On Wed, May 11, 2022 at 02:19:51PM +0800, Qi Zheng wrote:
> The 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.
>
> Syz-bot 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
>
> Fixes: b6da31b2c07c ("tty: Fix data race in tty_insert_flip_string_fixed_flag")
> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> Acked-by: Jiri Slaby <jirislaby@xxxxxxxxxx>
Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
