Messages in this thread Patch in this message | | | From | Sebastian Andrzej Siewior <> | Subject | [PATCH 2/2 v2] lib/vsprintf: Initialize vsprintf's pointer hash once the random core is ready. | Date | Fri, 29 Jul 2022 17:47:16 +0200 |
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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 disabled. 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.
Move the initializaion of ptr_key into a worker and schedule it from subsys_initcall(). This happens early but after the workqueue subsystem is ready. Use get_random_bytes_wait() to retrieve the random value which will block until random data is available.
Reported-by: Mike Galbraith <efault@gmx.de> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- lib/vsprintf.c | 44 +++++++++++++++++++++++++------------------- 1 file changed, 25 insertions(+), 19 deletions(-)
--- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -751,31 +751,37 @@ static int __init debug_boot_weak_hash_e early_param("debug_boot_weak_hash", debug_boot_weak_hash_enable); static bool filled_random_ptr_key; +static siphash_key_t ptr_key __read_mostly; + +static void fill_ptr_key_workfn(struct work_struct *work) +{ + int ret; + + ret = get_random_bytes_wait(&ptr_key, sizeof(ptr_key)); + if (WARN_ON(ret < 0)) + return; + /* Pairs with smp_rmb() before reading ptr_key. */ + smp_wmb(); + WRITE_ONCE(filled_random_ptr_key, true); +} + +static int vsprintf_init_hashval(void) +{ + static DECLARE_WORK(fill_ptr_key_work, fill_ptr_key_workfn); + + queue_work(system_unbound_wq, &fill_ptr_key_work); + return 0; +} +subsys_initcall(vsprintf_init_hashval) /* Maps a pointer to a 32 bit unique identifier. */ static inline int __ptr_to_hashval(const void *ptr, unsigned long *hashval_out) { - static siphash_key_t ptr_key __read_mostly; unsigned long hashval; - if (!READ_ONCE(filled_random_ptr_key)) { - static bool filled = false; - static DEFINE_SPINLOCK(filling); - unsigned long flags; - - if (!rng_is_initialized() || - !spin_trylock_irqsave(&filling, flags)) - return -EAGAIN; - - if (!filled) { - get_random_bytes(&ptr_key, sizeof(ptr_key)); - /* Pairs with smp_rmb() before reading ptr_key. */ - smp_wmb(); - WRITE_ONCE(filled_random_ptr_key, true); - filled = true; - } - spin_unlock_irqrestore(&filling, flags); - } + if (!READ_ONCE(filled_random_ptr_key)) + return -EBUSY; + /* Pairs with smp_wmb() after writting ptr_key. */ smp_rmb();
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