[PATCH 4/7] Protectable Memory

[Igor Stoppa]

On 24/02/18 02:10, J Freyensee wrote:
> On 2/23/18 6:48 AM, Igor Stoppa wrote:

[...]

>> +struct gen_pool *pmalloc_create_pool(const char *name,
>> +					 int min_alloc_order);
> 
> Same comments as earlier.  If this is new API with new code being 
> introduced into the kernel, can the variables be declared to avoid weird 
> problems?  Like min_alloc_order being a negative value makes little 
> sense (based on the description here), so can it be declared as size_t 
> or unsigned int?

in this case, yes, but I see it as different case

[...]

>> + * * NULL				- either no memory available or
>> + *					  pool already read-only
>> + */
> 
> I don't know if an errno value is being set, but setting a variable 
> somewhere using EROFS or ENOMEM would more accurate diagnose those two 
> NULL conditions.

I expect that the latter is highly unlikely to happen, because the user
of the API controls if the pool is locked or not.

I think it shouldn't come as a surprise to the one who locked the pool,
if the pool is locked.

If the pool is used with concurrent users, attention should be paid to
not lock it before ever user is happy (this is where the user of the API
has to provide own locking.)

Since the information if the pool is already protected is actually
present in the pmalloc_data structure associated with the pool, I was
tempted to make it available through the API, but that seemed wrong.

The user of the API should be very aware of the state of the pool, since
the user is the one who sets it. Why would it have to be read back?

>> +void *pmalloc(struct gen_pool *pool, size_t size, gfp_t gfp);
>> +
>> +
>> +/**
>> + * pzalloc() - zero-initialized version of pmalloc
>> + * @pool: handle to the pool to be used for memory allocation
>> + * @size: amount of memory (in bytes) requested
>> + * @gfp: flags for page allocation
>> + *
>> + * Executes pmalloc, initializing the memory requested to 0,
>> + * before returning the pointer to it.
>> + *
>> + * Return:
>> + * * pointer to the memory requested	- success
>> + * * NULL				- either no memory available or
>> + *					  pool already read-only
>> + */
> Same comment here, though that inline function below looks highly 
> optimized...

The same applies here.
I'm not very fond of the idea of returning the status elsewhere and in a
way that is not intrinsically connected with the action that has
determined the change of state.

AFAIK *alloc functions return either the memory requested or NULL.
I wonder how realistic this case is.

[...]

>> +	if (unlikely(!(pool && n && size)))
> Has this code been run through sparse?

I use "make C=1 W=1"

> I know one thing sparse looks at 
> is if NULL is being treated like a 0, and sparse does check cases when 0 
> is being used in place for NULL for pointer checks, and I'm wondering if 
> that line of code would pass.

It's a logical AND: wouldn't NULL translate to false, rather 0?
I can add an explicit check against NULL, it's probably more readable
too, but I don't think that the current construct treats NULL as 0.

[...]

>> +	if (unlikely(pool == NULL || s == NULL))
> Here, the right check is being done, so at the very least, I would make 
> the last line I commented on the same as this one for code continuity.

ok

[...]

>> +	if (unlikely(!(pool && chunk)))
> 
> Please make this check the same as the last line I commented on, 
> especially since it's the same struct being checked.

yes

[...]

>> +	if (!name) {
>> +		WARN_ON(1);
> ??  Maybe the name check should be in WARN_ON()?

true :-(

[...]

>> +	if (unlikely(!req_size || !pool))
> same unlikely() check problem mentioned before.
>> +		return -1;
> Can we use an errno value instead for better diagnosibility?
>> +
>> +	data = pool->data;
>> +
>> +	if (data == NULL)
>> +		return -1;
> Same here (ENOMEM or ENXIO comes to mind).
>> +
>> +	if (unlikely(data->protected)) {
>> +		WARN_ON(1);
> Maybe re-write this with the check inside WARN_ON()?
>> +		return -1;
> 
> Same here, how about a different errno value for this case?

yes, to all of the above

[...]

>> +static void pmalloc_chunk_set_protection(struct gen_pool *pool,
>> +
>> +					 struct gen_pool_chunk *chunk,
>> +					 void *data)
>> +{
>> +	const bool *flag = data;
>> +	size_t chunk_size = chunk->end_addr + 1 - chunk->start_addr;
>> +	unsigned long pages = chunk_size / PAGE_SIZE;
>> +
>> +	BUG_ON(chunk_size & (PAGE_SIZE - 1));
> Re-think WARN_ON() for BUG_ON()?  And also check chunk as well, as it's 
> being used below?

ok

>> +
>> +	if (*flag)
>> +		set_memory_ro(chunk->start_addr, pages);
>> +	else
>> +		set_memory_rw(chunk->start_addr, pages);
>> +}
>> +
>> +static int pmalloc_pool_set_protection(struct gen_pool *pool, bool protection)
>> +{
>> +	struct pmalloc_data *data;
>> +	struct gen_pool_chunk *chunk;
>> +
>> +	if (unlikely(!pool))
>> +		return -EINVAL;
> This is example of what I'd perfer seeing in check_alloc_params().

yes

>> +
>> +	data = pool->data;
>> +
>> +	if (unlikely(!data))
>> +		return -EINVAL;
> ENXIO or EIO or ENOMEM sound better?

Why? At least based on he description from errno-base.h, EINVAL seemed
the most appropriate:

#define	EIO		 5	/* I/O error */
#define	ENXIO		 6	/* No such device or address */
#define	ENOMEM		12	/* Out of memory */

#define	EINVAL		22	/* Invalid argument */

If I was really pressed to change it, I'd rather pick:

#define	EFAULT		14	/* Bad address */


>> +
>> +	if (unlikely(data->protected == protection)) {
>> +		WARN_ON(1);
> Better to put the check inside WARN_ON, me thinks...

yes, I have no idea why I wrote it like that  :-(

>> +		return 0;
>> +	}
>> +
>> +	data->protected = protection;
>> +	list_for_each_entry(chunk, &(pool)->chunks, next_chunk)
>> +		pmalloc_chunk_set_protection(pool, chunk, &protection);
>> +	return 0;
>> +}
>> +
>> +int pmalloc_protect_pool(struct gen_pool *pool)
>> +{
>> +	return pmalloc_pool_set_protection(pool, true);
> Is pool == NULL being checked somewhere, similar to previous functions 
> in this patch?

right.

>> +}
>> +
>> +
>> +static void pmalloc_chunk_free(struct gen_pool *pool,
>> +			       struct gen_pool_chunk *chunk, void *data)
>> +{
> Wat is 'data' being used for? Looks unused.  Should parameters be 
> checked, like other ones?


This is the iterator that is passed to genalloc
genalloc defines the format for the iterator, because it *will* want to
pass the pointer to the opaque data structure.

>> +	untag_chunk(chunk);
>> +	gen_pool_flush_chunk(pool, chunk);
>> +	vfree_atomic((void *)chunk->start_addr);
>> +}
>> +
>> +
>> +int pmalloc_destroy_pool(struct gen_pool *pool)
>> +{
>> +	struct pmalloc_data *data;
>> +
>> +	if (unlikely(pool == NULL))
>> +		return -EINVAL;
>> +
>> +	data = pool->data;
>> +
>> +	if (unlikely(data == NULL))
>> +		return -EINVAL;
> 
> I'd use a different errno value since you already used it for pool.

Thinking more about this, how about collapsing them?
I do need to check for the value of data, before I dereference it,
however the causes are very different:

* wrong pool pointer - definitely possible
* corrupted data structure (data member overwritten) - highly unlikely

>> +
>> +	mutex_lock(&pmalloc_mutex);
>> +	list_del(&data->node);
>> +	mutex_unlock(&pmalloc_mutex);
>> +
>> +	if (likely(data->pool_kobject))
>> +		pmalloc_disconnect(data, data->pool_kobject);
>> +
>> +	pmalloc_pool_set_protection(pool, false);
>> +	gen_pool_for_each_chunk(pool, pmalloc_chunk_free, NULL);
>> +	gen_pool_destroy(pool);
>> +	kfree(data);
> Does data need to be set to NULL in this case, as data is a member of 
> pool (pool->data)?  I'm worried about dangling pointer scenarios which 
> probably isn't good for security modules?

The pool was destroyed in the previous step.
There is nothing left that can be set to NULL.
Unless we are expecting an use-after-free of the pool structure.
But everything it was referring to is gone as well.

If we really want to go after this case, then basically everything that
has been allocated should be poisoned before being freed.

Isn't it a bit too much?

>> +	return 0;
>> +}
>> +
>> +/*
>> + * When the sysfs is ready to receive registrations, connect all the
>> + * pools previously created. Also enable further pools to be connected
>> + * right away.
>> + */
>> +static int __init pmalloc_late_init(void)
>> +{
>> +	struct pmalloc_data *data, *n;
>> +
>> +	pmalloc_kobject = kobject_create_and_add("pmalloc", kernel_kobj);
>> +
>> +	mutex_lock(&pmalloc_mutex);
>> +	pmalloc_list = &pmalloc_final_list;
>> +
>> +	if (likely(pmalloc_kobject != NULL)) {
>> +		list_for_each_entry_safe(data, n, &pmalloc_tmp_list, node) {
>> +			list_move(&data->node, &pmalloc_final_list);
>> +			pmalloc_connect(data);
>> +		}
>> +	}
> It would be nice to have the init() return an error value in case of 
> failure.

ok

--
igor
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