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

[Jason A. Donenfeld]

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?

> > +	pages_addr = vm_mmap(NULL, 0, alloc_size, PROT_READ | PROT_WRITE,
> > +			     MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED, 0);
> > +	if (IS_ERR_VALUE(pages_addr))
> > +		return pages_addr;
> 
> This will only succeed if the userspace process has permission to mlock pages,
> i.e. if there is space available in RLIMIT_MEMLOCK or if process has
> CAP_IPC_LOCK.  I suppose this is working as intended, as this syscall can be
> used to try to allocate and mlock arbitrary amounts of memory.
> 
> I wonder if this permission check will cause problems.  Maybe there could be a
> way to relax it for just one page per task?  I don't know how that would work,
> though, especially when the planned usage involves userspace allocating a single
> pool of these contexts per process that get handed out to threads.

Probably though, we don't want to create a mlock backdoor, right? I
suppose if a user is above RLIMIT_MEMLOCK, it'll just fallback to the
slowpath, which still works. That seems like an okay enough
circumstance.

Jason