Messages in this thread Patch in this message | | | Date | Fri, 29 Jul 2022 12:22:08 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH] random: Initialize vsprintf's pointer hash once the random core is ready. |
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On Fri 2022-07-29 10:52:58, Sebastian Andrzej Siewior wrote: > The printk code invokes vnsprintf in order to compute the complete > string before adding it into its buffer. This happens in an IRQ-off > region which leads to a warning on PREEMPT_RT in the random code if the > format strings contains a %p for pointer printing. This happens because > the random core acquires locks which become sleeping locks on PREEMPT_RT > which must not be acquired with disabled interrupts and or preemption. > By default the pointers are hashed which requires a random value on the > first invocation (either by printk or another user which comes first. > > One could argue that there is no need for printk to disable interrupts > during the vsprintf() invocation which would fix the just mentioned > problem. However printk itself can be invoked in a context with > disabled interrupts which would lead to the very same problem. > > This late init via printk can be avoided by explicitly initializing > vsprintf's random value once the random-core has been initialized. > > Remove the on demand init from __ptr_to_hashval() and keep the -EAGAIN if > the init has not yet been performed. Move the actual init bits to > vsprintf_init_hash_pointer() which are invoked from random-core once it > has been initialized and get_random_bytes() is available.
> --- a/drivers/char/random.c > +++ b/drivers/char/random.c > @@ -221,10 +222,15 @@ static void crng_reseed(void) > ++next_gen; > WRITE_ONCE(base_crng.generation, next_gen); > WRITE_ONCE(base_crng.birth, jiffies); > - if (!static_branch_likely(&crng_is_ready)) > + if (!static_branch_likely(&crng_is_ready)) { > crng_init = CRNG_READY; > + init_hash_pointer = true;
I am not familiar with the crng code. I wonder if the following would work:
if (!static_branch_likely(&crng_is_ready) && crng_init != CRNG_READY) { crng_init = CRNG_READY; init_hash_pointer = true; }
The point is that vsprintf_init_hash_pointer() will be called only by the first caller. It would allow to remove the @filling spin lock.
> + } > spin_unlock_irqrestore(&base_crng.lock, flags); > memzero_explicit(key, sizeof(key)); > + > + if (init_hash_pointer) > + vsprintf_init_hash_pointer(); > } > > /* > diff --git a/lib/vsprintf.c b/lib/vsprintf.c > index 3c1853a9d1c09..6fa2ebb9f9b9e 100644 > --- a/lib/vsprintf.c > +++ b/lib/vsprintf.c > @@ -751,36 +751,30 @@ static int __init debug_boot_weak_hash_enable(char *str) > early_param("debug_boot_weak_hash", debug_boot_weak_hash_enable); > > static DEFINE_STATIC_KEY_FALSE(filled_random_ptr_key); > +static siphash_key_t ptr_key __read_mostly; > > -static void enable_ptr_key_workfn(struct work_struct *work) > +void vsprintf_init_hash_pointer(void) > { > - static_branch_enable(&filled_random_ptr_key); > + static DEFINE_SPINLOCK(filling); > + unsigned long flags; > + static bool filled; > + > + spin_lock_irqsave(&filling, flags); > + if (!filled) { > + get_random_bytes(&ptr_key, sizeof(ptr_key)); > + filled = true; > + static_branch_enable(&filled_random_ptr_key);
This can't be called in an atomic context. Is crng_reseed() always called in a non-atomic context?
That said, the static branch is an overkill. vsprintf() is a slow path. It should be enough to use a simple boolean. It might require a simple memory barrier to serialize @ptr_key and the new boolean read&write.
> + } > + spin_unlock_irqrestore(&filling, flags); > }
Best Regards, Petr
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