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Date:   Thu, 5 Jul 2018 01:52:05 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Seung-Woo Kim <sw0312.kim@samsung.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        stable <stable@vger.kernel.org>, lwn@lwn.net,
        Jiri Slaby <jslaby@suse.cz>
Subject: Re: Linux 3.18.111
Message-ID: <20180705005205.GC30522@ZenIV.linux.org.uk>
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On Mon, Jul 02, 2018 at 10:01:25PM -0700, Linus Torvalds wrote:
> On Mon, Jul 2, 2018 at 9:43 PM Seung-Woo Kim <sw0312.kim@samsung.com> wrote:
> >
> > I think the commit itself is required. Simple, but not reliable,
> > workaround fix is like below:
> >
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index a34d401..7c751f2 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -1879,6 +1879,8 @@ void d_instantiate_new(struct dentry *entry,
> > struct inode *inode)
> >         BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
> >         BUG_ON(!inode);
> >         lockdep_annotate_inode_mutex_key(inode);
> > +       /* WORKAROUND for calling security_d_instantiate() */
> > +       entry->d_inode = inode;
> >         security_d_instantiate(entry, inode);
> >         spin_lock(&inode->i_lock);
> >         __d_instantiate(entry, inode);
> 
> Ugh. That looks horrible even if it might avoid the oops.
> 
> I think a much better solution is to back-port commit b296821a7c42
> ("xattr_handler: pass dentry and inode as separate arguments of
> ->get()") to older kernels. Then the inode is passed down all the way,
> and you don't have people try to get it from the (not yet initialized)
> dentry.
> 
> But there might be other parts missing too, and I didn't look at how
> easy/painful that backport would be.
> 
> Al - comments? This is all because of commit 1e2e547a93a0 ("do
> d_instantiate/unlock_new_inode combinations safely") being marked for
> stable, and various cases of security_d_instantiate() calling down to
> getxattr. Which used to not get the inode at all, so those older
> kernels use d_inode(dentry), which doesn't work in this path since
> dentry->d_inode hasn't been instantiated yet..

You also want b96809173e94 and ce23e6401334 there...