apply: reject patches larger than ~1 GiB

The apply code is not prepared to handle extremely large files. It uses
"int" in some places, and "unsigned long" in others.

This combination leads to unfortunate problems when switching between
the two types. Using "int" prevents us from handling large files, since
large offsets will wrap around and spill into small negative values,
which can result in wrong behavior (like accessing the patch buffer with
a negative offset).

Converting from "unsigned long" to "int" also has truncation problems
even on LLP64 platforms where "long" is the same size as "int", since
the former is unsigned but the latter is not.

To avoid potential overflow and truncation issues in `git apply`, apply
similar treatment as in dcd1742e56 (xdiff: reject files larger than
~1GB, 2015-09-24), where the xdiff code was taught to reject large
files for similar reasons.

The maximum size was chosen somewhat arbitrarily, but picking a value
just shy of a gigabyte allows us to double it without overflowing 2^31-1
(after which point our value would wrap around to a negative number).
To give ourselves a bit of extra margin, the maximum patch size is a MiB
smaller than a full GiB, which gives us some slop in case we allocate
"(records + 1) * sizeof(int)" or similar.

Luckily, the security implications of these conversion issues are
relatively uninteresting, because a victim needs to be convinced to
apply a malicious patch.

Reported-by: 정재우 <thebound7@gmail.com>
Suggested-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

apply.c

@@ -386,9 +386,19 @@ static void say_patch_name(FILE *output, const char *fmt, struct patch *patch)
 
 #define SLOP (16)
 
+/*
+ * apply.c isn't equipped to handle arbitrarily large patches, because
+ * it intermingles `unsigned long` with `int` for the type used to store
+ * buffer lengths.
+ *
+ * Only process patches that are just shy of 1 GiB large in order to
+ * avoid any truncation or overflow issues.
+ */
+#define MAX_APPLY_SIZE (1024UL * 1024 * 1023)
+
 static int read_patch_file(struct strbuf *sb, int fd)
 {
-	if (strbuf_read(sb, fd, 0) < 0)
+	if (strbuf_read(sb, fd, 0) < 0 || sb->len >= MAX_APPLY_SIZE)
 		return error_errno("git apply: failed to read");
 
 	/*

t/t4141-apply-too-large.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+test_description='git apply with too-large patch'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success EXPENSIVE 'git apply rejects patches that are too large' '
+	sz=$((1024 * 1024 * 1023)) &&
+	{
+		cat <<-\EOF &&
+		diff --git a/file b/file
+		new file mode 100644
+		--- /dev/null
+		+++ b/file
+		@@ -0,0 +1 @@
+		EOF
+		test-tool genzeros
+	} | test_copy_bytes $sz | test_must_fail git apply 2>err &&
+	grep "git apply: failed to read" err
+'
+
+test_done