Re: [PATCH v16 4/5] random: introduce generic vDSO getrandom() implementation

[Jason A. Donenfeld]

On Fri, May 31, 2024 at 04:06:37PM -0700, Andy Lutomirski wrote:
> > On May 28, 2024, at 5:25 AM, Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> >
> > Provide a generic C vDSO getrandom() implementation, which operates on
> > an opaque state returned by vgetrandom_alloc() and produces random bytes
> > the same way as getrandom(). This has a the API signature:
> >
> >  ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state);
> 
> > +/**
> > + * type vdso_kernel_ulong - unsigned long type that matches kernel's unsigned long
> > + *
> > + * Data shared between userspace and the kernel must operate the same way in both 64-bit code and in
> > + * 32-bit compat code, over the same potentially 64-bit kernel. This type represents the size of an
> > + * unsigned long as used by kernel code. This isn't necessarily the same as an unsigned long as used
> > + * by userspace, however.
> 
> Why is this better than using plain u64?  It’s certainly more
> complicated. It also rather fundamentally breaks CRIU on 32-bit
> userspace (although CRIU may well be unable to keep vgetrandom working
> after a restore onto a different kernel anyway).  Admittedly 32-bit
> userspace is a slowly dying breed, but still.

That came out of this conversation: https://lore.kernel.org/all/878rjs7mcx.fsf@xxxxxxxxxxxxxxxxxxxxxxxx/
(And I'd like single instruction increments, which means long, not u64
on 32-bit machines.)

> > +{
> > +    ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len);
> > +    struct vgetrandom_state *state = opaque_state;
> > +    size_t batch_len, nblocks, orig_len = len;
> > +    unsigned long current_generation;
> > +    void *orig_buffer = buffer;
> > +    u32 counter[2] = { 0 };
> > +    bool in_use, have_retried = false;
> > +
> > +    /* The state must not straddle a page, since pages can be zeroed at any time. */
> > +    if (unlikely(((unsigned long)opaque_state & ~PAGE_MASK) + sizeof(*state) > PAGE_SIZE))
> > +        goto fallback_syscall;
> 
> This is weird. Either the provided pointer is valid or it isn’t.
> Reasonable outcomes are a segfault if the pointer is bad or success
> (or fallback if needed for some reason) if the pointer is good.  Why
> is there specific code to catch a specific sort of pointer screwup
> here?

I guess I could make it return -EFAULT in this case, rather than
silently succeeding.

Jason