Re: possible deadlock in console_unlock
From: Sergey Senozhatsky
Date: Thu Jun 07 2018 - 01:10:33 EST
Cc-ing more people
Link: lkml.kernel.org/r/00000000000087008b056df8fbb3@xxxxxxxxxx
On (06/06/18 06:17), syzbot wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit: af6c5d5e01ad Merge branch 'for-4.18' of git://git.kernel.o..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=173d93ef800000
> kernel config: https://syzkaller.appspot.com/x/.config?x=12ff770540994680
> dashboard link: https://syzkaller.appspot.com/bug?extid=43e93968b964e369db0b
> compiler: gcc (GCC) 8.0.1 20180413 (experimental)
> userspace arch: i386
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=16e00bb7800000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+43e93968b964e369db0b@xxxxxxxxxxxxxxxxxxxxxxxxx
Thanks a ton!
So tty ioctl is known to be printk-deadlock prone and we don't know
how to handle this in printk, as of now.
ioctl
tty_ioctl
tty_port->lock
printk
call_console_driver
console_driver
uart_port->lock
The problem is that call_console_driver->console_driver also can do
this thing
uart_port->lock
tty_wakeup
tty_port->lock
So we can have the following:
ioctl
tty_ioctl
tty_port->lock
printk
call_console_driver
console_driver
uart_port->lock
tty_wakeup
tty_port->lock << deadlock
But lockdep is complaining about another scenario:
> the existing dependency chain (in reverse order) is:
>
> -> #2 (&(&port->lock)->rlock){-.-.}:
> __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
> _raw_spin_lock_irqsave+0x96/0xc0 kernel/locking/spinlock.c:152
> tty_port_tty_get+0x20/0x80 drivers/tty/tty_port.c:288
> tty_port_default_wakeup+0x15/0x40 drivers/tty/tty_port.c:47
> tty_port_tty_wakeup+0x5d/0x70 drivers/tty/tty_port.c:390
> uart_write_wakeup+0x44/0x60 drivers/tty/serial/serial_core.c:103
> serial8250_tx_chars+0x4be/0xb60
> drivers/tty/serial/8250/8250_port.c:1808
> serial8250_handle_irq.part.25+0x1ee/0x280
> drivers/tty/serial/8250/8250_port.c:1881
> serial8250_handle_irq drivers/tty/serial/8250/8250_port.c:1867
> [inline]
> serial8250_default_handle_irq+0xc8/0x150
> drivers/tty/serial/8250/8250_port.c:1897
> serial8250_interrupt+0xfa/0x1d0
> drivers/tty/serial/8250/8250_core.c:125
> __handle_irq_event_percpu+0x1c0/0xad0 kernel/irq/handle.c:149
> handle_irq_event_percpu+0x98/0x1c0 kernel/irq/handle.c:189
> handle_irq_event+0xa7/0x135 kernel/irq/handle.c:206
> handle_edge_irq+0x20f/0x870 kernel/irq/chip.c:791
> generic_handle_irq_desc include/linux/irqdesc.h:159 [inline]
> handle_irq+0x18c/0x2e7 arch/x86/kernel/irq_64.c:77
> do_IRQ+0x78/0x190 arch/x86/kernel/irq.c:245
> ret_from_intr+0x0/0x1e
> native_safe_halt+0x6/0x10 arch/x86/include/asm/irqflags.h:54
> arch_safe_halt arch/x86/include/asm/paravirt.h:94 [inline]
> default_idle+0xc2/0x440 arch/x86/kernel/process.c:500
> arch_cpu_idle+0x10/0x20 arch/x86/kernel/process.c:491
> default_idle_call+0x6d/0x90 kernel/sched/idle.c:93
> cpuidle_idle_call kernel/sched/idle.c:153 [inline]
> do_idle+0x395/0x560 kernel/sched/idle.c:262
> cpu_startup_entry+0x104/0x120 kernel/sched/idle.c:368
> start_secondary+0x42b/0x5c0 arch/x86/kernel/smpboot.c:265
> secondary_startup_64+0xa5/0xb0 arch/x86/kernel/head_64.S:242
So this one is IRQ ==> uart_port->lock ==> tty_port->lock
> -> #1 (&port_lock_key){-.-.}:
> __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
> _raw_spin_lock_irqsave+0x96/0xc0 kernel/locking/spinlock.c:152
> serial8250_console_write+0x8d5/0xb00
> drivers/tty/serial/8250/8250_port.c:3230
> univ8250_console_write+0x5f/0x70
> drivers/tty/serial/8250/8250_core.c:590
> call_console_drivers kernel/printk/printk.c:1718 [inline]
> console_unlock+0xac2/0x1100 kernel/printk/printk.c:2395
> vprintk_emit+0x6ad/0xdd0 kernel/printk/printk.c:1907
> vprintk_default+0x28/0x30 kernel/printk/printk.c:1947
> vprintk_func+0x7a/0xe7 kernel/printk/printk_safe.c:379
> printk+0x9e/0xba kernel/printk/printk.c:1980
> register_console+0x7e7/0xc00 kernel/printk/printk.c:2714
> univ8250_console_init+0x3f/0x4b
> drivers/tty/serial/8250/8250_core.c:685
> console_init+0x6d9/0xa38 kernel/printk/printk.c:2798
> start_kernel+0x608/0x92d init/main.c:661
> x86_64_start_reservations+0x29/0x2b arch/x86/kernel/head64.c:452
> x86_64_start_kernel+0x76/0x79 arch/x86/kernel/head64.c:433
> secondary_startup_64+0xa5/0xb0 arch/x86/kernel/head_64.S:242
This one is console_owner/console_sem ==> uart_port->lock
> -> #0 (console_owner){-...}:
> lock_acquire+0x1dc/0x520 kernel/locking/lockdep.c:3924
> console_lock_spinning_enable kernel/printk/printk.c:1581 [inline]
> console_unlock+0x5ef/0x1100 kernel/printk/printk.c:2392
> vprintk_emit+0x6ad/0xdd0 kernel/printk/printk.c:1907
> vprintk_default+0x28/0x30 kernel/printk/printk.c:1947
> vprintk_func+0x7a/0xe7 kernel/printk/printk_safe.c:379
> printk+0x9e/0xba kernel/printk/printk.c:1980
> fail_dump lib/fault-inject.c:44 [inline]
> should_fail+0x97a/0xbcd lib/fault-inject.c:149
> __should_failslab+0x124/0x180 mm/failslab.c:32
> should_failslab+0x9/0x14 mm/slab_common.c:1522
> slab_pre_alloc_hook mm/slab.h:423 [inline]
> slab_alloc mm/slab.c:3378 [inline]
> __do_kmalloc mm/slab.c:3716 [inline]
> __kmalloc+0x63/0x760 mm/slab.c:3727
> kmalloc include/linux/slab.h:517 [inline]
> tty_buffer_alloc drivers/tty/tty_buffer.c:170 [inline]
> __tty_buffer_request_room+0x2d2/0x7f0 drivers/tty/tty_buffer.c:268
> tty_insert_flip_string_fixed_flag+0x8d/0x1f0
> drivers/tty/tty_buffer.c:313
> tty_insert_flip_string include/linux/tty_flip.h:37 [inline]
> pty_write+0x12c/0x1f0 drivers/tty/pty.c:121
> tty_put_char+0x129/0x150 drivers/tty/tty_io.c:2865
> __process_echoes+0x4d9/0x8d0 drivers/tty/n_tty.c:703
> process_echoes+0xfc/0x170 drivers/tty/n_tty.c:781
> n_tty_set_termios+0xb56/0xe80 drivers/tty/n_tty.c:1837
> tty_set_termios+0x7a0/0xac0 drivers/tty/tty_ioctl.c:341
> set_termios+0x41e/0x7d0 drivers/tty/tty_ioctl.c:414
> tty_mode_ioctl+0x871/0xb50 drivers/tty/tty_ioctl.c:781
> n_tty_ioctl_helper+0x54/0x3b0 drivers/tty/tty_ioctl.c:940
> n_tty_ioctl+0x54/0x320 drivers/tty/n_tty.c:2441
> tty_ioctl+0x5e1/0x1870 drivers/tty/tty_io.c:2655
> vfs_ioctl fs/ioctl.c:46 [inline]
> file_ioctl fs/ioctl.c:500 [inline]
> do_vfs_ioctl+0x1cf/0x16f0 fs/ioctl.c:684
> __do_compat_sys_ioctl fs/compat_ioctl.c:1483 [inline]
> __se_compat_sys_ioctl fs/compat_ioctl.c:1407 [inline]
> __ia32_compat_sys_ioctl+0x43e/0x640 fs/compat_ioctl.c:1407
> do_syscall_32_irqs_on arch/x86/entry/common.c:323 [inline]
> do_fast_syscall_32+0x345/0xf9b arch/x86/entry/common.c:394
> entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
This one is tty IOCTL ==> tty_port->lock ==> console_owner/console_sem ==> uart_port->lock
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(&(&port->lock)->rlock);
> lock(&port_lock_key);
> lock(&(&port->lock)->rlock);
> lock(console_owner);
IOW
tty ioctl
tty_port->lock IRQ
printk uart_port->lock
console_owner
uart_port->lock tty_port->rlock
The simplest thing to do [not necessarily the right one, tho]
would be to break the IOCTL ==> tty_port->lock ==> printk ==> uart_port->lock
chain.
E.g. by adding __GFP_NOWARN
---
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 c996b6859c5e..71958ef6a831 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -167,7 +167,8 @@ 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;
---
Another way could be - switch to printk_safe mode around that
kmalloc():
__printk_safe_enter();
kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
__printk_safe_exit();
This will redirect all printk()-s from kmalloc() to a special per-CPU
buffer, which will be flushed later from a safe context (irq work).
Or, may be, we even can switch to printk_safe mode every time we grab
tty_port lock.
#define tty_port_lock_irqsave(l,f) \
do { \
spin_lock_irqsave((l), f); \
__printk_safe_enter(); \
} while (0)
#define tty_port_unlock_irqrestore(l,f) \
do { \
__printk_safe_exit(); \
spin_lock_irqrestore((l),f); \
} while (0)
This will require some "automatic" replacement of all port->lock
operation in drivers/tty/*.
Perhaps something like this should be done for uart_port->lock
as well. Because, technically, we can have the following
IRQ
uart_port->lock
tty_wakeup
printk
call_console_drivers
console_driver
uart_port->lock << deadlock
Which is totally possible.
E.g. tty_port_default_wakeup()->tty_port_tty_get()->refcount_inc()->WARN_ONCE()
Any opinions?
-ss