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Subject[RFC PATCH] random: add a spinlock in addition to get_cpu_var()
The per-CPU variable batched_entropy_uXX is protected by get_cpu_var().
This is just a preempt_disable() which ensures that the variable is only
protected against other CPUs. It does not protect against users from
interrupt context. It possible that a preemptible context read slot 0
and an interrupt read the same value.

By adding the spinlock we would deadlock in such a scenario and have
lockdep coverage to warn us about it. And it does:

| ================================
| WARNING: inconsistent lock state
| 5.1.0-rc3+ #42 Not tainted
| --------------------------------
| inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
| ksoftirqd/9/56 [HC0[0]:SC1[1]:HE0:SE0] takes:
| (____ptrval____) (batched_entropy_u32.lock){+.?.}, at: get_random_u32+0x3e/0xe0
| {SOFTIRQ-ON-W} state was registered at:
| _raw_spin_lock+0x2a/0x40
| get_random_u32+0x3e/0xe0
| new_slab+0x15c/0x7b0
| ___slab_alloc+0x492/0x620
| __slab_alloc.isra.73+0x53/0xa0
| kmem_cache_alloc_node+0xaf/0x2a0
| copy_process.part.41+0x1e1/0x2370
| _do_fork+0xdb/0x6d0
| kernel_thread+0x20/0x30
| kthreadd+0x1ba/0x220
| ret_from_fork+0x3a/0x50
| irq event stamp: 107
| hardirqs last enabled at (106): [<ffffffff9ddf72c4>] _raw_spin_unlock_irqrestore+0x54/0x70
| hardirqs last disabled at (107): [<ffffffff9d71c7bc>] __slab_alloc.isra.73+0x2c/0xa0
| softirqs last enabled at (94): [<ffffffff9e0003dc>] __do_softirq+0x3dc/0x48d
| softirqs last disabled at (99): [<ffffffff9d5489a7>] run_ksoftirqd+0x37/0x60
|
| other info that might help us debug this:
| Possible unsafe locking scenario:
|
| CPU0
| ----
| lock(batched_entropy_u32.lock);
| <Interrupt>
| lock(batched_entropy_u32.lock);
|
| *** DEADLOCK ***
|
| no locks held by ksoftirqd/9/56.
|
| stack backtrace:
| CPU: 9 PID: 56 Comm: ksoftirqd/9 Not tainted 5.1.0-rc3+ #42
| Hardware name: Intel Corporation S2600WFT/S2600WFT, BIOS SE5C620.86B.00.01.0016.020120190930 02/01/2019
| Call Trace:
| dump_stack+0x7d/0xbb
| print_usage_bug+0x1ca/0x1e0
| mark_lock+0x50f/0x580
| __lock_acquire+0x267/0x1100
| lock_acquire+0xa6/0x1b0
| _raw_spin_lock+0x2a/0x40
| get_random_u32+0x3e/0xe0
| new_slab+0x15c/0x7b0
| ___slab_alloc+0x492/0x620
| __slab_alloc.isra.73+0x53/0xa0
| kmem_cache_alloc_trace+0x20e/0x270
| ipmi_alloc_recv_msg+0x16/0x40
| i_ipmi_request+0x51c/0xaa0
| send_channel_info_cmd+0x80/0xa0
| channel_handler+0xd1/0x160
| deliver_response+0xd6/0x100
| deliver_local_response+0x9/0x20
| handle_one_recv_msg+0x11c/0x10b0
| handle_new_recv_msgs+0x1b1/0x270
| tasklet_action_common.isra.19+0x67/0x1d0
| __do_softirq+0xec/0x48d
| run_ksoftirqd+0x37/0x60
| smpboot_thread_fn+0x191/0x290
| kthread+0xfe/0x130
| ret_from_fork+0x3a/0x50

I had also another backtrace with in-IRQ usage. Should I resend the
patch with spinlock_irq_save() instead?

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
drivers/char/random.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 38c6d1af6d1c0..6b0dce50c2b92 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2211,6 +2211,7 @@ struct batched_entropy {
u32 entropy_u32[CHACHA_BLOCK_SIZE / sizeof(u32)];
};
unsigned int position;
+ spinlock_t lock;
};
static rwlock_t batched_entropy_reset_lock = __RW_LOCK_UNLOCKED(batched_entropy_reset_lock);

@@ -2222,7 +2223,10 @@ static rwlock_t batched_entropy_reset_lock = __RW_LOCK_UNLOCKED(batched_entropy_
* wait_for_random_bytes() should be called and return 0 at least once
* at any point prior.
*/
-static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u64);
+static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u64) = {
+ .lock = __SPIN_LOCK_UNLOCKED(batched_entropy_u64.lock),
+};
+
u64 get_random_u64(void)
{
u64 ret;
@@ -2243,7 +2247,8 @@ u64 get_random_u64(void)
warn_unseeded_randomness(&previous);

use_lock = READ_ONCE(crng_init) < 2;
- batch = &get_cpu_var(batched_entropy_u64);
+ batch = raw_cpu_ptr(&batched_entropy_u64);
+ spin_lock(&batch->lock);
if (use_lock)
read_lock_irqsave(&batched_entropy_reset_lock, flags);
if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
@@ -2253,12 +2258,14 @@ u64 get_random_u64(void)
ret = batch->entropy_u64[batch->position++];
if (use_lock)
read_unlock_irqrestore(&batched_entropy_reset_lock, flags);
- put_cpu_var(batched_entropy_u64);
+ spin_unlock(&batch->lock);
return ret;
}
EXPORT_SYMBOL(get_random_u64);

-static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u32);
+static DEFINE_PER_CPU(struct batched_entropy, batched_entropy_u32) = {
+ .lock = __SPIN_LOCK_UNLOCKED(batched_entropy_u32.lock),
+};
u32 get_random_u32(void)
{
u32 ret;
@@ -2273,7 +2280,8 @@ u32 get_random_u32(void)
warn_unseeded_randomness(&previous);

use_lock = READ_ONCE(crng_init) < 2;
- batch = &get_cpu_var(batched_entropy_u32);
+ batch = raw_cpu_ptr(&batched_entropy_u32);
+ spin_lock(&batch->lock);
if (use_lock)
read_lock_irqsave(&batched_entropy_reset_lock, flags);
if (batch->position % ARRAY_SIZE(batch->entropy_u32) == 0) {
@@ -2283,7 +2291,7 @@ u32 get_random_u32(void)
ret = batch->entropy_u32[batch->position++];
if (use_lock)
read_unlock_irqrestore(&batched_entropy_reset_lock, flags);
- put_cpu_var(batched_entropy_u32);
+ spin_unlock(&batch->lock);
return ret;
}
EXPORT_SYMBOL(get_random_u32);
--
2.11.0
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