwise-conforming header.

cryptsetup's digest check already stops this from yielding a usable key,
so it isn't exploitable on its own, but the validator shouldn't accept a
header pointing its key material somewhere arbitrary. Bound the area to
[2 * hdr_size, PAYLOAD_OFFSET) and reject anything outside it.

Also note why salts stay unchecked (high-entropy KMS-derived passphrase,
per #552) so the gap doesn't read as an oversight next time.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: h4x3rotab

dstack/dstack-util/src/system_setup.rs

@@ -2946,6 +2946,16 @@ fn validate_single_luks2_header(mut reader: impl std::io::Read, hdr_ind: u64) ->
         if area.encryption() != "aes-xts-plain64" {
             bail!("Invalid LUKS keyslot encryption: {}", area.encryption());
         }
+        // Pin where the encrypted key material is read from. The binary area
+        // must sit between the two header copies and the encrypted payload;
+        // otherwise a host with raw disk access could redirect it elsewhere.
+        if area.offset() < 2 * hdr_size || area.offset() + area.size() > PAYLOAD_OFFSET {
+            bail!(
+                "Invalid LUKS keyslot area: offset={} size={}",
+                area.offset(),
+                area.size()
+            );
+        }
         if *key_size != 64 {
             bail!("Invalid LUKS keyslot key size: {key_size}");
         }
@@ -2956,6 +2966,9 @@ fn validate_single_luks2_header(mut reader: impl std::io::Read, hdr_ind: u64) ->
             let LuksKdf::pbkdf2 {
                 hash,
                 iterations: _,
+                // Salts are left unchecked on purpose: the passphrase is
+                // high-entropy and KMS-derived, so an attacker-chosen salt
+                // buys nothing without it (see security report #552).
                 salt: _,
             } = kdf
             else {
@@ -3046,6 +3059,30 @@ fn test_validate_luks2_header() {
         .contains("Invalid LUKS keyslot encryption"));
 }
 
+#[test]
+fn test_validate_luks2_header_rejects_out_of_range_keyslot_area() {
+    // Redirect the keyslot binary area below the header region. Same length
+    // so the surrounding header stays intact; "00768" parses to 768, which is
+    // inside the header copies (< 2 * hdr_size) rather than the metadata gap.
+    let mut header = include_bytes!("../tests/fixtures/luks_header_good").to_vec();
+    let needle = br#""offset":"32768""#;
+    let replacement = br#""offset":"00768""#;
+    let mut patched = 0;
+    let mut i = 0;
+    while i + needle.len() <= header.len() {
+        if &header[i..i + needle.len()] == needle {
+            header[i..i + needle.len()].copy_from_slice(replacement);
+            patched += 1;
+            i += needle.len();
+        } else {
+            i += 1;
+        }
+    }
+    assert_eq!(patched, 2, "expected to patch both header copies");
+    let error = validate_luks2_headers(&mut &header[..]).unwrap_err();
+    assert!(error.to_string().contains("Invalid LUKS keyslot area"));
+}
+
 #[cfg(test)]
 fn test_app_compose(
     manifest_version: serde_json::Value,