fix(svg): refuse a document compressed twice and charge a refused embedded one (#1924)

One gzip layer is unpacked within the import budget, but usvg gunzips what
it is handed again with no limit, so a doubly compressed document or
embedded image expanded outside the budget. A compressed embedded image
that failed its budget also left the shared allowance untouched, so every
sibling image could repeat the work; a refusal now spends what was left.

crates/svg/src/import/budget.rs

@@ -44,6 +44,11 @@ fn inflate(data: &[u8], limit: usize) -> Result<Vec<u8>, String> {
     if out.len() > limit {
         return Err(format!("compressed SVG expands past {} MiB", limit >> 20));
     }
+    // usvg gunzips what it is handed, with no limit: one layer is unpacked here, within the
+    // budget, and what comes out must be the document itself.
+    if gzipped(&out) {
+        return Err("compressed SVG holds another compressed file, not a document".into());
+    }
     Ok(out)
 }
 
@@ -72,7 +77,15 @@ fn embedded(used: &Used, limits: &Limits, data: &[u8], opts: &Options) -> Option
         return None;
     }
     let left = limits.embedded.saturating_sub(used.bytes.load(Ordering::Relaxed));
-    let plain = inflate(data, left).ok()?;
+    let plain = match inflate(data, left) {
+        Ok(plain) => plain,
+        // What a refused document unpacked before it was refused was work too: it uses up what is
+        // left, so the next image can't repeat it.
+        Err(_) => {
+            used.bytes.fetch_add(left, Ordering::Relaxed);
+            return None;
+        }
+    };
     used.bytes.fetch_add(plain.len(), Ordering::Relaxed);
     if super::too_deep(&plain) {
         return None;

crates/svg/src/import/budget_tests.rs

@@ -111,3 +111,31 @@ fn linked_files_and_other_mime_types_stay_unresolved() {
     );
     assert_eq!(layers(data.as_bytes(), Limits::default()).unwrap(), layers(svg("").as_bytes(), Limits::default()).unwrap());
 }
+
+#[test]
+fn a_document_compressed_twice_is_refused_not_unpacked_again() {
+    // One gzip layer is unpacked within the budget; usvg would unpack a second one without any.
+    let twice = gzip(&gzip(svg(&" ".repeat(1 << 20)).as_bytes()));
+    let e = layers(&twice, Limits { document: 4 << 20, ..Limits::default() }).unwrap_err();
+    assert!(e.contains("another compressed file"), "{e}");
+    // Embedded, the same is dropped.
+    let inner = svg(r##"<rect width="10" height="10"/>"##);
+    let data = svg(&format!(
+        r##"<image width="10" height="10" xlink:href="data:image/svg+xml;base64,{}"/>"##,
+        base64::engine::general_purpose::STANDARD.encode(gzip(&gzip(inner.as_bytes())))
+    ))
+    .into_bytes();
+    assert_eq!(layers(&data, Limits::default()).unwrap(), layers(svg("").as_bytes(), Limits::default()).unwrap());
+}
+
+#[test]
+fn a_refused_embedded_bomb_uses_up_the_budget_for_the_images_after_it() {
+    let bomb = gzip(svg(&" ".repeat(1 << 20)).as_bytes());
+    let inner = svg(r##"<rect width="10" height="10"/>"##);
+    let image = |href: String| format!(r##"<image width="10" height="10" xlink:href="{href}"/>"##);
+    let bomb_uri = format!("data:image/svg+xml;base64,{}", base64::engine::general_purpose::STANDARD.encode(&bomb));
+    let data = svg(&format!("{}{}", image(bomb_uri).repeat(8), image(uri(inner.as_bytes())))).into_bytes();
+    let limits = Limits { embedded: 4096, ..Limits::default() };
+    let empty = layers(svg("").as_bytes(), limits).unwrap();
+    assert_eq!(layers(&data, limits).unwrap(), empty, "the first bomb spends the allowance; nothing after it unpacks");
+}