[Qemu-devel] [PATCH] usb-mtp: Limit filename to object information size

[Qemu-devel] [PATCH] usb-mtp: Limit filename to object information size

[Michael Hanselmann]

The filename length in MTP metadata is specified by the guest. By
trusting it directly it'd theoretically be possible to get the host to
write memory parts outside the filename buffer into a filename. In
practice though there are usually NUL bytes stopping the string
operations.

Also use the opportunity to not assign the filename member twice.

Signed-off-by: Michael Hanselmann <public@hansmi.ch>
---
 hw/usb/dev-mtp.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/hw/usb/dev-mtp.c b/hw/usb/dev-mtp.c
index 100b7171f4..360ca65ee4 100644
--- a/hw/usb/dev-mtp.c
+++ b/hw/usb/dev-mtp.c
@@ -1705,7 +1705,7 @@ free:
     s->write_pending = false;
 }
 
-static void usb_mtp_write_metadata(MTPState *s)
+static void usb_mtp_write_metadata(MTPState *s, uint64_t dlen)
 {
     MTPData *d = s->data_out;
     ObjectInfo *dataset = (ObjectInfo *)d->data;
@@ -1717,7 +1717,8 @@ static void usb_mtp_write_metadata(MTPState *s)
     assert(!s->write_pending);
     assert(p != NULL);
 
-    filename = utf16_to_str(dataset->length, dataset->filename);
+    filename = utf16_to_str(MIN(dataset->length, dlen - offsetof(ObjectInfo, filename)),
+                            dataset->filename);
 
     if (strchr(filename, '/')) {
         usb_mtp_queue_result(s, RES_PARAMETER_NOT_SUPPORTED, d->trans,
@@ -1733,7 +1734,6 @@ static void usb_mtp_write_metadata(MTPState *s)
     s->dataset.filename = filename;
     s->dataset.format = dataset->format;
     s->dataset.size = dataset->size;
-    s->dataset.filename = filename;
     s->write_pending = true;
 
     if (s->dataset.format == FMT_ASSOCIATION) {
@@ -1802,7 +1802,7 @@ static void usb_mtp_get_data(MTPState *s, mtp_container *container,
         if (d->offset == d->length) {
             /* The operation might have already failed */
             if (!s->result) {
-                usb_mtp_write_metadata(s);
+                usb_mtp_write_metadata(s, dlen);
             }
             usb_mtp_data_free(s->data_out);
             s->data_out = NULL;
-- 
2.11.0

Re: [Qemu-devel] [PATCH] usb-mtp: Limit filename to object information size

[Gerd Hoffmann]

On Thu, Dec 13, 2018 at 10:37:06PM +0000, Michael Hanselmann wrote:
> The filename length in MTP metadata is specified by the guest. By
> trusting it directly it'd theoretically be possible to get the host to
> write memory parts outside the filename buffer into a filename. In
> practice though there are usually NUL bytes stopping the string
> operations.
> 
> Also use the opportunity to not assign the filename member twice.

Added to usb patch queue.

thanks,
  Gerd