Re: [V9fs-developer] [PATCH kernel] 9p/trans_fd: Check file mode at opening

[Greg Kurz]

Hi Alexey,

Working on 9p now ?!? ;-)

Cc'ing Dominique Martinet who appears to be the person who takes care of 9p
these days.

On Tue, 28 Jul 2020 22:41:29 +1000
Alexey Kardashevskiy <aik@xxxxxxxxx> wrote:

> The "fd" transport layer uses 2 file descriptors passed externally
> and calls kernel_write()/kernel_read() on these. If files were opened
> without FMODE_WRITE/FMODE_READ, WARN_ON_ONCE() will fire.
> 
> This adds file mode checking in p9_fd_open; this returns -EBADF to
> preserve the original behavior.
> 

So this would cause open() to fail with EBADF, which might look a bit
weird to userspace since it didn't pass an fd... Is this to have a
different error than -EIO that is returned when either rfd or wfd
doesn't point to an open file descriptor ? If yes, why do we care ?

> Found by syzkaller.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
> ---
>  net/9p/trans_fd.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
> index 13cd683a658a..62cdfbd01f0a 100644
> --- a/net/9p/trans_fd.c
> +++ b/net/9p/trans_fd.c
> @@ -797,6 +797,7 @@ static int parse_opts(char *params, struct p9_fd_opts *opts)
>  
>  static int p9_fd_open(struct p9_client *client, int rfd, int wfd)
>  {
> +	bool perm;
>  	struct p9_trans_fd *ts = kzalloc(sizeof(struct p9_trans_fd),
>  					   GFP_KERNEL);
>  	if (!ts)
> @@ -804,12 +805,16 @@ static int p9_fd_open(struct p9_client *client, int rfd, int wfd)
>  
>  	ts->rd = fget(rfd);
>  	ts->wr = fget(wfd);
> -	if (!ts->rd || !ts->wr) {
> +	perm = ts->rd && (ts->rd->f_mode & FMODE_READ) &&
> +	       ts->wr && (ts->wr->f_mode & FMODE_WRITE);
> +	if (!ts->rd || !ts->wr || !perm) {
>  		if (ts->rd)
>  			fput(ts->rd);
>  		if (ts->wr)
>  			fput(ts->wr);
>  		kfree(ts);
> +		if (!perm)
> +			return -EBADF;
>  		return -EIO;
>  	}
>