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Date:   Fri, 02 Jul 2021 17:08:09 +0000
To:     John Wood <john.wood@gmx.com>
From:   Alexander Lobakin <alobakin@pm.me>
Cc:     Alexander Lobakin <alobakin@pm.me>,
        Kees Cook <keescook@chromium.org>,
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Reply-To: Alexander Lobakin <alobakin@pm.me>
Subject: Re: [PATCH v8 3/8] security/brute: Detect a brute force attack
Message-ID: <20210702170101.16116-1-alobakin@pm.me>
In-Reply-To: <20210702145954.GA4513@ubuntu>
References: <20210701234807.50453-1-alobakin@pm.me> <20210702145954.GA4513@ubuntu>
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From: John Wood <john.wood@gmx.com>
Date: Fri, 2 Jul 2021 16:59:54 +0200

> Hi,
>
> On Thu, Jul 01, 2021 at 11:55:14PM +0000, Alexander Lobakin wrote:
> > Hi,
> >
> > From: John Wood <john.wood@gmx.com>
> > Date: Sat, 5 Jun 2021 17:04:00 +0200
> >
> > > +static int brute_task_execve(struct linux_binprm *bprm, struct file =
*file)
> > > +{
> > > +=09struct dentry *dentry =3D file_dentry(bprm->file);
> > > +=09struct inode *inode =3D file_inode(bprm->file);
> > > +=09struct brute_stats stats;
> > > +=09int rc;
> > > +
> > > +=09inode_lock(inode);
> > > +=09rc =3D brute_get_xattr_stats(dentry, inode, &stats);
> > > +=09if (WARN_ON_ONCE(rc && rc !=3D -ENODATA))
> > > +=09=09goto unlock;
> >
> > I think I caught a problem here. Have you tested this with
> > initramfs?
>
> No, it has not been tested with initramfs :(
>
> > According to init/do_mount.c's
> > init_rootfs()/rootfs_init_fs_context(), when `root=3D` cmdline
> > parameter is not empty, kernel creates rootfs of type ramfs
> > (tmpfs otherwise).
> > The thing about ramfs is that it doesn't support xattrs.
>
> It is a known issue that systems without xattr support are not
> suitable for Brute (there are a note in the documentation).
> However, the purpose is not to panic the system :(
>
> > I'm running this v8 on a regular PC with initramfs and having
> > `root=3D` in cmdline, and Brute doesn't allow the kernel to run
> > any init processes (/init, /sbin/init, ...) with err =3D=3D -95
> > (-EOPNOTSUPP) -- I'm getting a
> >
> > WARNING: CPU: 0 PID: 173 at brute_task_execve+0x15d/0x200
> > <snip>
> > Failed to execute /init (error -95)
> >
> > and so on (and a panic at the end).
> >
> > If I omit `root=3D` from cmdline, then the kernel runs init process
> > just fine -- I guess because initramfs is then placed inside tmpfs
> > with xattr support.
> >
> > As for me, this ramfs/tmpfs selection based on `root=3D` presence
> > is ridiculous and I don't see or know any reasons behind that.
> > But that's another story, and ramfs might be not the only one
> > system without xattr support.
> > I think Brute should have a fallback here, e.g. it could simply
> > ignore files from xattr-incapable filesystems instead of such
> > WARNING splats and stuff.
>
> Ok, it seems reasonable to me: if the file system doesn't support
> xattr, but Brute is enabled, Brute will do nothing and the system
> will work normally.

On the other hand, it leaves a potentional window for attackers to
perform brute force from xattr-incapable filesystems. So at the end
of the day I think that the current implementation (a strong
rejection of such filesystems) is way more secure than having
a fallback I proposed.

I'm planning to make a patch which will eliminate such weird rootfs
type selection and just always use more feature-rich tmpfs if it's
compiled in. So, as an alternative, you could add it to your series
as a preparatory change and just add a Kconfig dependency on
CONFIG_TMPFS && CONFIG_TMPFS_XATTR to CONFIG_SECURITY_FORK_BRUTE
without messing with any fallbacks at all.
What do you think?

> I will work on it for the next version.
> Thanks for the feedback.
>
> John Wood

Thanks,
Al