Harden file resume prefix validation

src/bin/dosh-server.rs

@@ -2566,8 +2566,10 @@ struct UploadState {
     hasher: Sha256,
     written: u64,
     expected_size: u64,
+    mode: Option<u32>,
     modified_secs: Option<u64>,
     atomic: bool,
+    original_len: Option<u64>,
 }
 
 async fn run_file_stream_service(
@@ -2716,31 +2718,41 @@ async fn handle_file_request(
                 bail!("destination exists: {}", final_path.display());
             }
             let mut hasher = Sha256::new();
-            let (file, temp_path, written, atomic) = if resume && final_path.exists() {
-                let mut file = fs::OpenOptions::new()
-                    .read(true)
-                    .append(true)
-                    .open(&final_path)
-                    .with_context(|| format!("open {}", final_path.display()))?;
-                let written = file.metadata()?.len().min(size);
-                file.set_len(written)
-                    .with_context(|| format!("truncate {}", final_path.display()))?;
-                if written > 0 {
-                    let mut prefix = fs::File::open(&final_path)?;
-                    hash_exact_prefix(&mut prefix, written, &mut hasher)?;
-                }
-                file.seek(SeekFrom::Start(written))?;
-                (file, None, written, false)
-            } else {
-                let temp_path = upload_temp_path(&final_path, stream_id);
-                let file = fs::OpenOptions::new()
-                    .create(true)
-                    .truncate(true)
-                    .write(true)
-                    .open(&temp_path)
-                    .with_context(|| format!("create {}", temp_path.display()))?;
-                (file, Some(temp_path), 0, true)
-            };
+            let (file, temp_path, written, atomic, original_len, prefix_sha256) =
+                if resume && final_path.exists() {
+                    let original_len = fs::metadata(&final_path)
+                        .with_context(|| format!("stat {}", final_path.display()))?
+                        .len();
+                    let written = original_len.min(size);
+                    if written > 0 {
+                        let mut prefix = fs::File::open(&final_path)?;
+                        hash_exact_prefix(&mut prefix, written, &mut hasher)?;
+                    }
+                    let prefix_sha256 = (written > 0).then(|| hasher.clone().finalize().into());
+                    let mut file = fs::OpenOptions::new()
+                        .read(true)
+                        .append(true)
+                        .open(&final_path)
+                        .with_context(|| format!("open {}", final_path.display()))?;
+                    file.seek(SeekFrom::Start(written))?;
+                    (
+                        file,
+                        None,
+                        written,
+                        false,
+                        Some(original_len),
+                        prefix_sha256,
+                    )
+                } else {
+                    let temp_path = upload_temp_path(&final_path, stream_id);
+                    let file = fs::OpenOptions::new()
+                        .create(true)
+                        .truncate(true)
+                        .write(true)
+                        .open(&temp_path)
+                        .with_context(|| format!("create {}", temp_path.display()))?;
+                    (file, Some(temp_path), 0, true, None, None)
+                };
             *upload = Some(UploadState {
                 final_path,
                 temp_path,
@@ -2748,26 +2760,20 @@ async fn handle_file_request(
                 hasher,
                 written,
                 expected_size: size,
+                mode,
                 modified_secs,
                 atomic,
+                original_len,
             });
-            if let Some(mode) = mode {
-                let Some(active_upload) = upload.as_ref() else {
-                    return Err(anyhow!("upload state missing after start"));
-                };
-                let target = active_upload
-                    .temp_path
-                    .as_ref()
-                    .unwrap_or(&active_upload.final_path)
-                    .clone();
-                set_mode(&target, mode)?;
-            }
             send_file_response_to_client(
                 state,
                 socket,
                 client_id,
                 stream_id,
-                FileResponse::Resume { offset: written },
+                FileResponse::Resume {
+                    offset: written,
+                    prefix_sha256,
+                },
             )
             .await
         }
@@ -2789,8 +2795,6 @@ async fn handle_file_request(
             let Some(mut active) = upload.take() else {
                 bail!("no upload in progress");
             };
-            active.file.flush()?;
-            active.file.sync_all()?;
             anyhow::ensure!(
                 active.written == active.expected_size,
                 "upload size mismatch: got {}, expected {}",
@@ -2801,9 +2805,17 @@ async fn handle_file_request(
             if digest != sha256 {
                 if let Some(temp_path) = &active.temp_path {
                     let _ = fs::remove_file(temp_path);
+                } else if let Some(original_len) = active.original_len {
+                    let _ = active.file.set_len(original_len);
                 }
                 bail!("upload checksum mismatch");
             }
+            active
+                .file
+                .set_len(active.expected_size)
+                .with_context(|| format!("truncate {}", active.final_path.display()))?;
+            active.file.flush()?;
+            active.file.sync_all()?;
             drop(active.file);
             if active.atomic
                 && let Some(temp_path) = active.temp_path.take()
@@ -2816,6 +2828,9 @@ async fn handle_file_request(
                     )
                 })?;
             }
+            if let Some(mode) = active.mode {
+                set_mode(&active.final_path, mode)?;
+            }
             if let Some(modified_secs) = active.modified_secs {
                 filetime::set_file_mtime(
                     &active.final_path,
@@ -2843,9 +2858,20 @@ async fn handle_file_request(
                 "remote path is not a file: {}",
                 meta.path
             );
+            anyhow::ensure!(
+                offset <= meta.len,
+                "download offset {offset} is beyond file size {}",
+                meta.len
+            );
             let mut file =
                 fs::File::open(&path).with_context(|| format!("open {}", path.display()))?;
             file.seek(SeekFrom::Start(offset))?;
+            let mut hasher = Sha256::new();
+            if offset > 0 {
+                let mut prefix = fs::File::open(&path)?;
+                hash_exact_prefix(&mut prefix, offset, &mut hasher)?;
+            }
+            let prefix_sha256 = (offset > 0).then(|| hasher.clone().finalize().into());
             send_file_response_to_client(
                 state,
                 socket,
@@ -2854,14 +2880,10 @@ async fn handle_file_request(
                 FileResponse::Start {
                     meta: meta.clone(),
                     offset,
+                    prefix_sha256,
                 },
             )
             .await?;
-            let mut hasher = Sha256::new();
-            if offset > 0 {
-                let mut prefix = fs::File::open(&path)?;
-                hash_exact_prefix(&mut prefix, offset, &mut hasher)?;
-            }
             let mut sent = offset;
             let mut buf = vec![0u8; CHUNK_SIZE];
             loop {