From 09a14a16d91b6326cda53df117f012f890219ed8 Mon Sep 17 00:00:00 2001
From: Grzegorz Antoniak <ga@anadoxin.org>
Date: Sun, 2 Feb 2020 08:04:41 +0100
Subject: [PATCH] RAR5 reader: reject files that declare invalid header flags

One of the fields in RAR5's base block structure is the size of the
header. Some invalid files declare a 0 header size setting, which can
confuse the unpacker. Minimum header size for RAR5 base blocks is 7
bytes (4 bytes for CRC, and 3 bytes for the rest), so block size of 0
bytes should be rejected at header parsing stage.

The fix adds an error condition if header size of 0 bytes is detected.
In this case, the unpacker will not attempt to unpack the file, as the
header is corrupted.

The commit also adds OSSFuzz #20459 sample to test further regressions
in this area.
---
 Makefile.am                                             |  1 +
 libarchive/archive_read_support_format_rar5.c           | 17 +++++++++++++++--
 libarchive/test/test_read_format_rar5.c                 | 15 +++++++++++++++
 ...test_read_format_rar5_block_size_is_too_small.rar.uu |  8 ++++++++
 4 files changed, 39 insertions(+), 2 deletions(-)
 create mode 100644 libarchive/test/test_read_format_rar5_block_size_is_too_small.rar.uu

diff --git a/Makefile.am b/Makefile.am
index 781bbf7..c59466f 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -876,6 +876,7 @@ libarchive_test_EXTRA_DIST=\
 	libarchive/test/test_read_format_rar5_win32.rar.uu \
 	libarchive/test/test_read_format_rar5_arm_filter_on_window_boundary.rar.uu \
 	libarchive/test/test_read_format_rar5_different_winsize_on_merge.rar.uu \
+	libarchive/test/test_read_format_rar5_block_size_is_too_small.rar.uu \
 	libarchive/test/test_read_format_raw.bufr.uu \
 	libarchive/test/test_read_format_raw.data.gz.uu \
 	libarchive/test/test_read_format_raw.data.Z.uu \
diff --git a/libarchive/archive_read_support_format_rar5.c b/libarchive/archive_read_support_format_rar5.c
index ce38b1f..970a924 100644
--- a/libarchive/archive_read_support_format_rar5.c
+++ b/libarchive/archive_read_support_format_rar5.c
@@ -2080,6 +2080,8 @@ static int scan_for_signature(struct archive_read* a);
 static int process_base_block(struct archive_read* a,
     struct archive_entry* entry)
 {
+	const size_t SMALLEST_RAR5_BLOCK_SIZE = 3;
+
 	struct rar5* rar = get_context(a);
 	uint32_t hdr_crc, computed_crc;
 	size_t raw_hdr_size = 0, hdr_size_len, hdr_size;
@@ -2103,15 +2105,26 @@ static int process_base_block(struct archive_read* a,
 		return ARCHIVE_EOF;
 	}
 
+	hdr_size = raw_hdr_size + hdr_size_len;
+
 	/* Sanity check, maximum header size for RAR5 is 2MB. */
-	if(raw_hdr_size > (2 * 1024 * 1024)) {
+	if(hdr_size > (2 * 1024 * 1024)) {
 		archive_set_error(&a->archive, ARCHIVE_ERRNO_FILE_FORMAT,
 		    "Base block header is too large");
 
 		return ARCHIVE_FATAL;
 	}
 
-	hdr_size = raw_hdr_size + hdr_size_len;
+	/* Additional sanity checks to weed out invalid files. */
+	if(raw_hdr_size == 0 || hdr_size_len == 0 ||
+		hdr_size < SMALLEST_RAR5_BLOCK_SIZE)
+	{
+		archive_set_error(&a->archive, ARCHIVE_ERRNO_FILE_FORMAT,
+		    "Too small block encountered (%ld bytes)",
+		    raw_hdr_size);
+
+		return ARCHIVE_FATAL;
+	}
 
 	/* Read the whole header data into memory, maximum memory use here is
 	 * 2MB. */
diff --git a/libarchive/test/test_read_format_rar5.c b/libarchive/test/test_read_format_rar5.c
index bb94d4e..f91521e 100644
--- a/libarchive/test/test_read_format_rar5.c
+++ b/libarchive/test/test_read_format_rar5.c
@@ -1256,3 +1256,18 @@ DEFINE_TEST(test_read_format_rar5_different_winsize_on_merge)
 
 	EPILOGUE();
 }
+
+DEFINE_TEST(test_read_format_rar5_block_size_is_too_small)
+{
+	char buf[4096];
+	PROLOGUE("test_read_format_rar5_block_size_is_too_small.rar");
+
+	/* This file is damaged, so those functions should return failure.
+	 * Additionally, SIGSEGV shouldn't be raised during execution
+	 * of those functions. */
+
+	assertA(archive_read_next_header(a, &ae) != ARCHIVE_OK);
+	assertA(archive_read_data(a, buf, sizeof(buf)) <= 0);
+
+	EPILOGUE();
+}
diff --git a/libarchive/test/test_read_format_rar5_block_size_is_too_small.rar.uu b/libarchive/test/test_read_format_rar5_block_size_is_too_small.rar.uu
new file mode 100644
index 0000000..5cad219
--- /dev/null
+++ b/libarchive/test/test_read_format_rar5_block_size_is_too_small.rar.uu
@@ -0,0 +1,8 @@
+begin 644 test_read_format_rar5_block_size_is_too_small.rar
+M4F%R(1H'`0"-[P+2``+'(!P,("`@N`,!`B`@("`@("`@("`@("`@("#_("`@
+M("`@("`@("`@((:Q;2!4-'-^4B`!((WO`M(``O\@$/\@-R`@("`@("`@("`@
+M``X@("`@("`@____("`@("`@(/\@("`@("`@("`@("#_(+6U,2"UM;6UM[CU
+M)B`@*(0G(`!.`#D\3R``(/__(,+_````-0#_($&%*/HE=C+N`"```"```"`D
+J`)$#("#_("#__P`@__\@_R#_("`@("`@("#_("#__R`@(/__("#__R`"
+`
+end
-- 
1.8.3.1

