[PATCH 4.9 071/167] tty: fix deadlock caused by calling printk() under tty_port->lock

Greg Kroah-Hartman
2022-06-13T10:22:41Z

From: Qi Zheng <zhengqi.arch@bytedance.com>

[ Upstream commit 6b9dbedbe3499fef862c4dff5217cf91f34e43b3 ]

pty_write() invokes kmalloc() which may invoke a normal printk() to print
failure message. This can cause a deadlock in the scenario reported by
syz-bot below:

CPU0 CPU1 CPU2
---- ---- ----
lock(console_owner);
lock(&port_lock_key);
lock(&port->lock);
lock(&port_lock_key);
lock(&port->lock);
lock(console_owner);

As commit dbdda842fe96 ("printk: Add console owner and waiter logic to
load balance console writes") said, such deadlock can be prevented by
using printk_deferred() in kmalloc() (which is invoked in the section
guarded by the port->lock). But there are too many printk() on the
kmalloc() path, and kmalloc() can be called from anywhere, so changing
printk() to printk_deferred() is too complicated and inelegant.

Therefore, this patch chooses to specify __GFP_NOWARN to kmalloc(), so
that printk() will not be called, and this deadlock problem can be
avoided.

Syzbot reported the following lockdep error:

======================================================
WARNING: possible circular locking dependency detected
5.4.143-00237-g08ccc19a-dirty #10 Not tainted
------------------------------------------------------
syz-executor.4/29420 is trying to acquire lock:
ffffffff8aedb2a0 (console_owner){....}-{0:0}, at: console_trylock_spinning kernel/printk/printk.c:1752 [inline]
ffffffff8aedb2a0 (console_owner){....}-{0:0}, at: vprintk_emit+0x2ca/0x470 kernel/printk/printk.c:2023

but task is already holding lock:
ffff8880119c9158 (&port->lock){-.-.}-{2:2}, at: pty_write+0xf4/0x1f0 drivers/tty/pty.c:120

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (&port->lock){-.-.}-{2:2}:
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
_raw_spin_lock_irqsave+0x35/0x50 kernel/locking/spinlock.c:159
tty_port_tty_get drivers/tty/tty_port.c:288 [inline] <-- lock(&port->lock);
tty_port_default_wakeup+0x1d/0xb0 drivers/tty/tty_port.c:47
serial8250_tx_chars+0x530/0xa80 drivers/tty/serial/8250/8250_port.c:1767
serial8250_handle_irq.part.0+0x31f/0x3d0 drivers/tty/serial/8250/8250_port.c:1854
serial8250_handle_irq drivers/tty/serial/8250/8250_port.c:1827 [inline] <-- lock(&port_lock_key);
serial8250_default_handle_irq+0xb2/0x220 drivers/tty/serial/8250/8250_port.c:1870
serial8250_interrupt+0xfd/0x200 drivers/tty/serial/8250/8250_core.c:126
__handle_irq_event_percpu+0x109/0xa50 kernel/irq/handle.c:156
[...]

-> #1 (&port_lock_key){-.-.}-{2:2}:
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
_raw_spin_lock_irqsave+0x35/0x50 kernel/locking/spinlock.c:159
serial8250_console_write+0x184/0xa40 drivers/tty/serial/8250/8250_port.c:3198
<-- lock(&port_lock_key);
call_console_drivers kernel/printk/printk.c:1819 [inline]
console_unlock+0x8cb/0xd00 kernel/printk/printk.c:2504
vprintk_emit+0x1b5/0x470 kernel/printk/printk.c:2024 <-- lock(console_owner);
vprintk_func+0x8d/0x250 kernel/printk/printk_safe.c:394
printk+0xba/0xed kernel/printk/printk.c:2084
register_console+0x8b3/0xc10 kernel/printk/printk.c:2829
univ8250_console_init+0x3a/0x46 drivers/tty/serial/8250/8250_core.c:681
console_init+0x49d/0x6d3 kernel/printk/printk.c:2915
start_kernel+0x5e9/0x879 init/main.c:713
secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:241

-> #0 (console_owner){....}-{0:0}:
[...]
lock_acquire+0x127/0x340 kernel/locking/lockdep.c:4734
console_trylock_spinning kernel/printk/printk.c:1773 [inline] <-- lock(console_owner);
vprintk_emit+0x307/0x470 kernel/printk/printk.c:2023
vprintk_func+0x8d/0x250 kernel/printk/printk_safe.c:394
printk+0xba/0xed kernel/printk/printk.c:2084
fail_dump lib/fault-inject.c:45 [inline]
should_fail+0x67b/0x7c0 lib/fault-inject.c:144
__should_failslab+0x152/0x1c0 mm/failslab.c:33
should_failslab+0x5/0x10 mm/slab_common.c:1224
slab_pre_alloc_hook mm/slab.h:468 [inline]
slab_alloc_node mm/slub.c:2723 [inline]
slab_alloc mm/slub.c:2807 [inline]
__kmalloc+0x72/0x300 mm/slub.c:3871
kmalloc include/linux/slab.h:582 [inline]
tty_buffer_alloc+0x23f/0x2a0 drivers/tty/tty_buffer.c:175
__tty_buffer_request_room+0x156/0x2a0 drivers/tty/tty_buffer.c:273
tty_insert_flip_string_fixed_flag+0x93/0x250 drivers/tty/tty_buffer.c:318
tty_insert_flip_string include/linux/tty_flip.h:37 [inline]
pty_write+0x126/0x1f0 drivers/tty/pty.c:122 <-- lock(&port->lock);
n_tty_write+0xa7a/0xfc0 drivers/tty/n_tty.c:2356
do_tty_write drivers/tty/tty_io.c:961 [inline]
tty_write+0x512/0x930 drivers/tty/tty_io.c:1045
__vfs_write+0x76/0x100 fs/read_write.c:494
[...]

other info that might help us debug this:

Chain exists of:
console_owner --> &port_lock_key --> &port->lock

Link: https://lkml.kernel.org/r/20220511061951.1114-2-zhengqi.arch@bytedance.com
Link: https://lkml.kernel.org/r/20220510113809.80626-2-zhengqi.arch@bytedance.com
Fixes: b6da31b2c07c ("tty: Fix data race in tty_insert_flip_string_fixed_flag")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Jiri Slaby <jirislaby@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/tty/tty_buffer.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index dfccc102c1dd..e65faa98146e 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -166,7 +166,8 @@ static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
have queued and recycle that ? */
if (atomic_read(&port->buf.mem_used) > port->buf.mem_limit)
return NULL;
- p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
+ p = kmalloc(sizeof(struct tty_buffer) + 2 * size,
+ GFP_ATOMIC | __GFP_NOWARN);
if (p == NULL)
return NULL;

--
2.35.1