Re: [Linux-kernel-mentees] [PATCH] media/v4l2-core: Fix kernel-infoleak in video_put_user()

[Laurent Pinchart]

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