linux-kernel - Re: Mount options may be silently discarded

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Date:   Mon, 28 Sep 2020 21:00:54 +0300
From:   Dmitry Kasatkin <dmitry.kasatkin@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Mount options may be silently discarded

On Mon, Sep 28, 2020 at 5:36 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Dmitry Kasatkin
> > Sent: 28 September 2020 15:03
> >
> > "copy_mount_options" function came to my eyes.
> > It splits copy into 2 pieces - over page boundaries.
> > I wonder what is the real reason for doing this?
> > Original comment was that we need exact bytes and some user memcpy
> > functions  do not return correct number on page fault.
> >
> > But how would all other cases work?
> >
> > https://elixir.bootlin.com/linux/latest/source/fs/namespace.c#L3075
> >
> > if (size != PAGE_SIZE) {
> >        if (copy_from_user(copy + size, data + size, PAGE_SIZE - size))
> >             memset(copy + size, 0, PAGE_SIZE - size);
> > }
> >
> > This looks like some options may be just discarded?
> > What if it is an important security option?
> >
> > Why it does not return EFAULT, but just memset?
>

> The user doesn't supply the transfer length, the max size
> is a page.
> Since the copy can only start to fail on a page boundary
> reading in two pieces is exactly the same as knowing the
> address at which the transfer started to fail.
>
> Since the actual mount options can be much smaller than
> a page (and usually are) zero-filling is best.
>

Hi David,

Ok. This is now obvious that it is done for "proper" memseting...

But why "we" should allow "discarding" failed part instead of failing
with EFAULT as a whole?

Thanks,

>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)



-- 
Thanks,
Dmitry