[PATCH 4.9 021/264] random: add a spinlock_t to struct batched_entropy

Greg Kroah-Hartman
2022-06-23T16:47:15Z

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

[ Upstream commit b7d5dc21072cda7124d13eae2aefb7343ef94197 ]

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
from the local CPU. It does not protect against users on the same CPU
from another context. It is possible that a preemptible context reads
slot 0 and then an interrupt occurs and the same value is read again.

The above scenario is confirmed by lockdep if we add a spinlock:
| ================================
| 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
…
| 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 ***
|
| stack backtrace:
| Call Trace:
…
| kmem_cache_alloc_trace+0x20e/0x270
| ipmi_alloc_recv_msg+0x16/0x40
…
| __do_softirq+0xec/0x48d
| run_ksoftirqd+0x37/0x60
| smpboot_thread_fn+0x191/0x290
| kthread+0xfe/0x130
| ret_from_fork+0x3a/0x50

Add a spinlock_t to the batched_entropy data structure and acquire the
lock while accessing it. Acquire the lock with disabled interrupts
because this function may be used from interrupt context.

Remove the batched_entropy_reset_lock lock. Now that we have a lock for
the data scructure, we can access it from a remote CPU.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/random.c | 52 +++++++++++++++++++++++++-------------------------
1 file changed, 27 insertions(+), 25 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2228,8 +2228,8 @@ struct batched_entropy {
u32 entropy_u32[CHACHA20_BLOCK_SIZE / sizeof(u32)];
};
unsigned int position;
+ spinlock_t batch_lock;
};
-static rwlock_t batched_entropy_reset_lock = __RW_LOCK_UNLOCKED(batched_entropy_reset_lock);

/*
* Get a random word for internal kernel use only. The quality of the random
@@ -2239,12 +2239,14 @@ static rwlock_t batched_entropy_reset_lo
* 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) = {
+ .batch_lock = __SPIN_LOCK_UNLOCKED(batched_entropy_u64.lock),
+};
+
u64 get_random_u64(void)
{
u64 ret;
- bool use_lock;
- unsigned long flags = 0;
+ unsigned long flags;
struct batched_entropy *batch;
static void *previous;

@@ -2259,28 +2261,25 @@ u64 get_random_u64(void)

warn_unseeded_randomness(&previous);

- use_lock = READ_ONCE(crng_init) < 2;
- batch = &get_cpu_var(batched_entropy_u64);
- if (use_lock)
- read_lock_irqsave(&batched_entropy_reset_lock, flags);
+ batch = raw_cpu_ptr(&batched_entropy_u64);
+ spin_lock_irqsave(&batch->batch_lock, flags);
if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
extract_crng((u8 *)batch->entropy_u64);
batch->position = 0;
}
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_irqrestore(&batch->batch_lock, flags);
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) = {
+ .batch_lock = __SPIN_LOCK_UNLOCKED(batched_entropy_u32.lock),
+};
u32 get_random_u32(void)
{
u32 ret;
- bool use_lock;
- unsigned long flags = 0;
+ unsigned long flags;
struct batched_entropy *batch;
static void *previous;

@@ -2289,18 +2288,14 @@ u32 get_random_u32(void)

warn_unseeded_randomness(&previous);

- use_lock = READ_ONCE(crng_init) < 2;
- batch = &get_cpu_var(batched_entropy_u32);
- if (use_lock)
- read_lock_irqsave(&batched_entropy_reset_lock, flags);
+ batch = raw_cpu_ptr(&batched_entropy_u32);
+ spin_lock_irqsave(&batch->batch_lock, flags);
if (batch->position % ARRAY_SIZE(batch->entropy_u32) == 0) {
extract_crng((u8 *)batch->entropy_u32);
batch->position = 0;
}
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_irqrestore(&batch->batch_lock, flags);
return ret;
}
EXPORT_SYMBOL(get_random_u32);
@@ -2314,12 +2309,19 @@ static void invalidate_batched_entropy(v
int cpu;
unsigned long flags;

- write_lock_irqsave(&batched_entropy_reset_lock, flags);
for_each_possible_cpu (cpu) {
- per_cpu_ptr(&batched_entropy_u32, cpu)->position = 0;
- per_cpu_ptr(&batched_entropy_u64, cpu)->position = 0;
+ struct batched_entropy *batched_entropy;
+
+ batched_entropy = per_cpu_ptr(&batched_entropy_u32, cpu);
+ spin_lock_irqsave(&batched_entropy->batch_lock, flags);
+ batched_entropy->position = 0;
+ spin_unlock(&batched_entropy->batch_lock);
+
+ batched_entropy = per_cpu_ptr(&batched_entropy_u64, cpu);
+ spin_lock(&batched_entropy->batch_lock);
+ batched_entropy->position = 0;
+ spin_unlock_irqrestore(&batched_entropy->batch_lock, flags);
}
- write_unlock_irqrestore(&batched_entropy_reset_lock, flags);
}

/**