Re: [PATCH 1/2] tty: vcc: zero-initialize control packet in vcc_send_ctl()

On Fri, Jul 31, 2026 at 11:16:57AM +0200, Jiri Slaby wrote:
> On 31. 07. 26, 10:32, Greg Kroah-Hartman wrote:
> > On Fri, Jul 31, 2026 at 10:29:14AM +0200, Jiri Slaby wrote:
> > > On 31. 07. 26, 10:15, Greg Kroah-Hartman wrote:
> > > > From: Joshua Rogers <[email protected]>
> > > > 
> > > > The stack-allocated struct vio_vcc pkt was partially initialized,
> > > > leaving the tag.stype_env field uninitialized before being sent via
> > > > ldc_write(), potentially leaking kernel stack data to the LDC peer.
> > > > 
> > > > Assisted-by: AISLE:Snapshot
> > > > Cc: stable <[email protected]>
> > > > Signed-off-by: Joshua Rogers <[email protected]>
> > > > Signed-off-by: Greg Kroah-Hartman <[email protected]>
> > > > ---
> > > >    drivers/tty/vcc.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/tty/vcc.c b/drivers/tty/vcc.c
> > > > index 27a55465bf5e..3947bd2b75ac 100644
> > > > --- a/drivers/tty/vcc.c
> > > > +++ b/drivers/tty/vcc.c
> > > > @@ -492,7 +492,7 @@ static ssize_t domain_show(struct device *dev,
> > > >    static int vcc_send_ctl(struct vcc_port *port, int ctl)
> > > >    {
> > > > -	struct vio_vcc pkt;
> > > > +	struct vio_vcc pkt = {};
> > > 
> > > Note this needs not initialize holes.
> > 
> > Does it?  I keep forgetting this if it's really true or not.  I think
> > we've been through this in the past but I can't find the archives.
> > 
> > > I believe there are none here, but memset() is usually
> > > preferred/safer.
> > 
> > Last I remember, a long time ago, the compiler would just use memset()
> > for this, and hit the holes.  Or is that a compiler-specific issue?
> > 
> > Yes, it's more obvious, and I'll be glad to change it, but figuring this
> > out for real would be great :)
> 
> OK, so the standard (C17) does not guarantee zeroing.
> 
> What I found though:
> 
> * both clang and gcc zero the full struct for "{}". There was a bug in gcc <
> 8 where it did not.
> 
> * this is not true for designated initializer, cf. ce50039be49e ("net:
> sched: act_ife: initialize struct tc_ife to fix KMSAN kernel-infoleak")).

Ok, as our compiler is newer than these versions, and this isn't a
designated initializer, we should be ok.

> 
> Adding Kees who is the author of:
> ====
> Memory initialization
> ---------------------
> 
> Memory copied to userspace must always be fully initialized. If not
> explicitly memset(), this will require changes to the compiler to make
> sure structure holes are cleared.
> ====
> 
> He might aware of more bugs.
> 
> 
> 
> FWIW, {} here should be safe for two reasons:
> * not a designated initializer
> * no holes (IMO)

Great, I'll keep this as-is.  Now if only anyone can tell me if this
code is even used anymore :)

thanks,

greg k-h