Re: [patch V4 01/32] mm/slab: Fix broken stack trace storage

Thomas Gleixner
2019-04-15T16:07:56Z

On Mon, 15 Apr 2019, Josh Poimboeuf wrote:
> On Mon, Apr 15, 2019 at 11:02:58AM +0200, Thomas Gleixner wrote:
> > addr = (unsigned long *)&((char *)addr)[obj_offset(cachep)];
> >
> > - if (size < 5 * sizeof(unsigned long))
> > + if (size < 5)
> > return;
> >
> > *addr++ = 0x12345678;
> > *addr++ = caller;
> > *addr++ = smp_processor_id();
> > - size -= 3 * sizeof(unsigned long);
> > + size -= 3;
> > +#ifdef CONFIG_STACKTRACE
> > {
> > - unsigned long *sptr = &caller;
> > - unsigned long svalue;
> > -
> > - while (!kstack_end(sptr)) {
> > - svalue = *sptr++;
> > - if (kernel_text_address(svalue)) {
> > - *addr++ = svalue;
> > - size -= sizeof(unsigned long);
> > - if (size <= sizeof(unsigned long))
> > - break;
> > - }
> > - }
> > + struct stack_trace trace = {
> > + /* Leave one for the end marker below */
> > + .max_entries = size - 1,
> > + .entries = addr,
> > + .skip = 3,
> > + };
> >
> > + save_stack_trace(&trace);
> > + addr += trace.nr_entries;
> > }
> > - *addr++ = 0x87654321;
> > +#endif
> > + *addr = 0x87654321;
>
> Looks like stack_trace.nr_entries isn't initialized? (though this code
> gets eventually replaced by a later patch)

struct initializer initialized the non mentioned fields to 0, if I'm not
totally mistaken.

> Who actually reads this stack trace? I couldn't find a consumer.

It's stored directly in the memory pointed to by @addr and that's the freed
cache memory. If that is used later (UAF) then the stack trace can be
printed to see where it was freed.

Thanks,

tglx