Dubios pointer casting with put_user()

Dubios pointer casting with put_user()

[Richard Sailer]

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Hi,

In the following example from net/dccp/proto.c the pointer given
put_user() is casted to (int __user *) although the value to copy is a
unsigned long. Is this (correctness and security wise) sane? Because as
I understand it put_user() determines the amount it copies from the
pointer type.

```
		unsigned long amount = 0;

		skb = skb_peek(&sk->sk_receive_queue);
		if (skb != NULL) {
			amount = skb->len;
		}
		rc = put_user(amount, (int __user *)arg);
```

Also skb->len is an unsigned int realisticly in most cases < 9000 (and
in all cases I can imagine < int_max (with 16 bit)).

I would like to declare amount as int outside of the switch case
statement (because I  need it in another case statement as signed int)
would it be safe to do so?

Thanks,
-- Richard


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Re: Dubios pointer casting with put_user()

[Richard Sailer]

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Link to full source:
https://elixir.bootlin.com/linux/latest/source/net/dccp/proto.c#L390


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Re: Dubios pointer casting with put_user()

[Valdis Klētnieks]

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On Fri, 17 Jul 2020 02:13:34 +0200, Richard Sailer said:

> unsigned long. Is this (correctness and security wise) sane? Because as
> I understand it put_user() determines the amount it copies from the
> pointer type.

> 		rc = put_user(amount, (int __user *)arg);

If that were true, you wouldn't need to pass the 'amount' variable....

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Re: Dubios pointer casting with put_user()

[Richard Sailer]

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On 18/07/2020 00:46, Valdis Klētnieks wrote:
> On Fri, 17 Jul 2020 02:13:34 +0200, Richard Sailer said:
> 
>> unsigned long. Is this (correctness and security wise) sane? Because as
>> I understand it put_user() determines the amount it copies from the
>> pointer type.
> 
>> 		rc = put_user(amount, (int __user *)arg);
> 
> If that were true, you wouldn't need to pass the 'amount' variable....
> 
Hmm, that would make no sense to me. arg is a pointer to user space
memory, put_user would still need the value to copy to that memory.

And my understanding of put_user() comes from its definition in uaccess.h:

#define put_user(x, ptr)					\
({								\
	int __ret_pu;						\
	__typeof__(*(ptr)) __pu_val;				\
	__chk_user_ptr(ptr);					\
	might_fault();						\
	__pu_val = x;						\
	switch (sizeof(*(ptr))) {				\
	case 1:							\
		__put_user_x(1, __pu_val, ptr, __ret_pu);	\
		break;						\
	case 2:							\
		__put_user_x(2, __pu_val, ptr, __ret_pu);	\
		break;						\
	case 4:							\
		__put_user_x(4, __pu_val, ptr, __ret_pu);	\
		break;						\
	case 8:							\
		__put_user_x8(__pu_val, ptr, __ret_pu);		\
		break;						\
[...]

But please tell me if I got anything wrong here, I'm still not 100% sure

-- Richard


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Re: Dubios pointer casting with put_user()

[Valdis Klētnieks]

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On Sun, 19 Jul 2020 14:15:36 +0200, Richard Sailer said:

> > If that were true, you wouldn't need to pass the 'amount' variable....
> >
> Hmm, that would make no sense to me. arg is a pointer to user space
> memory, put_user would still need the value to copy to that memory.

-ENOCAFFEINE. :)

I was thinking copy_(to/from)_user, not get/put.

I will now depart for the grocery store and obtain a supply of caffeine :)


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