On Thu, Dec 10, 2020 at 10:25:07AM +0800, Yan.Gao wrote: > 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 > tty->termios_rwsem. > > 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 use tty->termios_rwsem 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.y...@h3c.com> > Reviewed-by: Xianting Tian <tian.xiant...@h3c.com> > --- > drivers/tty/n_tty.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c > index 7e5e36315..e78124ce1 100644 > --- a/drivers/tty/n_tty.c > +++ b/drivers/tty/n_tty.c > @@ -1892,8 +1892,10 @@ static void n_tty_close(struct tty_struct *tty) > if (tty->link) > n_tty_packet_mode_flush(tty); > > + down_write(&tty->termios_rwsem); > vfree(ldata); > tty->disc_data = NULL; > + up_write(&tty->termios_rwsem); > } > > /**
So does this solve your problem in testing? Do you have a reproducer for this problem? thanks, greg k-h
