Explain why there is no use after free vulnerability

When looking at the code I initially thought that commit
b8f0031 introduced a use after free
vulnerability.  It does not, but this was not clear from reading the
code, so add comments to explain why the buffer (unlike stdin_buf) can
just be uninitialized memory and will always be valid throughout
process_io() until it is freed.

Also avoid allocating two buffers when one will do.

libqrexec/process_io.c

@@ -110,12 +110,6 @@ int process_io(const struct process_io_request *req) {
     };
     if (remote_buffer.data == NULL)
         handle_vchan_error("remote buffer alloc");
-    struct buffer stdin_buffer = {
-        .data = malloc(max_chunk_size),
-        .buflen = max_chunk_size,
-    };
-    if (stdin_buffer.data == NULL)
-        handle_vchan_error("stdin buffer alloc");
 
     sigemptyset(&pollmask);
     sigaddset(&pollmask, SIGCHLD);
@@ -279,7 +273,7 @@ int process_io(const struct process_io_request *req) {
         if (prefix.len > 0 || (stdout_fd >= 0 && fds[FD_STDOUT].revents)) {
             switch (handle_input_v2(
                         vchan, stdout_fd, stdout_msg_type,
-                        &prefix, &stdin_buffer)) {
+                        &prefix, &remote_buffer)) {
                 case REMOTE_ERROR:
                     handle_vchan_error("send(handle_input stdout)");
                     break;
@@ -291,7 +285,7 @@ int process_io(const struct process_io_request *req) {
         if (stderr_fd >= 0 && fds[FD_STDERR].revents) {
             switch (handle_input_v2(
                         vchan, stderr_fd, MSG_DATA_STDERR,
-                        &empty, &stdin_buffer)) {
+                        &empty, &remote_buffer)) {
                 case REMOTE_ERROR:
                     handle_vchan_error("send(handle_input stderr)");
                     break;
@@ -321,7 +315,6 @@ int process_io(const struct process_io_request *req) {
     }
 
     free(remote_buffer.data);
-    free(stdin_buffer.data);
 
     if (!is_service && remote_status)
         return remote_status;

libqrexec/remote.h

@@ -40,6 +40,17 @@
  * Options:
  *   replace_chars_stdout, replace_chars_stderr - remove non-printable
  *     characters from stdout/stderr
+ *
+ * stdin_buf is the buffer holding data to be written to stdin, and may
+ * be modified or reallocated.  Its content must be valid data, and on
+ * return it will be a valid buffer pointing to valid (generally different)
+ * data.
+ *
+ * buffer is scratch space _only_.  Its size must be the maximum data
+ * chunk size.  Its contents (the data pointed to) does _not_ need to be
+ * initialized, and will _not_ be anything meaningful on return.  The
+ * buffer pointer and length will not be changed and will remain valid,
+ * though.  In Rust terms: this is an &mut [MaybeUninit<u8>].
  */
 int handle_remote_data_v2(
     libvchan_t *data_vchan, int stdin_fd, int *status,
@@ -56,6 +67,12 @@ int handle_remote_data_v2(
  *   REMOTE_EOF - EOF received, do not access this FD again
  *   REMOTE_OK - some data processed, call it again when buffer space and
  *     more data availabla
+ *
+ * buffer is scratch space _only_.  Its size must be the maximum data
+ * chunk size.  Its contents (the data pointed to) does _not_ need to be
+ * initialized, and will _not_ be anything meaningful on return.  The
+ * buffer pointer and length will not be freed or reallocated, though.
+ * In Rust terms: this is an &mut [MaybeUninit<u8>].
  */
 int handle_input_v2(
     libvchan_t *vchan, int fd, int msg_type,