On Sat, Jul 31, 2021 at 04:32:41AM +0300, Laurent Pinchart wrote:

strncpy() is widely regarded as unsafe due to the fact that it may leave the destination string without a nul-termination when the source string size is too large. When compiling the kernel with W=1, the gcc warns about this:

drivers/gpu/drm/drm_property.c: In function ‘drm_property_create’: drivers/gpu/drm/drm_property.c:130:2: warning: ‘strncpy’ specified bound 32 equals destination size [-Wstringop-truncation] 130 | strncpy(property->name, name, DRM_PROP_NAME_LEN); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are three occurrences of strncpy() in drm_property.c. None of them are actually unsafe, as the very next line forces nul-termination of the destination buffer. The warning is thus a false positive, but adds noise to the kernel log. It can easily be silenced by using strscpy_pad() instead. Do so.

One of the three occurrences, in drm_property_add_enum(), fills a char array that is later copied to userspace with copy_to_user() in drm_mode_getproperty_ioctl(). To avoid leaking kernel data, strscpy_pad() is required. Similarly, a second occurrence, in drm_mode_getproperty_ioctl(), copies the string to an ioctl data buffer that isn't previously zero'ed, to strscpy_pad() is also required. The last occurrence, in drm_property_create(), would be safe to replace with strscpy(), as the destination buffer is copied to userspace with strscpy_pad(). However, given that this isn't in a hot path, let's avoid future data leaks in case someone copies the whole char array blindly.

+1 on just playing it safe.

Signed-off-by: Laurent Pinchart laurent.pinchart+renesas@ideasonboard.com

Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch

---

drivers/gpu/drm/drm_property.c | 9 +++------ 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_property.c b/drivers/gpu/drm/drm_property.c index 27c824a6eb60..32404891446e 100644 --- a/drivers/gpu/drm/drm_property.c +++ b/drivers/gpu/drm/drm_property.c @@ -127,8 +127,7 @@ struct drm_property *drm_property_create(struct drm_device *dev, property->num_values = num_values; INIT_LIST_HEAD(&property->enum_list);

• strncpy(property->name, name, DRM_PROP_NAME_LEN);
• property->name[DRM_PROP_NAME_LEN-1] = '\0';

• strscpy_pad(property->name, name, DRM_PROP_NAME_LEN);

  list_add_tail(&property->head, &dev->mode_config.property_list);

@@ -421,8 +420,7 @@ int drm_property_add_enum(struct drm_property *property, if (!prop_enum) return -ENOMEM;

• strncpy(prop_enum->name, name, DRM_PROP_NAME_LEN);
• prop_enum->name[DRM_PROP_NAME_LEN-1] = '\0';

• strscpy_pad(prop_enum->name, name, DRM_PROP_NAME_LEN); prop_enum->value = value;

  property->values[index] = value;

@@ -475,8 +473,7 @@ int drm_mode_getproperty_ioctl(struct drm_device *dev, if (!property) return -ENOENT;

• strncpy(out_resp->name, property->name, DRM_PROP_NAME_LEN);
• out_resp->name[DRM_PROP_NAME_LEN-1] = 0;

• strscpy_pad(out_resp->name, property->name, DRM_PROP_NAME_LEN); out_resp->flags = property->flags;

  value_count = property->num_values;

-- Regards,

Laurent Pinchart