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

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Message-ID: <20231003-bespielbar-tarnt-c61162656db5@brauner>
Date:   Tue, 3 Oct 2023 15:38:06 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     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>,
        Mimi Zohar <zohar@...ux.ibm.com>
Subject: Re: [PATCH] fs: Pass AT_GETATTR_NOSEC flag to getattr interface
 function

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...