linux-kernel - Re: [PATCH 3/3] Check for write permission on FD based posix-clocks

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Date:	Fri, 4 Mar 2011 11:13:49 +0100
From:	torbenh <torbenh@....de>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	linux-kernel@...r.kernel.org, richard.cochran@...cron.at,
	johnstul@...ibm.com, tglx@...utronix.de
Subject: Re: [PATCH 3/3] Check for write permission on FD based posix-clocks

On Fri, Mar 04, 2011 at 08:22:39AM +0100, Richard Cochran wrote:
> On Thu, Mar 03, 2011 at 06:26:14PM +0100, Torben Hohn wrote:
> > pc_clock_settime() and pc_clock_adjtime() did not check
> > whether the fd was opened in write mode.
> > 
> > It was possible to set a clock, when we only had read
> > permissions.
> > 
> > for completeness, we would also need to check for Read permissions
> > on the read operations. but that would be a bit paranoid, probably.
> 
> I have no objection to this form of clock access control, but I would
> like to get agreement about it from the list.
> 
> > diff --git a/kernel/time/posix-clock.c b/kernel/time/posix-clock.c
> > index 04498cb..25028dd 100644
> > --- a/kernel/time/posix-clock.c
> > +++ b/kernel/time/posix-clock.c
> > @@ -287,11 +287,16 @@ static int pc_clock_adjtime(clockid_t id, struct timex *tx)
> >  	if (err)
> >  		return err;
> >  
> > +	if ((cd.fp->f_mode & FMODE_WRITE) == 0) {
> > +		err = -EACCES;
> 
> Looks like clock_settime and adjtimex are supposed to return EPERM in
> this case.

well... this is more similar to calling write(2) on an fd not opened
with FMODE_WRITE... 


ssize_t vfs_writev(struct file *file, const struct iovec __user *vec,
		   unsigned long vlen, loff_t *pos)
{
	if (!(file->f_mode & FMODE_WRITE))
		return -EBADF;
	if (!file->f_op || (!file->f_op->aio_write && !file->f_op->write))
		return -EINVAL;

	return do_readv_writev(WRITE, file, vec, vlen, pos);
}

so probably -EBADF is also a candidate :)
however, since the syscall is not really fd based, EPERM is probably
closer to the current man page.


-- 
torben Hohn
--
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