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

Jason A. Donenfeld
2022-11-20T01:45:46Z

On Sat, Nov 19, 2022 at 05:40:04PM -0800, Eric Biggers wrote:
> 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));

Thanks, that's much nicer.

Jason