fix(detector): cap decompressed content-stream size #418
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,145 @@ | ||
| //! Bounded stream decompression for detector scans. | ||
| //! | ||
| //! `lopdf::Stream::decompressed_content` materializes the full decoded buffer | ||
| //! before any caller can apply a limit. A few megabytes of Flate-compressed | ||
| //! zeros can therefore expand to gigabytes. These helpers stop inflate once | ||
| //! the decoded budget is reached. | ||
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| use flate2::read::{DeflateDecoder, ZlibDecoder}; | ||
| use lopdf::Stream; | ||
| use std::io::Read; | ||
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| /// Maximum decoded bytes held for a single content stream during detection. | ||
| pub(crate) const MAX_DECOMPRESSED_STREAM_BYTES: usize = 32 * 1024 * 1024; | ||
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| /// Decode `stream` for scanning, or return an empty buffer when the decoded | ||
| /// size would exceed `max_bytes`. | ||
| pub(crate) fn stream_content_for_scan(stream: &Stream) -> Vec<u8> { | ||
| match decompressed_content_bounded(stream, MAX_DECOMPRESSED_STREAM_BYTES) { | ||
| Some(data) => data, | ||
| None => Vec::new(), | ||
| } | ||
| } | ||
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| /// Incremental decode with a hard output cap. `None` means the stream is | ||
| /// larger than `max_bytes` (or not safely decodable within that budget). | ||
| pub(crate) fn decompressed_content_bounded(stream: &Stream, max_bytes: usize) -> Option<Vec<u8>> { | ||
| let filters = match stream.filters() { | ||
| Ok(filters) => filters, | ||
| Err(_) => { | ||
| return take_if_within_budget(&stream.content, max_bytes); | ||
| } | ||
| }; | ||
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| if filters.is_empty() { | ||
| return take_if_within_budget(&stream.content, max_bytes); | ||
| } | ||
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| // Plain Flate is the highly compressible case. Detector scans only need | ||
| // the inflated operator bytes; skip PNG predictors here so inflate can | ||
| // stop at the budget instead of materializing the full buffer first. | ||
| if filters.len() == 1 && filters[0] == b"FlateDecode" { | ||
| return inflate_flate_bounded(&stream.content, max_bytes); | ||
| } | ||
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| if stream.content.len() > max_bytes { | ||
| return None; | ||
| } | ||
| match stream.decompressed_content() { | ||
|
There was a problem hiding this comment. P1: When a content stream uses a filter chain, this branch materializes the entire decoded buffer before checking its size. Decode every supported filter incrementally or reject the stream before this call, otherwise a small multi-filter stream bypasses the 32 MiB protection. Prompt for AI agents
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| Ok(data) if data.len() <= max_bytes => Some(data), | ||
| Ok(_) => None, | ||
| Err(_) => take_if_within_budget(&stream.content, max_bytes), | ||
| } | ||
| } | ||
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| fn take_if_within_budget(bytes: &[u8], max_bytes: usize) -> Option<Vec<u8>> { | ||
| if bytes.len() > max_bytes { | ||
| None | ||
| } else { | ||
| Some(bytes.to_vec()) | ||
| } | ||
| } | ||
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| fn inflate_flate_bounded(input: &[u8], max_bytes: usize) -> Option<Vec<u8>> { | ||
| if input.is_empty() { | ||
| return Some(Vec::new()); | ||
| } | ||
| match read_bounded(ZlibDecoder::new(input), max_bytes) { | ||
| Some(data) => Some(data), | ||
| None if input.len() > 2 => read_bounded(DeflateDecoder::new(&input[2..]), max_bytes), | ||
| None => None, | ||
| } | ||
| } | ||
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| fn read_bounded<R: Read>(mut decoder: R, max_bytes: usize) -> Option<Vec<u8>> { | ||
| let mut output = Vec::new(); | ||
| let mut buf = [0u8; 16 * 1024]; | ||
| loop { | ||
| match decoder.read(&mut buf) { | ||
| Ok(0) => return Some(output), | ||
| Ok(n) => { | ||
| if output.len().saturating_add(n) > max_bytes { | ||
| return None; | ||
| } | ||
| output.extend_from_slice(&buf[..n]); | ||
| } | ||
| Err(_) => return None, | ||
| } | ||
| } | ||
| } | ||
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| #[cfg(test)] | ||
| mod tests { | ||
| use super::*; | ||
| use flate2::write::ZlibEncoder; | ||
| use flate2::Compression; | ||
| use lopdf::dictionary; | ||
| use std::io::Write; | ||
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| fn flate_stream(plain: &[u8]) -> Stream { | ||
| let mut encoder = ZlibEncoder::new(Vec::new(), Compression::best()); | ||
| encoder.write_all(plain).unwrap(); | ||
| let compressed = encoder.finish().unwrap(); | ||
| Stream::new(dictionary! { "Filter" => "FlateDecode" }, compressed) | ||
| } | ||
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| #[test] | ||
| fn small_flate_stream_round_trips() { | ||
| let plain = b"BT /F1 12 Tf (Hello world) Tj ET"; | ||
| let stream = flate_stream(plain); | ||
| assert_eq!( | ||
| decompressed_content_bounded(&stream, MAX_DECOMPRESSED_STREAM_BYTES).as_deref(), | ||
| Some(plain.as_slice()) | ||
| ); | ||
| } | ||
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| #[test] | ||
| fn highly_compressible_flate_stops_at_budget() { | ||
| let plain = vec![0u8; 256 * 1024]; | ||
| let stream = flate_stream(&plain); | ||
| assert!( | ||
| stream.content.len() < 8 * 1024, | ||
| "fixture must stay compact on disk, got {} compressed bytes", | ||
| stream.content.len() | ||
| ); | ||
| assert!(decompressed_content_bounded(&stream, 16 * 1024).is_none()); | ||
| assert_eq!( | ||
| decompressed_content_bounded(&stream, 256 * 1024).as_deref(), | ||
| Some(plain.as_slice()) | ||
| ); | ||
| } | ||
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| #[test] | ||
| fn uncompressed_over_budget_is_skipped() { | ||
| let stream = Stream::new(dictionary! {}, vec![b'x'; 64]); | ||
| assert!(decompressed_content_bounded(&stream, 32).is_none()); | ||
| assert_eq!(decompressed_content_bounded(&stream, 64).unwrap().len(), 64); | ||
| } | ||
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| #[test] | ||
| fn scan_helper_returns_empty_when_capped() { | ||
| let stream = flate_stream(&vec![0u8; 64 * 1024]); | ||
| // Production cap is far above 64 KiB, so this still decodes. | ||
| assert_eq!(stream_content_for_scan(&stream).len(), 64 * 1024); | ||
| } | ||
| } | ||
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P2: When a Flate content stream has PNG predictor parameters, this branch scans predictor-encoded bytes instead of applying predictor reversal. Apply predictor decoding incrementally or route predictor streams through a correctly bounded decoder, or text pages using them can be misclassified.
Prompt for AI agents