buffer: fix printing into out-of-memory buffer

Before printing into a `git_buf` structure, we always call `ENSURE_SIZE`
first. This macro will reallocate the buffer as-needed depending on
whether the current amount of allocated bytes is sufficient or not. If
`asize` is big enough, then it will just do nothing, otherwise it will
call out to `git_buf_try_grow`. But in fact, it is insufficient to only
check `asize`.

When we fail to allocate any more bytes e.g. via `git_buf_try_grow`,
then we set the buffer's pointer to `git_buf__oom`. Note that we touch
neither `asize` nor `size`. So if we just check `asize > targetsize`,
then we will happily let the caller of `ENSURE_SIZE` proceed with an
out-of-memory buffer. As a result, we will print all bytes into the
out-of-memory buffer instead, resulting in an out-of-bounds write.

Fix the issue by having `ENSURE_SIZE` verify that the buffer is not
marked as OOM. Add a test to verify that we're not writing into the OOM
buffer.

src/buffer.c

@@ -18,7 +18,8 @@ char git_buf__initbuf[1];
 char git_buf__oom[1];
 
 #define ENSURE_SIZE(b, d) \
-	if ((d) > (b)->asize && git_buf_grow((b), (d)) < 0)\
+	if ((b)->ptr == git_buf__oom || \
+	    ((d) > (b)->asize && git_buf_grow((b), (d)) < 0))\
 		return -1;
 
 

tests/core/buffer.c

@@ -1220,3 +1220,23 @@ void test_core_buffer__dont_hit_infinite_loop_when_resizing(void)
 
 	git_buf_dispose(&buf);
 }
+
+void test_core_buffer__avoid_printing_into_oom_buffer(void)
+{
+	git_buf buf = GIT_BUF_INIT;
+
+	/* Emulate OOM situation with a previous allocation */
+	buf.asize = 8;
+	buf.ptr = git_buf__oom;
+
+	/*
+	 * Print the same string again. As the buffer still has
+	 * an `asize` of 8 due to the previous print,
+	 * `ENSURE_SIZE` would not try to reallocate the array at
+	 * all. As it didn't explicitly check for `git_buf__oom`
+	 * in earlier versions, this would've resulted in it
+	 * returning successfully and thus `git_buf_puts` would
+	 * just print into the `git_buf__oom` array.
+	 */
+	cl_git_fail(git_buf_puts(&buf, "foobar"));
+}