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

Joe Perches
2013-07-30T07:12:32Z

On Tue, 2013-07-30 at 09:55 +0300, Dan Carpenter wrote:
> On Mon, Jul 29, 2013 at 01:12:31PM -0700, Joe Perches wrote:
> > 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?
> >
>
> Because I would assume if these aren't aligned the same way we have
> far more serious problems than just this one case. It would change
> the user space API and break network protocols.

<shrug>

I didn't say it was necessary to be done here, I said it
was a correctness issue. I still believe that's true.

The nla_put here is by structure, the struct is unpacked,
and it's local to the arch, not a particular endian type.

btw: to answer David's question, gcc 4.7 is smart enough
to elide resetting values when the struct is initialized
to 0 either with a memset or using {0}.

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

struct foo {
int a;
char b;
long c;
};

void init1(void)
{
struct foo bar = {0};
bar.a = 1;
bar.b = 2;
bar.c = 3;
printf("%p\n", &bar);
}

void init2(void)
{
struct foo bar;
memset(&bar, 0, sizeof(bar));
bar.a = 1;
bar.b = 2;
bar.c = 3;
printf("%p\n", &bar);

$ gcc -S -O2 t.c
$ cat t.s
.file "t.c"
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "%p\n"
.text
.p2align 4,,15
.globl init1
.type init1, @function
init1:
.LFB60:
.cfi_startproc
subl $44, %esp
.cfi_def_cfa_offset 48
leal 20(%esp), %eax
movl %eax, 8(%esp)
movl $.LC0, 4(%esp)
movl $1, (%esp)
movl $0, 24(%esp)
movl $1, 20(%esp)
movb $2, 24(%esp)
movl $3, 28(%esp)
call __printf_chk
addl $44, %esp
.cfi_def_cfa_offset 4
ret
.cfi_endproc
.LFE60:
.size init1, .-init1
.p2align 4,,15
.globl init2
.type init2, @function
init2:
.LFB61:
.cfi_startproc
subl $44, %esp
.cfi_def_cfa_offset 48
leal 20(%esp), %eax
movl %eax, 8(%esp)
movl $.LC0, 4(%esp)
movl $1, (%esp)
movl $0, 24(%esp)
movl $1, 20(%esp)
movb $2, 24(%esp)
movl $3, 28(%esp)
call __printf_chk
addl $44, %esp
.cfi_def_cfa_offset 4
ret
.cfi_endproc
.LFE61:
.size init2, .-init2
.ident "GCC: (Ubuntu/Linaro 4.7.3-1ubuntu1) 4.7.3"
.section .note.GNU-stack,"",@progbits