Subject: Re: [PATCH v5 19/34] random: vDSO: only access vDSO datapage after random_init()

From: "Jason A. Donenfeld" <[email-masked]>
Date: Mon, 10 Nov 2025 11:37:07 +0100

On Mon, Nov 10, 2025 at 10:04:17AM +0100, Thomas Weißschuh wrote:
> On Sat, Nov 08, 2025 at 12:46:05AM +0100, Jason A. Donenfeld wrote:
> > I'm not a huge fan of this change:
> > 
> > On Thu, Nov 06, 2025 at 11:02:12AM +0100, Thomas Weißschuh wrote:
> > > +static DEFINE_STATIC_KEY_FALSE(random_vdso_is_ready);
> > >  
> > >  /* Control how we warn userspace. */
> > >  static struct ratelimit_state urandom_warning =
> > > @@ -252,6 +253,9 @@ static void random_vdso_update_generation(unsigned long next_gen)
> > >  	if (!IS_ENABLED(CONFIG_VDSO_GETRANDOM))
> > >  		return;
> > >  
> > > +	if (!static_branch_likely(&random_vdso_is_ready))
> > > +		return;
> > > +
> > >  	/* base_crng.generation's invalid value is ULONG_MAX, while
> > >  	 * vdso_k_rng_data->generation's invalid value is 0, so add one to the
> > >  	 * former to arrive at the latter. Use smp_store_release so that this
> > > @@ -274,6 +278,9 @@ static void random_vdso_set_ready(void)
> > >  	if (!IS_ENABLED(CONFIG_VDSO_GETRANDOM))
> > >  		return;
> > >  
> > > +	if (!static_branch_likely(&random_vdso_is_ready))
> > > +		return;
> > > +
> > >  	WRITE_ONCE(vdso_k_rng_data->is_ready, true);
> > >  }
> > >  
> > > @@ -925,6 +932,9 @@ void __init random_init(void)
> > >  	_mix_pool_bytes(&entropy, sizeof(entropy));
> > >  	add_latent_entropy();
> > >  
> > > +	if (IS_ENABLED(CONFIG_VDSO_GETRANDOM))
> > > +		static_branch_enable(&random_vdso_is_ready);
> > > +
> > >  	/*
> > >  	 * If we were initialized by the cpu or bootloader before jump labels
> > >  	 * or workqueues are initialized, then we should enable the static
> > > @@ -934,8 +944,10 @@ void __init random_init(void)
> > >  		crng_set_ready(NULL);
> > >  
> > >  	/* Reseed if already seeded by earlier phases. */
> > > -	if (crng_ready())
> > > +	if (crng_ready()) {
> > >  		crng_reseed(NULL);
> > > +		random_vdso_set_ready();
> > > +	}
> > 
> > The fact that the vdso datapage is set up by the time random_init() is
> > called seems incredibly contingent on init details. Why not, instead,
> > make this a necessary part of the structure of vdso setup code, which
> > can actually know about what happens when?
> 
> The whole early init is "carefully" ordered in any case. I would have been
> happy to allocate the data pages before the random initialization, but the
> allocator is not yet usable by then.
> We could also make the ordering more visible by having the vDSO datastore call
> into a dedicated function to allow the random core to touch the data pages:
> random_vdso_enable_datapages().
> 
> > For example, one clean way of
> > doing that would be to make vdso_k_rng_data always valid by having it
> > initially point to __initdata memory, and then when it's time to
> > initialize the real datapage, memcpy() the __initdata memory to the new
> > specially allocated memory. Then we don't need the complex state
> > tracking that this commit and the prior one introduce.
> 
> Wouldn't that require synchronization between the update path and the memcpy()
> path? Also if the pointer is going to change at some point we'll probably need
> to use READ_ONCE()/WRITE_ONCE(). In general I would be happy about a cleaner
> solution for this but didn't find a great one.

This is still before userspace has started, and interrupts are disabled,
so I don't think so? Also, you only care about being after
mm_core_init(), right? So move your thing before sched_init() and then
you'll really have nothing to worry about.

But I think globally I agree with Andy/Arnd -- this is kind of ugly and
not worth it. Disable vDSO for these old CPUs with cache aliasing
issues.

Jason