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Date: Thu, 10 Dec 2020 02:52:49 +0000 From: Gaoyan <gao.yanB@....com> To: Greg KH <gregkh@...uxfoundation.org> CC: "jirislaby@...nel.org" <jirislaby@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Tianxianting <tian.xianting@....com> Subject: 答复: [PATCH] tty: Protect disc_data in n_tty_close and n_tty_flush_buffer Hi Greg KH, Thank you for reviewing the patch, it helped me a lot. According to your suggestion, I change the code. Please help me to review the v2 patch. Thanks again. https://lkml.org/lkml/2020/12/9/1412 ----- Original mail ----- 发件人: Greg KH [mailto:gregkh@...uxfoundation.org] 发送时间: 2020年12月9日 22:38 收件人: gaoyan (RD) <gao.yanB@....com> 抄送: jirislaby@...nel.org; linux-kernel@...r.kernel.org; tianxianting (RD) <tian.xianting@....com> 主题: Re: [PATCH] tty: Protect disc_data in n_tty_close and n_tty_flush_buffer On Wed, Dec 09, 2020 at 05:59:21PM +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 > 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@....com> > Reviewed-by: Xianting Tian <tian.xianting@....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); We want to lock data, not code, and this is locking code. Why can't we use the tty's lock here? > + > 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); We already have the termios_rwsem lock here, why do we need another one? > > 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); Why can't you just grab the termios_rwsem lock? thanks, greg k-h
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