fix: 主题包校验 gzip 校验和，被改过的包不再能装上

src/frontend.rs

@@ -257,6 +257,11 @@ const MAX_ENTRIES: usize = 2_000;
 const MAX_FILE: u64 = 8 << 20;
 const MAX_EXPANDED: u64 = 64 << 20;
 
+/// What may follow tar's end marker, which is padding to a whole record: 10 KiB
+/// by default, and a mebibyte covers any blocking factor in use. Unbounded,
+/// zeros there would inflate at 1 GiB per MiB uploaded, 1.4 s of CPU each.
+const MAX_PADDING: u64 = 1 << 20;
+
 /// Installs a theme from its published `theme.tar.gz`, under the name its own
 /// manifest carries.
 ///
@@ -335,6 +340,14 @@ fn unpack<R: Read>(archive: R, into: &Path) -> Result<()> {
             refuse!("主题包里的路径越出了主题目录");
         }
     }
+    // Read to the end, where gzip keeps its checksum. The entries stop at tar's
+    // end marker, short of it, so otherwise an archive whose bytes changed in
+    // transit would install as long as its headers survived.
+    let mut rest = archive.into_inner();
+    std::io::copy(&mut (&mut rest).take(MAX_PADDING), &mut std::io::sink()).map_err(archive_error)?;
+    if rest.read(&mut [0]).map_err(archive_error)? != 0 {
+        refuse!("主题包在 tar 结尾之后还有超过 1 MiB 的数据，包本身有问题，请联系主题作者");
+    }
     Ok(())
 }
 
@@ -534,6 +547,28 @@ mod tests {
             panic!("half an archive installed")
         };
         assert_eq!(e.downcast_ref::<crate::Shown>().map(|s| s.0.as_str()), Some(DAMAGED), "{e:#}");
+        // Cut past tar's end marker, or with one byte changed on the way: every
+        // header reads, so only the gzip checksum can refuse either.
+        let mut altered = whole.clone();
+        let crc = altered.len() - 8;
+        altered[crc] ^= 1;
+        for damaged in [&whole[..whole.len() - 4], &altered[..]] {
+            let Err(e) = install(&base, damaged, None) else { panic!("a damaged archive installed") };
+            assert_eq!(e.downcast_ref::<crate::Shown>().map(|s| s.0.as_str()), Some(DAMAGED), "{e:#}");
+        }
+        // Past the end marker, padding and nothing more.
+        let mut padded = tar::Builder::new(Vec::new());
+        let mut header = tar::Header::new_gnu();
+        header.set_size(2);
+        header.set_mode(0o644);
+        padded.append_data(&mut header, "theme.json", &b"{}"[..]).unwrap();
+        let mut gz = flate2::write::GzEncoder::new(Vec::new(), flate2::Compression::fast());
+        std::io::Write::write_all(&mut gz, &padded.into_inner().unwrap()).unwrap();
+        std::io::Write::write_all(&mut gz, &vec![0; (MAX_PADDING + 1) as usize]).unwrap();
+        let Err(e) = install(&base, &gz.finish().unwrap()[..], None) else {
+            panic!("an oversized tail installed")
+        };
+        assert!(e.to_string().contains("tar 结尾之后"), "{e:#}");
 
         // None of that affected the theme being served or left a staging
         // directory behind.