linux-kernel - Re: [PATCH v5 1/3] random: add vgetrandom

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Date:   Sun, 20 Nov 2022 02:45:37 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>
Subject: Re: [PATCH v5 1/3] random: add vgetrandom_alloc() syscall

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