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Subject[PATCH] tty: Protect disc_data in n_tty_close and n_tty_flush_buffer
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n_tty_flush_buffer can happen in parallel with n_tty_close that the
tty->disc_data will be set to NULL. n_tty_flush_buffer accesses
tty->disc_data, so we must prevent n_tty_close clear tty->disc_data
while n_tty_flush_buffer has a non-NULL view of tty->disc_data.

So we need to make sure that accesses to disc_data are atomic using
spinlock.

There is an example I meet:
When n_tty_flush_buffer accesses tty struct, the disc_data is right.
However, then reset_buffer_flags accesses tty->disc_data, disc_data
become NULL, So kernel crash when accesses tty->disc_data->real_tail.
I guess there could be another thread change tty->disc_data to NULL,
and during N_TTY line discipline, n_tty_close will set tty->disc_data
to be NULL. So add spinlock to protect disc_data between close and
flush_buffer.

IP: reset_buffer_flags+0x9/0xf0
PGD 0 P4D 0
Oops: 0002 [#1] SMP
CPU: 23 PID: 2087626 Comm: (agetty) Kdump: loaded Tainted: G
Hardware name: UNISINSIGHT X3036P-G3/ST01M2C7S, BIOS 2.00.13 01/11/2019
task: ffff9c4e9da71e80 task.stack: ffffb30cfe898000
RIP: 0010:reset_buffer_flags+0x9/0xf0
RSP: 0018:ffffb30cfe89bca8 EFLAGS: 00010246
RAX: ffff9c4e9da71e80 RBX: ffff9c368d1bac00 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff9c4ea17b50f0 RDI: 0000000000000000
RBP: ffffb30cfe89bcc8 R08: 0000000000000100 R09: 0000000000000001
R10: 0000000000000001 R11: 0000000000000000 R12: ffff9c368d1bacc0
R13: ffff9c20cfd18428 R14: ffff9c4ea17b50f0 R15: ffff9c368d1bac00
FS: 00007f9fbbe97940(0000) GS:ffff9c375c740000(0000)
knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000002260 CR3: 0000002f72233003 CR4: 00000000007606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
? n_tty_flush_buffer+0x2a/0x60
tty_buffer_flush+0x76/0x90
tty_ldisc_flush+0x22/0x40
vt_ioctl+0x5a7/0x10b0
? n_tty_ioctl_helper+0x27/0x110
tty_ioctl+0xef/0x8c0
do_vfs_ioctl+0xa7/0x5e0
? __audit_syscall_entry+0xaf/0x100
? syscall_trace_enter+0x1d0/0x2b0
SyS_ioctl+0x79/0x90
do_syscall_64+0x6c/0x1b0
entry_SYSCALL64_slow_path+0x25/0x25

n_tty_flush_buffer --->tty->disc_data is OK
->reset_buffer_flags -->tty->disc_data is NULL

Signed-off-by: Yan.Gao <gao.yanB@h3c.com>
Reviewed-by: Xianting Tian <tian.xianting@h3c.com>
---
drivers/tty/n_tty.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 7e5e36315..f4b152f20 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -87,6 +87,8 @@
# define n_tty_trace(f, args...) no_printk(f, ##args)
#endif

+static DEFINE_SPINLOCK(disc_data_lock);
+
struct n_tty_data {
/* producer-published */
size_t read_head;
@@ -371,8 +373,10 @@ static void n_tty_packet_mode_flush(struct tty_struct *tty)
static void n_tty_flush_buffer(struct tty_struct *tty)
{
down_write(&tty->termios_rwsem);
+ spin_lock(&disc_data_lock);
reset_buffer_flags(tty->disc_data);
n_tty_kick_worker(tty);
+ spin_unlock(&disc_data_lock);

if (tty->link)
n_tty_packet_mode_flush(tty);
@@ -1892,8 +1896,10 @@ static void n_tty_close(struct tty_struct *tty)
if (tty->link)
n_tty_packet_mode_flush(tty);

+ spin_lock_irq(&disc_data_lock);
vfree(ldata);
tty->disc_data = NULL;
+ spin_unlock_irq(&disc_data_lock);
}

/**
--
2.17.1
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