Re: [PATCH] tty: tty_io: fix race between tty_fops and hung_up_tty_fops
From: Christian Brauner
Date: Mon Jul 22 2024 - 10:41:38 EST
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.
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?).
Something like the completely untested below:
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
>