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    Subject[PATCH 4.19 154/287] tty: n_gsm: fix race condition in gsmld_write()
    Date
    From: Daniel Starke <daniel.starke@siemens.com>

    [ Upstream commit 32dd59f96924f45e33bc79854f7a00679c0fa28e ]

    The function may be used by the user directly and also by the n_gsm
    internal functions. They can lead into a race condition which results in
    interleaved frames if both are writing at the same time. The receiving side
    is not able to decode those interleaved frames correctly.

    Add a lock around the low side tty write to avoid race conditions and frame
    interleaving between user originated writes and n_gsm writes.

    Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
    Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
    Link: https://lore.kernel.org/r/20220701061652.39604-9-daniel.starke@siemens.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/tty/n_gsm.c | 21 +++++++++++++++++----
    1 file changed, 17 insertions(+), 4 deletions(-)

    diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
    index 2c34a024b75f..3d45999fad1b 100644
    --- a/drivers/tty/n_gsm.c
    +++ b/drivers/tty/n_gsm.c
    @@ -2519,11 +2519,24 @@ static ssize_t gsmld_read(struct tty_struct *tty, struct file *file,
    static ssize_t gsmld_write(struct tty_struct *tty, struct file *file,
    const unsigned char *buf, size_t nr)
    {
    - int space = tty_write_room(tty);
    + struct gsm_mux *gsm = tty->disc_data;
    + unsigned long flags;
    + int space;
    + int ret;
    +
    + if (!gsm)
    + return -ENODEV;
    +
    + ret = -ENOBUFS;
    + spin_lock_irqsave(&gsm->tx_lock, flags);
    + space = tty_write_room(tty);
    if (space >= nr)
    - return tty->ops->write(tty, buf, nr);
    - set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
    - return -ENOBUFS;
    + ret = tty->ops->write(tty, buf, nr);
    + else
    + set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
    + spin_unlock_irqrestore(&gsm->tx_lock, flags);
    +
    + return ret;
    }

    /**
    --
    2.35.1


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