Re: [PATCH v11 19/19] kasan: enable instrumentation of global variables

[Rusty Russell]

Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> writes:
> On 02/16/2015 05:58 AM, Rusty Russell wrote:
>> Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> writes:
>>> This feature let us to detect accesses out of bounds of
>>> global variables. This will work as for globals in kernel
>>> image, so for globals in modules. Currently this won't work
>>> for symbols in user-specified sections (e.g. __init, __read_mostly, ...)
>>> @@ -1807,6 +1808,7 @@ static void unset_module_init_ro_nx(struct module *mod) { }
>>>  void __weak module_memfree(void *module_region)
>>>  {
>>>  	vfree(module_region);
>>> +	kasan_module_free(module_region);
>>>  }
>> 
>> This looks racy (memory reuse?).  Perhaps try other order?
>> 
>
> You are right, it's racy. Concurrent kasan_module_alloc() could fail because
> kasan_module_free() wasn't called/finished yet, so whole module_alloc() will fail
> and module loading will fail.
> However, I just find out that this race is not the worst problem here.
> When vfree(addr) called in interrupt context, memory at addr will be reused for
> storing 'struct llist_node':
>
> void vfree(const void *addr)
> {
> ...
> 	if (unlikely(in_interrupt())) {
> 		struct vfree_deferred *p = this_cpu_ptr(&vfree_deferred);
> 		if (llist_add((struct llist_node *)addr, &p->list))
> 			schedule_work(&p->wq);
>
>
> In this case we have to free shadow *after* freeing 'module_region', because 'module_region'
> is still used in llist_add() and in free_work() latter.
> free_work() (in mm/vmalloc.c) processes list in LIFO order, so to free shadow after freeing
> 'module_region' kasan_module_free(module_region); should be called before vfree(module_region);
>
> It will be racy still, but this is not so bad as potential crash that we have now.
> Honestly, I have no idea how to fix this race nicely. Any suggestions?

I think you need to take over the rcu callback for the kasan case.

Perhaps we rename that __module_memfree(), and do:

void module_memfree(void *p)
{
#ifdef CONFIG_KASAN
        ...
#endif
        __module_memfree(p);        
}

Note: there are calls to module_memfree from other code (BPF and
kprobes).  I assume you looked at those too...

Cheers,
Rusty.
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