Re: [PATCH v5 1/3] random: add vgetrandom_alloc() syscall

[Eric Biggers]

On Sun, Nov 20, 2022 at 12:59:07AM +0100, Jason A. Donenfeld wrote:
> Hi Eric,
> 
> On Sat, Nov 19, 2022 at 12:39:26PM -0800, Eric Biggers wrote:
> > On Sat, Nov 19, 2022 at 01:09:27PM +0100, Jason A. Donenfeld wrote:
> > > +SYSCALL_DEFINE3(vgetrandom_alloc, unsigned long __user *, num,
> > > +		unsigned long __user *, size_per_each, unsigned int, flags)
> > > +{
> > > +	unsigned long alloc_size;
> > > +	unsigned long num_states;
> > > +	unsigned long pages_addr;
> > > +	int ret;
> > > +
> > > +	if (flags)
> > > +		return -EINVAL;
> > > +
> > > +	if (get_user(num_states, num))
> > > +		return -EFAULT;
> > > +
> > > +	alloc_size = size_mul(num_states, sizeof(struct vgetrandom_state));
> > > +	if (alloc_size == SIZE_MAX)
> > > +		return -EOVERFLOW;
> > > +	alloc_size = roundup(alloc_size, PAGE_SIZE);
> > 
> > Small detail: the roundup to PAGE_SIZE can make alloc_size overflow to 0.
> > 
> > Also, 'roundup(alloc_size, PAGE_SIZE)' could be 'PAGE_ALIGN(alloc_size)'.
> 
> Good catch, thanks. So perhaps this?
> 
>         alloc_size = size_mul(num_states, sizeof(struct vgetrandom_state));
>         if (alloc_size > SIZE_MAX - PAGE_SIZE + 1)
>                 return -EOVERFLOW;
>         alloc_size = PAGE_ALIGN(alloc_size);
> 
> Does that look right?

Yes.  Maybe use 'SIZE_MAX & PAGE_MASK'?

Another alternative is the following:

	if (num_states >
	    (SIZE_MAX & PAGE_MASK) / sizeof(struct vgetrandom_state))
		return -EOVERFLOW;
	alloc_size = PAGE_ALIGN(num_states * sizeof(struct vgetrandom_state));

- Eric