From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2CB401D5151; Thu, 29 Jan 2026 14:36:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769697417; cv=none; b=qbUOMtzZepQ1NUKAyCR3+wpmaqbbE7zXa3+FEOQTvIwsnKxSGbatCsFZa+yWL/Yxrw9nugcQeNt390X1VIJaIH1FA8q1Mh/uYYC3zj9fNdBvP4V6p4IuL241tkk2XfYwGVvShXOwpQZ8kFlTGNkoOzZftrvEGkksggaMELCXyZA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769697417; c=relaxed/simple; bh=WAPkqVcktWNKHPYOyzWmWAwOC2J2vl/Ho/9F7ZSqjtw=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=bWU5e5uWPECGUGvzZ8ABpfRJae8/dk/td1fvDcnv+PiCtYP7gZcSH0Yp4cWai0sbq5VHj2ZuJYepMoVwtwkG5onb7VCq+LGLc/BeI0SfG5473i7KsnFdCAyQsanXnDNWApFjaMLiTh6OI9agVI6a3zQ13jzrVr0641qT2VBqifs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mi+YVOQ9; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mi+YVOQ9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C3734C4CEF7; Thu, 29 Jan 2026 14:36:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1769697416; bh=WAPkqVcktWNKHPYOyzWmWAwOC2J2vl/Ho/9F7ZSqjtw=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=mi+YVOQ9JHZnA9Z7sVVYJ8t3UFl+pkBNB0mefkKKChqiHQ5llWoH4tuzcJ77DFHxq 56w2hSfiivZSgTvfPkEry7jcUtzCH6ONv07GyBj1zOuzXR6uPnzrWJv0VdNGAcpCIr Kuk+KmH99OOhl4/KXObiPp10EJ4/RxsZDSyKu4n/cKbgyZOX+kZEUyeJxmf7Fmj/R5 maZwbLXEcDKnglPL7FSRSfSJS55eKZJUqBjM0wSSJtDRhgdXNta5hAB71V2oyupp+B +24KSxYsyM+8oAnHsc0gs9cDU66VMb3oA9/p5EC8NzmbKwzG5qICd/wh41DIw9KjsA uM66qcLCHvuRQ== Message-ID: Subject: Re: [PATCH] fs: don't allow non-init s_user_ns for filesystems without FS_USERNS_MOUNT From: Jeff Layton To: "Seth Forshee (DigitalOcean)" , Alexander Viro , Christian Brauner , Jan Kara Cc: Amir Goldstein , Aleksa Sarai , Alexander Mikhalitsyn , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org Date: Thu, 29 Jan 2026 09:36:54 -0500 In-Reply-To: <20240724-s_user_ns-fix-v1-1-895d07c94701@kernel.org> References: <20240724-s_user_ns-fix-v1-1-895d07c94701@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Wed, 2024-07-24 at 09:53 -0500, Seth Forshee (DigitalOcean) wrote: > Christian noticed that it is possible for a privileged user to mount > most filesystems with a non-initial user namespace in sb->s_user_ns. > When fsopen() is called in a non-init namespace the caller's namespace > is recorded in fs_context->user_ns. If the returned file descriptor is > then passed to a process priviliged in init_user_ns, that process can > call fsconfig(fd_fs, FSCONFIG_CMD_CREATE), creating a new superblock > with sb->s_user_ns set to the namespace of the process which called > fsopen(). >=20 > This is problematic. We cannot assume that any filesystem which does not > set FS_USERNS_MOUNT has been written with a non-initial s_user_ns in > mind, increasing the risk for bugs and security issues. >=20 > Prevent this by returning EPERM from sget_fc() when FS_USERNS_MOUNT is > not set for the filesystem and a non-initial user namespace will be > used. sget() does not need to be updated as it always uses the user > namespace of the current context, or the initial user namespace if > SB_SUBMOUNT is set. >=20 > Fixes: cb50b348c71f ("convenience helpers: vfs_get_super() and sget_fc()"= ) > Reported-by: Christian Brauner > Signed-off-by: Seth Forshee (DigitalOcean) > --- > fs/super.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) >=20 > diff --git a/fs/super.c b/fs/super.c > index 095ba793e10c..d681fb7698d8 100644 > --- a/fs/super.c > +++ b/fs/super.c > @@ -736,6 +736,17 @@ struct super_block *sget_fc(struct fs_context *fc, > struct user_namespace *user_ns =3D fc->global ? &init_user_ns : fc->use= r_ns; > int err; > =20 > + /* > + * Never allow s_user_ns !=3D &init_user_ns when FS_USERNS_MOUNT is > + * not set, as the filesystem is likely unprepared to handle it. > + * This can happen when fsconfig() is called from init_user_ns with > + * an fs_fd opened in another user namespace. > + */ > + if (user_ns !=3D &init_user_ns && !(fc->fs_type->fs_flags & FS_USERNS_M= OUNT)) { > + errorfc(fc, "mounting from non-initial user namespace is not allowed")= ; > + return ERR_PTR(-EPERM); > + } > + > retry: > spin_lock(&sb_lock); > if (test) { >=20 > --- > base-commit: 256abd8e550ce977b728be79a74e1729438b4948 > change-id: 20240723-s_user_ns-fix-b00c31de1cb8 >=20 > Best regards, I sent an incorrect RFC patch for this yesterday, but this patch breaks NFS mounting in containers for us, as the prohibited activity is exactly the process we use to do them. We basically have a task in the container do an fsopen() and then pass the fd to a daemon in the init namespace via unix socket. The daemon vets the NFS mount parameters (ensuring that the mount options are sane, and that we trust the server), and then does the mount inside the container. We don't want to set FS_USERNS_MOUNT on NFS, because that would give the container carte blanche to mount anything it likes, even a malicious server. Do we need to split that flag into two? Maybe FS_USERNS_SAFE and FS_USERNS_MOUNT? -- Jeff Layton