This is a note to let you know that I've just added the patch titled

vt: selection, push sel_lock up

to my tty git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From e8c75a30a23c6ba63f4ef6895cbf41fd42f21aa2 Mon Sep 17 00:00:00 2001

From: Jiri Slaby jslaby@suse.cz Date: Fri, 28 Feb 2020 12:54:06 +0100 Subject: vt: selection, push sel_lock up

sel_lock cannot nest in the console lock. Thanks to syzkaller, the kernel states firmly:

WARNING: possible circular locking dependency detected 5.6.0-rc3-syzkaller #0 Not tainted

---

syz-executor.4/20336 is trying to acquire lock: ffff8880a2e952a0 (&tty->termios_rwsem){++++}, at: tty_unthrottle+0x22/0x100 drivers/tty/tty_ioctl.c:136

but task is already holding lock: ffffffff89462e70 (sel_lock){+.+.}, at: paste_selection+0x118/0x470 drivers/tty/vt/selection.c:374

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (sel_lock){+.+.}: mutex_lock_nested+0x1b/0x30 kernel/locking/mutex.c:1118 set_selection_kernel+0x3b8/0x18a0 drivers/tty/vt/selection.c:217 set_selection_user+0x63/0x80 drivers/tty/vt/selection.c:181 tioclinux+0x103/0x530 drivers/tty/vt/vt.c:3050 vt_ioctl+0x3f1/0x3a30 drivers/tty/vt/vt_ioctl.c:364

This is ioctl(TIOCL_SETSEL). Locks held on the path: console_lock -> sel_lock

-> #1 (console_lock){+.+.}: console_lock+0x46/0x70 kernel/printk/printk.c:2289 con_flush_chars+0x50/0x650 drivers/tty/vt/vt.c:3223 n_tty_write+0xeae/0x1200 drivers/tty/n_tty.c:2350 do_tty_write drivers/tty/tty_io.c:962 [inline] tty_write+0x5a1/0x950 drivers/tty/tty_io.c:1046

This is write(). Locks held on the path: termios_rwsem -> console_lock

-> #0 (&tty->termios_rwsem){++++}: down_write+0x57/0x140 kernel/locking/rwsem.c:1534 tty_unthrottle+0x22/0x100 drivers/tty/tty_ioctl.c:136 mkiss_receive_buf+0x12aa/0x1340 drivers/net/hamradio/mkiss.c:902 tty_ldisc_receive_buf+0x12f/0x170 drivers/tty/tty_buffer.c:465 paste_selection+0x346/0x470 drivers/tty/vt/selection.c:389 tioclinux+0x121/0x530 drivers/tty/vt/vt.c:3055 vt_ioctl+0x3f1/0x3a30 drivers/tty/vt/vt_ioctl.c:364

This is ioctl(TIOCL_PASTESEL). Locks held on the path: sel_lock -> termios_rwsem

other info that might help us debug this:

Chain exists of: &tty->termios_rwsem --> console_lock --> sel_lock

Clearly. From the above, we have: console_lock -> sel_lock sel_lock -> termios_rwsem termios_rwsem -> console_lock

Fix this by reversing the console_lock -> sel_lock dependency in ioctl(TIOCL_SETSEL). First, lock sel_lock, then console_lock.

Signed-off-by: Jiri Slaby jslaby@suse.cz Reported-by: syzbot+26183d9746e62da329b8@syzkaller.appspotmail.com Fixes: 07e6124a1a46 ("vt: selection, close sel_buffer race") Cc: stable stable@vger.kernel.org Link: https://lore.kernel.org/r/20200228115406.5735-2-jslaby@suse.cz Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/vt/selection.c | 16 +++++++--------- 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c index 9126a01290ea..d7d2e4b844bc 100644 --- a/drivers/tty/vt/selection.c +++ b/drivers/tty/vt/selection.c @@ -214,7 +214,6 @@ static int __set_selection_kernel(struct tiocl_selection *v, struct tty_struct * if (ps > pe) /* make sel_start <= sel_end */ swap(ps, pe);

- mutex_lock(&sel_lock); if (sel_cons != vc_cons[fg_console].d) { clear_selection(); sel_cons = vc_cons[fg_console].d; @@ -260,10 +259,9 @@ static int __set_selection_kernel(struct tiocl_selection *v, struct tty_struct * break; case TIOCL_SELPOINTER: highlight_pointer(pe); - goto unlock; + return 0; default: - ret = -EINVAL; - goto unlock; + return -EINVAL; }

/* remove the pointer */ @@ -285,7 +283,7 @@ static int __set_selection_kernel(struct tiocl_selection *v, struct tty_struct * else if (new_sel_start == sel_start) { if (new_sel_end == sel_end) /* no action required */ - goto unlock; + return 0; else if (new_sel_end > sel_end) /* extend to right */ highlight(sel_end + 2, new_sel_end); else /* contract from right */ @@ -313,8 +311,7 @@ static int __set_selection_kernel(struct tiocl_selection *v, struct tty_struct * if (!bp) { printk(KERN_WARNING "selection: kmalloc() failed\n"); clear_selection(); - ret = -ENOMEM; - goto unlock; + return -ENOMEM; } kfree(sel_buffer); sel_buffer = bp; @@ -339,8 +336,7 @@ static int __set_selection_kernel(struct tiocl_selection *v, struct tty_struct * } } sel_buffer_lth = bp - sel_buffer; -unlock: - mutex_unlock(&sel_lock); + return ret; }

@@ -348,9 +344,11 @@ int set_selection_kernel(struct tiocl_selection *v, struct tty_struct *tty) { int ret;

+ mutex_lock(&sel_lock); console_lock(); ret = __set_selection_kernel(v, tty); console_unlock(); + mutex_unlock(&sel_lock);

return ret; }