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 7FFF139C63E for ; Fri, 15 May 2026 16:24:41 +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=1778862281; cv=none; b=EIKywBD6fyXHcazDdTTkkXkkoXHQaKt21d5Vp6fhUDIvXkUHGSXu+f2Vu+vwzDAbZNUyiphAZCHm7QE0WX8oa4lrj0b0HJ7Aa+dKu+O57xoGdcLO/eKGmABcoV3wXhZROVxhSVROpLN7SwSCWf3P38LJiBAxRE1wgsZvH40vz9A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778862281; c=relaxed/simple; bh=t1qQBrK+cRgELnlc05S5CALyOzD0Mv//pL3ZWuidWzg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=IV8B76WubfB7qDzFB0MNGyyUm4DnUt6mwtXUYuEiseXXV1urFVs3gl0hzd2JVqBPLq6KIxUGYUV5jwzxH77DTi+NawI7QLTvpvGY41flaDyqG2NWJ+bn057Pe0Wqf3fxi3wof8nM4GxG3hHqP7IYXuaxHtnJ7qCnb7rRZes3+L4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WF9K/295; 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="WF9K/295" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2D5C2C2BCC9; Fri, 15 May 2026 16:24:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778862281; bh=t1qQBrK+cRgELnlc05S5CALyOzD0Mv//pL3ZWuidWzg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=WF9K/295BaAk1Jngiv7IBaRPDuJQj2ZN/96vvgHe6b2WQVhfMj4v+sOcRKRQHgTG8 SdiQyctwlDBreXx8cw0d/ofycblCIVbvTJyrpI0IOLZR8Sb5zvkUzpnyKRopH8r36h qNMls9R0pHVuSlcQIrH4066/73QDQAbbMww/KZCcuv3vaQegVPTq3s0RYLjAe98AFq oI4M9ZQeGbXKhdFPxVKlFdzRFHkHJ3I6WE4gsWON4iNV3kSqJf+t9Soqh2os9o24qo IJTph/jqq5QfIbddElE3P6L7z9D2p17cSBySYABeec8Jg1kQG7I7Gm/Cuv/fNnuJGk VUXVsH0N3mmHQ== Date: Sat, 16 May 2026 00:24:36 +0800 From: Zorro Lang To: Christian Brauner Cc: fstests@vger.kernel.org Subject: Re: [PATCH] vfs: fix dumpable race in create_userns_hierarchy() Message-ID: Mail-Followup-To: Christian Brauner , fstests@vger.kernel.org References: <20260323-fix-vfs-bugs-v1-1-76a6aed624d9@kernel.org> Precedence: bulk X-Mailing-List: fstests@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260323-fix-vfs-bugs-v1-1-76a6aed624d9@kernel.org> On Mon, Mar 23, 2026 at 02:32:15PM +0100, Christian Brauner wrote: > All processes in the userns hierarchy share the same mm_struct due to > CLONE_VM. When a child calls setresuid() in switch_ids(), the kernel > clears the dumpable flag on the shared mm. The child immediately > re-sets it via prctl(PR_SET_DUMPABLE, 1), but there is a window > between the two where the parent may be opening /proc/PID/ns/user for > the previous level's child. That open hits ptrace_may_access() which > checks get_dumpable(mm) and, finding it zero, falls back to > ptrace_has_cap(mm->user_ns, mode). Since mm->user_ns is init_user_ns > (the mm was created by real root) and the parent is only ns-root, the > capability check fails and the open returns -EACCES. > > Fix this by extending the existing two-way socketpair handshake into a > three-way handshake: the child signals the parent after becoming > dumpable, then blocks until the parent has opened /proc/PID/ns/user > and signals back. Only then does the child proceed to > create_userns_hierarchy() for the next level. This guarantees no > concurrent setresuid() can clear dumpable while the parent's open() is > in flight. > > Signed-off-by: Christian Brauner > --- Sorry, a few unforeseen things came up on my end and I lost track of this patch. Reviewed-by: Zorro Lang > src/vfs/utils.c | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/src/vfs/utils.c b/src/vfs/utils.c > index 52bb7e42..0b435afe 100644 > --- a/src/vfs/utils.c > +++ b/src/vfs/utils.c > @@ -312,6 +312,18 @@ static int userns_fd_cb(void *data) > if (ret < 0) > return syserror("failure: write to socketpair"); > > + /* > + * Wait for the parent to open our /proc/PID/ns/user before > + * proceeding to create the next userns level. Without this, > + * setresuid() in the next-level child clears the dumpable flag > + * on the shared mm (CLONE_VM) and the parent's open() fails the > + * ptrace_may_access() check with -EACCES because mm->user_ns is > + * init_user_ns where userns processes lack CAP_SYS_PTRACE. > + */ > + ret = read_nointr(h->fd_event, &c, 1); > + if (ret < 0) > + return syserror("failure: read from socketpair"); > + > ret = create_userns_hierarchy(++h); > if (ret < 0) > return syserror("failure: userns level %d", h->level); > @@ -377,6 +389,14 @@ int create_userns_hierarchy(struct userns_hierarchy *h) > goto out_wait; > } > > + /* Tell the child it can now proceed to create the next level. */ > + bytes = write_nointr(fd_socket[0], "1", 1); > + if (bytes < 0) { > + kill(pid, SIGKILL); > + syserror("failure: write to socketpair"); > + goto out_wait; > + } > + > fret = 0; > > out_wait: > > --- > base-commit: 3ded3e13c008326d197d11ac975049ed1f8ec922 > change-id: 20260323-fix-vfs-bugs-b4f8a492d6d7 > >