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

[Christian Brauner <brauner@kernel.org>]

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@linux.vnet.ibm.com> wrote:
> >
> > From: Stefan Berger <stefanb@linux.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@syzkaller.appspotmail.com
> > Fixes: db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the i_version")
> > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> > Cc: linux-fsdevel@vger.kernel.org
> > Cc: Miklos Szeredi <miklos@szeredi.hu>
> > Cc: Amir Goldstein <amir73il@gmail.com>
> > Cc: Tyler Hicks <code@tyhicks.com>
> > Cc: Mimi Zohar <zohar@linux.ibm.com>
> > Suggested-by: Christian Brauner <brauner@kernel.org>
> > Co-developed-by: Amir Goldstein <amir73il@gmail.com>
> > Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> > ---
> 
> Reviewed-by: Amir Goldstein <amir73il@gmail.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...