Hi Peilin, On Sun, Jul 26, 2020 at 02:07:52PM -0400, Peilin Ye wrote: > On Sun, Jul 26, 2020 at 08:30:44PM +0300, Laurent Pinchart wrote: > > Hi Peilin, > > > > Thank you for the patch. > > > > On Sun, Jul 26, 2020 at 12:44:39PM -0400, Peilin Ye wrote: > > > video_put_user() is copying uninitialized stack memory to userspace. Fix > > > it by initializing `vb32` using memset(). > > > > What makes you think this will fix the issue ? When initializing a > > structure at declaration time, the fields that are not explicitly > > specified should be initialized to 0 by the compiler. See > > https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.cbclx01/strin.htm: > > Hi Mr. Pinchart! > > First of all, syzbot tested this patch, and it says it's "OK": > > https://syzkaller.appspot.com/bug?extid=79d751604cb6f29fbf59 > > > If a structure variable is partially initialized, all the uninitialized > > structure members are implicitly initialized to zero no matter what the > > storage class of the structure variable is. See the following example: > > > > struct one { > > int a; > > int b; > > int c; > > }; > > > > void main() { > > struct one z1; // Members in z1 do not have default initial values. > > static struct one z2; // z2.a=0, z2.b=0, and z2.c=0. > > struct one z3 = {1}; // z3.a=1, z3.b=0, and z3.c=0. > > } > > Yes, I understand that. I can safely printk() all members of that struct > without triggering a KMSAN warning, which means they have been properly > initialized. > > However, if I do something like: > > char *p = (char *)&vb32; > int i; > > for (i = 0; i < sizeof(struct vb32); i++, p++) > printk("*(p + i): %d", *(p + i)); > > This tries to print out `vb32` as "raw memory" one byte at a time, and > triggers a KMSAN warning somewhere in the middle (when `i` equals to 25 > or 26). > > According to a previous discussion with Mr. Kroah-Hartman, as well as > this LWN article: > > "Structure holes and information leaks" > https://lwn.net/Articles/417989/ > > Initializing a struct by assigning (both partially or fully) leaves the > "padding" part of it uninitialized, thus potentially leads to kernel > information leak if the structure in question is going to be copied to > userspace. > > memset() sets these "uninitialized paddings" to zero, therefore (I > think) should solve the problem. You're absolutely right. I wasn't aware the compiler wouldn't initialize holes in the structure. Thank you for educating me :-) For the patch, Reviewed-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> -- Regards, Laurent Pinchart
