Re: [bug report] livepatch: Initialize shadow variables safely by a custom callback

[Joe Lawrence <joe.lawrence@redhat.com>]

On 1/7/20 10:23 AM, Dan Carpenter wrote:
> On Tue, Jan 07, 2020 at 10:06:21AM -0500, Joe Lawrence wrote:
>> On 1/7/20 8:29 AM, Dan Carpenter wrote:
>>> Hello Petr Mladek,
>>>
>>> The patch e91c2518a5d2: "livepatch: Initialize shadow variables
>>> safely by a custom callback" from Apr 16, 2018, leads to the
>>> following static checker warning:
>>>
>>> 	samples/livepatch/livepatch-shadow-fix1.c:86 livepatch_fix1_dummy_alloc()
>>> 	error: 'klp_shadow_alloc()' 'leak' too small (4 vs 8)
>>>
>>> samples/livepatch/livepatch-shadow-fix1.c
>>>       53  static int shadow_leak_ctor(void *obj, void *shadow_data, void *ctor_data)
>>>       54  {
>>>       55          void **shadow_leak = shadow_data;
>>>       56          void *leak = ctor_data;
>>>       57
>>>       58          *shadow_leak = leak;
>>>       59          return 0;
>>>       60  }
>>>       61
>>>       62  static struct dummy *livepatch_fix1_dummy_alloc(void)
>>>       63  {
>>>       64          struct dummy *d;
>>>       65          void *leak;
>>>       66
>>>       67          d = kzalloc(sizeof(*d), GFP_KERNEL);
>>>       68          if (!d)
>>>       69                  return NULL;
>>>       70
>>>       71          d->jiffies_expire = jiffies +
>>>       72                  msecs_to_jiffies(1000 * EXPIRE_PERIOD);
>>>       73
>>>       74          /*
>>>       75           * Patch: save the extra memory location into a SV_LEAK shadow
>>>       76           * variable.  A patched dummy_free routine can later fetch this
>>>       77           * pointer to handle resource release.
>>>       78           */
>>>       79          leak = kzalloc(sizeof(int), GFP_KERNEL);
>>>       80          if (!leak) {
>>>       81                  kfree(d);
>>>       82                  return NULL;
>>>       83          }
>>>       84
>>>       85          klp_shadow_alloc(d, SV_LEAK, sizeof(leak), GFP_KERNEL,
>>>                                                ^^^^^^^^^^^^
>>> This doesn't seem right at all?  Leak is a pointer.  Why is leak a void
>>> pointer instead of an int pointer?
>>>
>>
>> Hi Dan,
>>
>> If I remember this code correctly, the shadow variable is tracking the
>> pointer value itself and not its contents, so sizeof(leak) should be correct
>> for the shadow variable data size.
>>
>> (For kernel/livepatch/shadow.c :: __klp_shadow_get_or_alloc() creates new
>> struct klp_shadow with .data[size] to accommodate its meta-data plus the
>> desired data).
>>
>> Why isn't leak an int pointer?  I don't remember why, according to git
>> history it's been that way since the beginning.  I think it was coded to
>> say, "Give me some storage, any size an int will do.  I'm not going to touch
>> it, but I want to demonstrate a memory leak".
>>
>> Would modifying the pointer type satisfy the static code complaint?
>>
>> Since the warning is about a size mismatch, what are the parameters that it
>> is keying on?  [ ... snip ... ]
> 
> It looks at places which call klp_shadow_alloc() and says that sometimes
> the third argument is the size of the last argument.  Then it complains
> when a caller doesn't match.
> 
> I could just make klp_shadow_alloc() an exception.
> 

Ah, I see.  It must also be checking that the last two arguments (ctor, 
ctor_data) are non-null, as that's the simplified calling sequence.

/me runs cscope to find an example ...

Well that's interesting, there really aren't any other 
klp_shadow_alloc() callers aside from 
lib/livepatch/test_klp_shadow_vars.c, which hides it in a wrapper 
routine that probably dodges the static code check.

For a typical out-of-tree example [1], we do a lot of this:

	newpid = klp_shadow_get_or_alloc(p, 0, sizeof(*newpid),
				     GFP_KERNEL, NULL, NULL);
	if (newpid)
		*newpid = ctr++;

as we don't always need the constructor / destructor callbacks for 
simple shadow variables.

Since the ctor/dtor callback part of the API was provided by Petr, I'll 
let him chime in what the code checker should be warning about here.  I 
think it may be more complicated than it's worth, but maybe he has 
another idea.

Regards,

-- Joe

[1] 
https://github.com/dynup/kpatch/blob/master/test/integration/centos-7/shadow-newpid.patch