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

[Greg Kurz]

On Wed, 29 Jul 2020 09:50:21 +1000
Alexey Kardashevskiy <aik@xxxxxxxxx> wrote:

> 
> 
> On 29/07/2020 03:42, Greg Kurz wrote:
> > Hi Alexey,
> > 
> > Working on 9p now ?!? ;-)
> 
> No, I am running syzkaller and seeing things :)
> 

:)

> 
> > 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 ?
> 
> This is only to preserve the existing behavior.
> 
> > If yes, why do we care ?
> 
> 
> Without the patch, p9_fd_open() produces a kernel warning which is not
> great by itself and becomes crash with panic_on_warn.
> 

I don't question the patch, just the errno. Why not returning -EIO ?

> 
> 
> > 
> >> 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;
> >>  	}
> >>  
> > 
>