[2/3] usb-mtp: fix bounds check for guest provided filename
diff mbox series

Message ID 20190415154503.6758-3-berrange@redhat.com
State New
Headers show
Series
  • usb-mtp: fix ObjectInfo request handling
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Commit Message

Daniel P. Berrangé April 15, 2019, 3:45 p.m. UTC
The ObjectInfo struct has a variable length array containing the UTF-16
encoded filename. The number of characters of trailing data is given by
the 'length' field in the struct and this must be validated against the
size of the data packet received from the guest.

Since the data is UTF-16, we must convert the byte count we have to a
character count before validating. This must take care to truncate if
a malicious guest sent an odd number of bytes.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 hw/usb/dev-mtp.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

Comments

Bandan Das April 16, 2019, 7:41 p.m. UTC | #1
Daniel P. Berrangé <berrange@redhat.com> writes:

> The ObjectInfo struct has a variable length array containing the UTF-16
> encoded filename. The number of characters of trailing data is given by
> the 'length' field in the struct and this must be validated against the
> size of the data packet received from the guest.
>
> Since the data is UTF-16, we must convert the byte count we have to a
> character count before validating. This must take care to truncate if
> a malicious guest sent an odd number of bytes.
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>  hw/usb/dev-mtp.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/hw/usb/dev-mtp.c b/hw/usb/dev-mtp.c
> index 838cd74da6..6b7d1296e4 100644
> --- a/hw/usb/dev-mtp.c
> +++ b/hw/usb/dev-mtp.c
> @@ -1699,12 +1699,19 @@ static void usb_mtp_write_metadata(MTPState *s, uint64_t dlen)
>      MTPObject *o;
>      MTPObject *p = usb_mtp_object_lookup(s, s->dataset.parent_handle);
>      uint32_t next_handle = s->next_handle;
> +    size_t filename_chars = dlen - offsetof(ObjectInfo, filename);
> +
> +    /*
> +     * filename is utf-16. We're intentionally doing
> +     * integer division to truncate if malicious guest
> +     * sent an odd number of bytes.
> +     */
> +    filename_chars /= 2;
>  
>      assert(!s->write_pending);
>      assert(p != NULL);
>  
> -    filename = utf16_to_str(MIN(dataset->length,
> -                                dlen - offsetof(ObjectInfo, filename)),
> +    filename = utf16_to_str(MIN(dataset->length, filename_chars),
>                              dataset->filename);
>  
>      if (strchr(filename, '/')) {

Reviewed-by: Bandan Das <bsd@redhat.com>

Patch
diff mbox series

diff --git a/hw/usb/dev-mtp.c b/hw/usb/dev-mtp.c
index 838cd74da6..6b7d1296e4 100644
--- a/hw/usb/dev-mtp.c
+++ b/hw/usb/dev-mtp.c
@@ -1699,12 +1699,19 @@  static void usb_mtp_write_metadata(MTPState *s, uint64_t dlen)
     MTPObject *o;
     MTPObject *p = usb_mtp_object_lookup(s, s->dataset.parent_handle);
     uint32_t next_handle = s->next_handle;
+    size_t filename_chars = dlen - offsetof(ObjectInfo, filename);
+
+    /*
+     * filename is utf-16. We're intentionally doing
+     * integer division to truncate if malicious guest
+     * sent an odd number of bytes.
+     */
+    filename_chars /= 2;
 
     assert(!s->write_pending);
     assert(p != NULL);
 
-    filename = utf16_to_str(MIN(dataset->length,
-                                dlen - offsetof(ObjectInfo, filename)),
+    filename = utf16_to_str(MIN(dataset->length, filename_chars),
                             dataset->filename);
 
     if (strchr(filename, '/')) {