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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 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.yanB@h3c.com> Reviewed-by: Xianting Tian <tian.xianting@h3c.com> Link: https://lore.kernel.org/r/20201210022507.30729-1-gao.yanB@h3c.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.