Re: [PATCH] tty: fix when iov_iter_count() returns 0 in tty_write()

[Greg KH]

On Wed, Feb 17, 2021 at 08:43:47PM +0600, Sabyrzhan Tasbolatov wrote:
> syzbot found WARNING in iov_iter_revert[1] when iov_iter_count() returns 0,
> therefore INT_MAX is passed to iov_iter_revert() causing > MAX_RW_COUNT
> warning.
> 
> static inline ssize_t do_tty_write()
> {
> ..
> 	size_t count = iov_iter_count(from);
> ..
> 		size_t size = count;
> 		if (ret != size)
> 			iov_iter_revert(from, size-ret);
> 
> [1] WARNING: lib/iov_iter.c:1090
> Call Trace:
>  do_tty_write drivers/tty/tty_io.c:967 [inline]
>  file_tty_write.constprop.0+0x55f/0x8f0 drivers/tty/tty_io.c:1048
>  call_write_iter include/linux/fs.h:1901 [inline]
>  new_sync_write+0x426/0x650 fs/read_write.c:518
>  vfs_write+0x791/0xa30 fs/read_write.c:605
>  ksys_write+0x12d/0x250 fs/read_write.c:658
> 
> Fixes: 494e63ee9c("tty: implement write_iter")

Nit, you need a ' ' before your '(' character here, otherwise the
linux-next scripts will complain.

> Reported-by: syzbot+3d2c27c2b7dc2a94814d@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@xxxxxxxxx>
> ---
>  drivers/tty/tty_io.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 816e709afa56..8d6d579ecc3b 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -905,6 +905,9 @@ static inline ssize_t do_tty_write(
>  	ssize_t ret, written = 0;
>  	unsigned int chunk;
>  
> +	if (!count)
> +		return -EINVAL;

According to the man page for write(2), I think this is the wrong error
value to return, unless that is the value that was returned on kernels
before the commit listed above.

Can you verify this?

thanks,

greg k-h