drm: fix leak of uninitialized data to userspace

...so drm_getunique() is trying to copy some uninitialized data to
userspace. The ECX register contains the number of words that are
left to copy -- so there are 5 * 4 = 20 bytes left. The offset of the
first uninitialized byte (counting from the start of the string) is
also 20 (i.e. 0xf65d2294&((1 << 5)-1) == 20). So somebody tried to
copy 40 bytes when the string was only 19 long.

In drm_set_busid() we have this code:

        dev->unique_len = 40;
        dev->unique = drm_alloc(dev->unique_len + 1, DRM_MEM_DRIVER);
      ...
        len = snprintf(dev->unique, dev->unique_len, pci:%04x:%02x:%02x.%d",

...so it seems that dev->unique is never updated to reflect the
actual length of the string. The remaining bytes (20 in this case)
are random uninitialized bytes that are copied into userspace.

This patch fixes the problem by setting dev->unique_len after the
snprintf().

airlied- I've had to fix this up to store the alloced size so
we have it for drm_free later.
Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Vegard Nossum <vegardno@thuin.ifi.uio.no>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_ioctl.c

@@ -92,7 +92,8 @@ int drm_setunique(struct drm_device *dev, void *data,
 		return -EINVAL;
 
 	master->unique_len = u->unique_len;
-	master->unique = drm_alloc(u->unique_len + 1, DRM_MEM_DRIVER);
+	master->unique_size = u->unique_len + 1;
+	master->unique = drm_alloc(master->unique_size, DRM_MEM_DRIVER);
 	if (!master->unique)
 		return -ENOMEM;
 	if (copy_from_user(master->unique, u->unique, master->unique_len))
@@ -136,7 +137,8 @@ static int drm_set_busid(struct drm_device *dev, struct drm_file *file_priv)
 		return -EBUSY;
 
 	master->unique_len = 40;
-	master->unique = drm_alloc(master->unique_len + 1, DRM_MEM_DRIVER);
+	master->unique_size = master->unique_len;
+	master->unique = drm_alloc(master->unique_size, DRM_MEM_DRIVER);
 	if (master->unique == NULL)
 		return -ENOMEM;
 
@@ -145,8 +147,10 @@ static int drm_set_busid(struct drm_device *dev, struct drm_file *file_priv)
 		       dev->pdev->bus->number,
 		       PCI_SLOT(dev->pdev->devfn),
 		       PCI_FUNC(dev->pdev->devfn));
-	if (len > master->unique_len)
+	if (len >= master->unique_len)
 		DRM_ERROR("buffer overflow");
+	else
+		master->unique_len = len;
 
 	dev->devname =
 	    drm_alloc(strlen(dev->driver->pci_driver.name) + master->unique_len +

drivers/gpu/drm/drm_stub.c

@@ -117,7 +117,7 @@ static void drm_master_destroy(struct kref *kref)
 		dev->driver->master_destroy(dev, master);
 
 	if (master->unique) {
-		drm_free(master->unique, strlen(master->unique) + 1, DRM_MEM_DRIVER);
+		drm_free(master->unique, master->unique_size, DRM_MEM_DRIVER);
 		master->unique = NULL;
 		master->unique_len = 0;
 	}

include/drm/drmP.h

@@ -627,6 +627,7 @@ struct drm_master {
 
 	char *unique;			/**< Unique identifier: e.g., busid */
 	int unique_len;			/**< Length of unique field */
+	int unique_size;		/**< amount allocated */
 
 	int blocked;			/**< Blocked due to VC switch? */