Re: [patch] net_sched: stack info leak in cbq_dump_wrr()

Joe Perches
2013-07-29T21:50:42Z

On Mon, 2013-07-29 at 14:17 -0700, David Miller wrote:
> From: Joe Perches <joe@perches.com>
> Date: Mon, 29 Jul 2013 13:12:31 -0700
>
> > On Mon, 2013-07-29 at 23:01 +0300, Dan Carpenter wrote:
> >> On Mon, Jul 29, 2013 at 12:44:32PM -0700, Joe Perches wrote:
> >> > On Mon, 2013-07-29 at 22:36 +0300, Dan Carpenter wrote:
> >> > > opt.__reserved isn't cleared so we leak a byte of stack information.
> >> > []
> >> > > diff --git a/net/sched/sch_cbq.c b/net/sched/sch_cbq.c
> >> > []
> >> > > @@ -1469,6 +1469,7 @@ static int cbq_dump_wrr(struct sk_buff *skb, struct cbq_class *cl)
> >> > > opt.allot = cl->allot;
> >> > > opt.priority = cl->priority + 1;
> >> > > opt.cpriority = cl->cpriority + 1;
> >> > > + opt.__reserved = 0;
> >> > > opt.weight = cl->weight;
> >> > > if (nla_put(skb, TCA_CBQ_WRROPT, sizeof(opt), &opt))
> >> > > goto nla_put_failure;
> >> >
> >> > Alignment isn't guaranteed here so it'd
> >> > probably be better with a memset.
> >> >
> >>
> >> Hm... Which arches would align it differently?
> >
> > Hey Dan.
> >
> > None so far as I know, but what difference does it make
> > when it's a general correctness issue?
>
> Should see if the compiler optimizes the spurious stores away,
> and if not we can use an initializer.

If the initializer is

struct foo = {0};

then as far as I know, the compiler is free to
not initialize any padding.

However, it looks like gcc 4.7 generates the same
code for this with or without the __aligned__ use.

(with gcc -O2 -S t.c)

$ cat t.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct foo {
int a;
char b __attribute__((__aligned__(256)));
long c;
};

void init1(void)
{
struct foo bar = {0};
printf("%p\n", &bar);
}

void init2(void)
{
struct foo bar;
memset(&bar, 0, sizeof(bar));
printf("%p\n", &bar);
}