delta: fix out-of-bounds read of delta

When computing the offset and length of the delta base, we repeatedly
increment the `delta` pointer without checking whether we have advanced
past its end already, which can thus result in an out-of-bounds read.
Fix this by repeatedly checking whether we have reached the end. Add a
test which would cause Valgrind to produce an error.

Reported-by: Riccardo Schirone <rschiron@redhat.com>
Test-provided-by: Riccardo Schirone <rschiron@redhat.com>

src/delta.c

@@ -568,15 +568,17 @@ int git_delta_apply(
 			/* cmd is a copy instruction; copy from the base. */
 			size_t off = 0, len = 0;
 
-			if (cmd & 0x01) off = *delta++;
-			if (cmd & 0x02) off |= *delta++ << 8UL;
-			if (cmd & 0x04) off |= *delta++ << 16UL;
-			if (cmd & 0x08) off |= ((unsigned) *delta++ << 24UL);
-
-			if (cmd & 0x10) len = *delta++;
-			if (cmd & 0x20) len |= *delta++ << 8UL;
-			if (cmd & 0x40) len |= *delta++ << 16UL;
+#define ADD_DELTA(o, shift) { if (delta < delta_end) (o) |= ((unsigned) *delta++ << shift); else goto fail; }
+			if (cmd & 0x01) ADD_DELTA(off, 0UL);
+			if (cmd & 0x02) ADD_DELTA(off, 8UL);
+			if (cmd & 0x04) ADD_DELTA(off, 16UL);
+			if (cmd & 0x08) ADD_DELTA(off, 24UL);
+
+			if (cmd & 0x10) ADD_DELTA(len, 0UL);
+			if (cmd & 0x20) ADD_DELTA(len, 8UL);
+			if (cmd & 0x40) ADD_DELTA(len, 16UL);
 			if (!len)       len = 0x10000;
+#undef ADD_DELTA
 
 			if (base_len < off + len || res_sz < len)
 				goto fail;

tests/delta/apply.c

@@ -10,3 +10,12 @@ void test_delta_apply__read_at_off(void)
 
 	cl_git_fail(git_delta_apply(&out, &outlen, base, sizeof(base), delta, sizeof(delta)));
 }
+
+void test_delta_apply__read_after_limit(void)
+{
+	unsigned char base[16] = { 0 }, delta[] = { 0x10, 0x70, 0xff };
+	void *out;
+	size_t outlen;
+
+	cl_git_fail(git_delta_apply(&out, &outlen, base, sizeof(base), delta, sizeof(delta)));
+}