linux-kernel - Re: [PATCH] fs: Pass AT_GETATTR_NOSEC flag to getattr interface function

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Date:   Mon, 09 Oct 2023 21:14:02 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Christian Brauner <brauner@...nel.org>,
        Amir Goldstein <amir73il@...il.com>
Cc:     Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-unionfs@...r.kernel.org, miklos@...redi.hu,
        Stefan Berger <stefanb@...ux.ibm.com>,
        syzbot+a67fc5321ffb4b311c98@...kaller.appspotmail.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Tyler Hicks <code@...icks.com>
Subject: Re: [PATCH] fs: Pass AT_GETATTR_NOSEC flag to getattr interface
 function

On Tue, 2023-10-03 at 15:38 +0200, Christian Brauner wrote:
> On Mon, Oct 02, 2023 at 04:22:25PM +0300, Amir Goldstein wrote:
> > On Mon, Oct 2, 2023 at 3:57 PM Stefan Berger <stefanb@...ux.vnet.ibm.com> wrote:
> > >
> > > From: Stefan Berger <stefanb@...ux.ibm.com>
> > >
> > > When vfs_getattr_nosec() calls a filesystem's getattr interface function
> > > then the 'nosec' should propagate into this function so that
> > > vfs_getattr_nosec() can again be called from the filesystem's gettattr
> > > rather than vfs_getattr(). The latter would add unnecessary security
> > > checks that the initial vfs_getattr_nosec() call wanted to avoid.
> > > Therefore, introduce the getattr flag GETATTR_NOSEC and allow to pass
> > > with the new getattr_flags parameter to the getattr interface function.
> > > In overlayfs and ecryptfs use this flag to determine which one of the
> > > two functions to call.
> > >
> > > In a recent code change introduced to IMA vfs_getattr_nosec() ended up
> > > calling vfs_getattr() in overlayfs, which in turn called
> > > security_inode_getattr() on an exiting process that did not have
> > > current->fs set anymore, which then caused a kernel NULL pointer
> > > dereference. With this change the call to security_inode_getattr() can
> > > be avoided, thus avoiding the NULL pointer dereference.
> > >
> > > Reported-by: syzbot+a67fc5321ffb4b311c98@...kaller.appspotmail.com
> > > Fixes: db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the i_version")
> > > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > > Cc: linux-fsdevel@...r.kernel.org
> > > Cc: Miklos Szeredi <miklos@...redi.hu>
> > > Cc: Amir Goldstein <amir73il@...il.com>
> > > Cc: Tyler Hicks <code@...icks.com>
> > > Cc: Mimi Zohar <zohar@...ux.ibm.com>
> > > Suggested-by: Christian Brauner <brauner@...nel.org>
> > > Co-developed-by: Amir Goldstein <amir73il@...il.com>
> > > Signed-off-by: Stefan Berger <stefanb@...ux.ibm.com>
> > > ---
> > 
> > Reviewed-by: Amir Goldstein <amir73il@...il.com>
> > 
> > Now let's see what vfs maintainers think about this...
> 
> Seems fine overall. We kind of need to propagate the knowledge through
> the layers. But I don't like that we need something like it...

So at this point there are two options.  Either revert commit
db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the i_version") or
this patch to fix it.  Christian, what do you prefer?

Mimi