On Mon 22-07-24 16:41:22, Christian Brauner wrote:
> On Fri, Jul 19, 2024 at 10:37:47PM GMT, Tetsuo Handa wrote:
> > syzbot is reporting data race between __tty_hangup() and __fput(), and
> > Dmitry Vyukov mentioned that this race has possibility of NULL pointer
> > dereference, for tty_fops implements e.g. splice_read callback whereas
> > hung_up_tty_fops does not.
> >
> > CPU0 CPU1
> > ---- ----
> > do_splice_read() {
> > __tty_hangup() {
> > // f_op->splice_read was copy_splice_read
> > if (unlikely(!in->f_op->splice_read))
> > return warn_unsupported(in, "read");
> > filp->f_op = &hung_up_tty_fops;
> > // f_op->splice_read is now NULL
> > return in->f_op->splice_read(in, ppos, pipe, len, flags);
> > }
> > }
> >
> > Fix possibility of NULL pointer dereference by implementing missing
> > callbacks, and suppress KCSAN messages by adding __data_racy qualifier
> > to "struct file"->f_op .
>
> This f_op replacing without synchronization seems really iffy imho.
Yeah, when I saw this I was also going "ouch". I was just waiting whether a
tty maintainer will comment ;). Anyway this replacement of ops in file /
inode has proven problematic in almost every single case where it was used
leading to subtle issues.
> Why can't the hangup just be recorded in tty_file_private and then
> checked in the corresponding f_op->$method()?
>
> And if that's not possible for some reason I'd be willing to sacrifice
> one of the FMODE_* bits I recently freed up and add e.g.,
> FMODE_TTY_HANGUP instead of this f_op raciness (Why wasn't this using
> replace_fops anyway so it'd be easy to grep for it?).
I don't think burning FMODE bit is really warranted here. Tty layer has its
own structure at filp->private_data and in fact
filp->private_data->tty->flags has TTY_HUPPING and TTY_HUPPED bits already
which we could presumably use.
Honza
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index bc9aebcb873f..219bf6391fed 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -583,6 +583,26 @@ static struct file *tty_release_redirect(struct tty_struct *tty)
> return f;
> }
>
> +static noinline void noinline tty_mark_hungup(struct file *file)
> +{
> + fmode_t fmode = READ_ONCE(file->f_mode);
> +
> + do {
> + if (fmode & FMODE_TTY_HUNGUP)
> + break;
> + } while (!try_cmpxchg(&file->f_mode, &fmode, fmode | FMODE_TTY_HUNGUP));
> +}
> +
> +static noinline void noinline tty_clear_hungup(struct file *file)
> +{
> + fmode_t fmode = READ_ONCE(file->f_mode);
> +
> + do {
> + if (!(fmode & FMODE_TTY_HUNGUP))
> + break;
> + } while (!try_cmpxchg(&file->f_mode, &fmode, fmode & ~FMODE_TTY_HUNGUP));
> +}
> +
> /**
> * __tty_hangup - actual handler for hangup events
> * @tty: tty device
> @@ -652,7 +672,7 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
> continue;
> closecount++;
> __tty_fasync(-1, filp, 0); /* can't block */
> - filp->f_op = &hung_up_tty_fops;
> + tty_mark_hungup(filp);
> }
> spin_unlock(&tty->files_lock);
>
> @@ -776,7 +796,7 @@ void tty_vhangup_session(struct tty_struct *tty)
> */
> int tty_hung_up_p(struct file *filp)
> {
> - return (filp && filp->f_op == &hung_up_tty_fops);
> + return (filp && (READ_ONCE(filp->f_mode) & FMODE_TTY_HUNGUP));
> }
> EXPORT_SYMBOL(tty_hung_up_p);
>
> @@ -2204,7 +2224,7 @@ static int tty_open(struct inode *inode, struct file *filp)
> * Need to reset f_op in case a hangup happened.
> */
> if (tty_hung_up_p(filp))
> - filp->f_op = &tty_fops;
> + tty_clear_hungup(filp);
> goto retry_open;
> }
> clear_bit(TTY_HUPPED, &tty->flags);
> @@ -2718,6 +2738,10 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> int retval;
> struct tty_ldisc *ld;
>
> + /* Check whether the tty hung up. */
> + if (tty_hung_up_p(file))
> + return hung_up_tty_ioctl(file, cmd, arg);
> +
> if (tty_paranoia_check(tty, file_inode(file), "tty_ioctl"))
> return -EINVAL;
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index ea3df718c53e..e96b86bab356 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -128,7 +128,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
> /* File supports atomic writes */
> #define FMODE_CAN_ATOMIC_WRITE ((__force fmode_t)(1 << 7))
>
> -/* FMODE_* bit 8 */
> +/* File driver hung up (tty specific) */
> +#define FMODE_TTY_HUNGUP ((__force fmode_t)(1 << 8))
>
> /* 32bit hashes as llseek() offset (for directories) */
> #define FMODE_32BITHASH ((__force fmode_t)(1 << 9))
>
>
> >
> > Reported-by: syzbot <syzbot+b7c3ba8cdc2f6cf83c21@xxxxxxxxxxxxxxxxxxxxxxxxx>
> > Closes: https://syzkaller.appspot.com/bug?extid=b7c3ba8cdc2f6cf83c21
> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Marco Elver <elver@xxxxxxxxxx>
> > Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> > ---
> > This patch has been tested using linux-next tree via my tomoyo tree since 20240611,
> > and there was no response on
> > [fs] Are you OK with updating "struct file"->f_op value dynamically?
> > at https://lkml.kernel.org/r/b221d2cf-7dc0-4624-a040-85c131ed72a1@xxxxxxxxxxxxxxxxxxx .
> > Thus, I guess we can go with this approach.
> >
> > drivers/tty/tty_io.c | 34 ++++++++++++++++++++++++++++++++++
> > include/linux/fs.h | 2 +-
> > 2 files changed, 35 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> > index 407b0d87b7c1..bc9aebcb873f 100644
> > --- a/drivers/tty/tty_io.c
> > +++ b/drivers/tty/tty_io.c
> > @@ -430,6 +430,24 @@ static ssize_t hung_up_tty_write(struct kiocb *iocb, struct iov_iter *from)
> > return -EIO;
> > }
> >
> > +static ssize_t hung_up_copy_splice_read(struct file *in, loff_t *ppos,
> > + struct pipe_inode_info *pipe,
> > + size_t len, unsigned int flags)
> > +{
> > + return -EINVAL;
> > +}
> > +
> > +static ssize_t hung_up_iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
> > + loff_t *ppos, size_t len, unsigned int flags)
> > +{
> > + return -EINVAL;
> > +}
> > +
> > +static int hung_up_no_open(struct inode *inode, struct file *file)
> > +{
> > + return -ENXIO;
> > +}
> > +
> > /* No kernel lock held - none needed ;) */
> > static __poll_t hung_up_tty_poll(struct file *filp, poll_table *wait)
> > {
> > @@ -462,6 +480,12 @@ static void tty_show_fdinfo(struct seq_file *m, struct file *file)
> > }
> >
> > static const struct file_operations tty_fops = {
> > + /*
> > + * WARNING: You must implement all callbacks defined in tty_fops in
> > + * hung_up_tty_fops, for tty_fops and hung_up_tty_fops are toggled
> > + * after "struct file" is published. Failure to synchronize has a risk
> > + * of NULL pointer dereference bug.
> > + */
> > .llseek = no_llseek,
> > .read_iter = tty_read,
> > .write_iter = tty_write,
> > @@ -491,14 +515,24 @@ static const struct file_operations console_fops = {
> > };
> >
> > static const struct file_operations hung_up_tty_fops = {
> > + /*
> > + * WARNING: You must implement all callbacks defined in hung_up_tty_fops
> > + * in tty_fops, for tty_fops and hung_up_tty_fops are toggled after
> > + * "struct file" is published. Failure to synchronize has a risk of
> > + * NULL pointer dereference bug.
> > + */
> > .llseek = no_llseek,
> > .read_iter = hung_up_tty_read,
> > .write_iter = hung_up_tty_write,
> > + .splice_read = hung_up_copy_splice_read,
> > + .splice_write = hung_up_iter_file_splice_write,
> > .poll = hung_up_tty_poll,
> > .unlocked_ioctl = hung_up_tty_ioctl,
> > .compat_ioctl = hung_up_tty_compat_ioctl,
> > + .open = hung_up_no_open,
> > .release = tty_release,
> > .fasync = hung_up_tty_fasync,
> > + .show_fdinfo = tty_show_fdinfo,
> > };
> >
> > static DEFINE_SPINLOCK(redirect_lock);
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 0283cf366c2a..636bcc59a3f5 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -1008,7 +1008,7 @@ struct file {
> > struct file_ra_state f_ra;
> > struct path f_path;
> > struct inode *f_inode; /* cached value */
> > - const struct file_operations *f_op;
> > + const struct file_operations *__data_racy f_op;
> >
> > u64 f_version;
> > #ifdef CONFIG_SECURITY
> > --
> > 2.43.5
> >
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
