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Subject[PATCH] mm/slub: fix a deadlock in shuffle_freelist()
Date
The commit b7d5dc21072c ("random: add a spinlock_t to struct
batched_entropy") insists on acquiring "batched_entropy_u32.lock" in
get_random_u32() which introduced the lock chain,

"&rq->lock --> batched_entropy_u32.lock"

even after crng init. As the result, it could result in deadlock below.
Fix it by using get_random_bytes() in shuffle_freelist() which does not
need to take on the batched_entropy locks.

WARNING: possible circular locking dependency detected
5.3.0-rc7-mm1+ #3 Tainted: G L
------------------------------------------------------
make/7937 is trying to acquire lock:
ffff900012f225f8 (random_write_wait.lock){....}, at:
__wake_up_common_lock+0xa8/0x11c

but task is already holding lock:
ffff0096b9429c00 (batched_entropy_u32.lock){-.-.}, at:
get_random_u32+0x6c/0x1dc

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (batched_entropy_u32.lock){-.-.}:
lock_acquire+0x31c/0x360
_raw_spin_lock_irqsave+0x7c/0x9c
get_random_u32+0x6c/0x1dc
new_slab+0x234/0x6c0
___slab_alloc+0x3c8/0x650
kmem_cache_alloc+0x4b0/0x590
__debug_object_init+0x778/0x8b4
debug_object_init+0x40/0x50
debug_init+0x30/0x29c
hrtimer_init+0x30/0x50
init_dl_task_timer+0x24/0x44
__sched_fork+0xc0/0x168
init_idle+0x78/0x26c
fork_idle+0x12c/0x178
idle_threads_init+0x108/0x178
smp_init+0x20/0x1bc
kernel_init_freeable+0x198/0x26c
kernel_init+0x18/0x334
ret_from_fork+0x10/0x18

-> #2 (&rq->lock){-.-.}:
lock_acquire+0x31c/0x360
_raw_spin_lock+0x64/0x80
task_fork_fair+0x5c/0x1b0
sched_fork+0x15c/0x2dc
copy_process+0x9e0/0x244c
_do_fork+0xb8/0x644
kernel_thread+0xc4/0xf4
rest_init+0x30/0x238
arch_call_rest_init+0x10/0x18
start_kernel+0x424/0x52c

-> #1 (&p->pi_lock){-.-.}:
lock_acquire+0x31c/0x360
_raw_spin_lock_irqsave+0x7c/0x9c
try_to_wake_up+0x74/0x8d0
default_wake_function+0x38/0x48
pollwake+0x118/0x158
__wake_up_common+0x130/0x1c4
__wake_up_common_lock+0xc8/0x11c
__wake_up+0x3c/0x4c
account+0x390/0x3e0
extract_entropy+0x2cc/0x37c
_xfer_secondary_pool+0x35c/0x3c4
push_to_pool+0x54/0x308
process_one_work+0x4f4/0x950
worker_thread+0x390/0x4bc
kthread+0x1cc/0x1e8
ret_from_fork+0x10/0x18

-> #0 (random_write_wait.lock){....}:
validate_chain+0xd10/0x2bcc
__lock_acquire+0x7f4/0xb8c
lock_acquire+0x31c/0x360
_raw_spin_lock_irqsave+0x7c/0x9c
__wake_up_common_lock+0xa8/0x11c
__wake_up+0x3c/0x4c
account+0x390/0x3e0
extract_entropy+0x2cc/0x37c
crng_reseed+0x60/0x2f8
_extract_crng+0xd8/0x164
crng_reseed+0x7c/0x2f8
_extract_crng+0xd8/0x164
get_random_u32+0xec/0x1dc
new_slab+0x234/0x6c0
___slab_alloc+0x3c8/0x650
kmem_cache_alloc+0x4b0/0x590
getname_flags+0x44/0x1c8
user_path_at_empty+0x3c/0x68
vfs_statx+0xa4/0x134
__arm64_sys_newfstatat+0x94/0x120
el0_svc_handler+0x170/0x240
el0_svc+0x8/0xc

other info that might help us debug this:

Chain exists of:
random_write_wait.lock --> &rq->lock --> batched_entropy_u32.lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(batched_entropy_u32.lock);
lock(&rq->lock);
lock(batched_entropy_u32.lock);
lock(random_write_wait.lock);

*** DEADLOCK ***

1 lock held by make/7937:
#0: ffff0096b9429c00 (batched_entropy_u32.lock){-.-.}, at:
get_random_u32+0x6c/0x1dc

stack backtrace:
CPU: 220 PID: 7937 Comm: make Tainted: G L 5.3.0-rc7-mm1+
Hardware name: HPE Apollo 70 /C01_APACHE_MB , BIOS
L50_5.13_1.11 06/18/2019
Call trace:
dump_backtrace+0x0/0x248
show_stack+0x20/0x2c
dump_stack+0xd0/0x140
print_circular_bug+0x368/0x380
check_noncircular+0x248/0x250
validate_chain+0xd10/0x2bcc
__lock_acquire+0x7f4/0xb8c
lock_acquire+0x31c/0x360
_raw_spin_lock_irqsave+0x7c/0x9c
__wake_up_common_lock+0xa8/0x11c
__wake_up+0x3c/0x4c
account+0x390/0x3e0
extract_entropy+0x2cc/0x37c
crng_reseed+0x60/0x2f8
_extract_crng+0xd8/0x164
crng_reseed+0x7c/0x2f8
_extract_crng+0xd8/0x164
get_random_u32+0xec/0x1dc
new_slab+0x234/0x6c0
___slab_alloc+0x3c8/0x650
kmem_cache_alloc+0x4b0/0x590
getname_flags+0x44/0x1c8
user_path_at_empty+0x3c/0x68
vfs_statx+0xa4/0x134
__arm64_sys_newfstatat+0x94/0x120
el0_svc_handler+0x170/0x240
el0_svc+0x8/0xc

Signed-off-by: Qian Cai <cai@lca.pw>
---
mm/slub.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 8834563cdb4b..96cdd36f9380 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1598,8 +1598,15 @@ static bool shuffle_freelist(struct kmem_cache *s, struct page *page)
if (page->objects < 2 || !s->random_seq)
return false;

+ /*
+ * Don't use get_random_int() here as it might deadlock due to
+ * "&rq->lock --> batched_entropy_u32.lock" chain.
+ */
+ if (!arch_get_random_int((int *)&pos))
+ get_random_bytes(&pos, sizeof(int));
+
freelist_count = oo_objects(s->oo);
- pos = get_random_int() % freelist_count;
+ pos %= freelist_count;

page_limit = page->objects * s->size;
start = fixup_red_left(s, page_address(page));
--
1.8.3.1
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