linux-kernel - Re: [RESEND PATCH] vfs: add RWF

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Date:   Mon, 31 Aug 2020 11:15:57 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Rich Felker <dalias@...c.org>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RESEND PATCH] vfs: add RWF_NOAPPEND flag for pwritev2

On Mon, Aug 31, 2020 at 3:46 AM Rich Felker <dalias@...c.org> wrote:
> On Mon, Aug 31, 2020 at 03:15:04AM +0200, Jann Horn wrote:
> > On Sun, Aug 30, 2020 at 10:00 PM Rich Felker <dalias@...c.org> wrote:
> > > On Sun, Aug 30, 2020 at 09:02:31PM +0200, Jann Horn wrote:
> > > > On Sun, Aug 30, 2020 at 8:43 PM Rich Felker <dalias@...c.org> wrote:
> > > > > On Sun, Aug 30, 2020 at 08:31:36PM +0200, Jann Horn wrote:
> > > > > > On Sun, Aug 30, 2020 at 6:36 PM Rich Felker <dalias@...c.org> wrote:
> > > > > > > So just checking IS_APPEND in the code paths used by
> > > > > > > pwritev2 (and erroring out rather than silently writing output at the
> > > > > > > wrong place) should suffice to preserve all existing security
> > > > > > > invariants.
> > > > > >
> > > > > > Makes sense.
> > > > >
> > > > > There are 3 places where kiocb_set_rw_flags is called with flags that
> > > > > seem to be controlled by userspace: aio.c, io_uring.c, and
> > > > > read_write.c. Presumably each needs to EPERM out on RWF_NOAPPEND if
> > > > > the underlying inode is S_APPEND. To avoid repeating the same logic in
> > > > > an error-prone way, should kiocb_set_rw_flags's signature be updated
> > > > > to take the filp so that it can obtain the inode and check IS_APPEND
> > > > > before accepting RWF_NOAPPEND? It's inline so this should avoid
> > > > > actually loading anything except in the codepath where
> > > > > flags&RWF_NOAPPEND is nonzero.
> > > >
> > > > You can get the file pointer from ki->ki_filp. See the RWF_NOWAIT
> > > > branch of kiocb_set_rw_flags().
> > >
> > > Thanks. I should have looked for that. OK, so a fixup like this on top
> > > of the existing patch?
> > >
> > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > index 473289bff4c6..674131e8d139 100644
> > > --- a/include/linux/fs.h
> > > +++ b/include/linux/fs.h
> > > @@ -3457,8 +3457,11 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
> > >                 ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
> > >         if (flags & RWF_APPEND)
> > >                 ki->ki_flags |= IOCB_APPEND;
> > > -       if (flags & RWF_NOAPPEND)
> > > +       if (flags & RWF_NOAPPEND) {
> > > +               if (IS_APPEND(file_inode(ki->ki_filp)))
> > > +                       return -EPERM;
> > >                 ki->ki_flags &= ~IOCB_APPEND;
> > > +       }
> > >         return 0;
> > >  }
> > >
> > > If this is good I'll submit a v2 as the above squashed with the
> > > original patch.
> >
> > Looks good to me.
>
> Actually it's not quite. I think it should be:
>
>         if ((flags & RWF_NOAPPEND) & (ki->ki_flags & IOCB_APPEND)) {
>                 if (IS_APPEND(file_inode(ki->ki_filp)))
>                         return -EPERM;
>                 ki->ki_flags &= ~IOCB_APPEND;
>         }
>
> i.e. don't refuse RWF_NOAPPEND on a file that was already successfully
> opened without O_APPEND that only subsequently got chattr +a. The
> permission check should only be done if it's overriding the default
> action for how the file is open.
>
> This is actually related to the fcntl corner case mentioned before.
>
> Are you ok with this change? If so I'll go ahead and prepare a v2.

Ah, yeah, I guess that makes sense to keep things more consistent.

(You'll have to write that as "(flags & RWF_NOAPPEND) && (ki->ki_flags
& IOCB_APPEND)" though (logical AND, not bitwise AND).)