From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [62.89.141.173]) (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 21482302CCC; Wed, 13 Aug 2025 18:56:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=62.89.141.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755111368; cv=none; b=RJBhvqlm0p+xsiiJLLqib+8WqIgXONs5zP7aDcKkMCFyGOEmZvUJ7OgHLAC+Oumz2eEvaNobP4l8SxiGQ73BQ2xdUiHOA4eHvWluGg2O36u9xm3v+pxHPatUjRlPyNVKkoHKtJ5bMkfF6u5BgDa0Wkld8mDmqO0pl+RUZp5N/3w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755111368; c=relaxed/simple; bh=Fe19/ip8+CVxXl2aEKak9LtudYaMiDlEcAjExYvKYTk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=TfpowyCzyzn6FmXQtENFNq1SkbRDEJFowX5UqvflcflM7gRUv9j3sxGa4oFxiMcofOnHt1fa1LWW9HFmLLVMFbMkw6weaVrw9kn7V1qL0jwcwJro6bGmr2+LWEHO8vS8EzoomhWFvwD1gibCjTJ48tXjeB3NPO4ybLE6bhMk24s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zeniv.linux.org.uk; spf=none smtp.mailfrom=ftp.linux.org.uk; dkim=pass (2048-bit key) header.d=linux.org.uk header.i=@linux.org.uk header.b=w0i5Dd/Q; arc=none smtp.client-ip=62.89.141.173 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zeniv.linux.org.uk Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=ftp.linux.org.uk Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linux.org.uk header.i=@linux.org.uk header.b="w0i5Dd/Q" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=5lpkJos7qWPnLKHUIhAIUhJWzGvW6Gy5HTrpgDY7qFs=; b=w0i5Dd/QeMZm9yCMAj4ji+KQB4 hwaBPlNk992pRnUJXE8xcTRUBKlwpyVCXGGvQBjfsxUENoHMpscRc9L0Z4xYrhA29/BZCxR7u39yw JONiv5w6hqmajGUsgftfEWBIxUNBjc9qrtij6s4ljff/rsbHC8W78GrpTtUbwB/RxSISLb9iDeXn9 GN/bMDaj/B3GX0S7OEds2zY2TdIPqLtdXVy2cFvSt4SXYnYYHmlPrAJ47TVFpDgFnVnQqts5LDTzi 4WJiaXKIeRdx59rrlKgpUUk797RMFE+lXFx1tKjj8e9f/JISGEFMjl1CO7ybKN3WiKAUOXRhFsFR6 6sD5MWBA==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1umGdl-0000000CPrt-1AOC; Wed, 13 Aug 2025 18:56:01 +0000 Date: Wed, 13 Aug 2025 19:56:01 +0100 From: Al Viro To: Andrei Vagin Cc: Andrei Vagin , Christian Brauner , linux-fsdevel , LKML , criu@lists.linux.dev, Linux API , stable Subject: Re: do_change_type(): refuse to operate on unmounted/not ours mounts Message-ID: <20250813185601.GJ222315@ZenIV> References: <20250724230052.GW2580412@ZenIV> <20250726175310.GB222315@ZenIV> Precedence: bulk X-Mailing-List: linux-api@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: Sender: Al Viro On Sat, Jul 26, 2025 at 02:01:20PM -0700, Andrei Vagin wrote: > > For a very mild example of fun to be had there: > > mount("none", "/mnt", "tmpfs", 0, ""); > > chdir("/mnt"); > > umount2(".", MNT_DETACH); > > mount(NULL, ".", NULL, MS_SHARED, NULL); > > Repeat in a loop, watch mount group id leak. That's a trivial example > > of violating the assertion ("a mount that had been through umount_tree() > > is out of propagation graph and related data structures for good"). > > I wasn't referring to detached mounts. CRIU modifies mounts from > non-current namespaces. > > > > > As for the "CAP_SYS_ADMIN within the mount user namespace" - which > > userns do you have in mind? > > > > The user namespace of the target mount: > ns_capable(mnt->mnt_ns->user_ns, CAP_SYS_ADMIN) To bring that thread back: how about the following? If nobody objects, I'm going to throw it into viro/vfs.git #fixes... [PATCH] use uniform permission checks for all mount propagation changes do_change_type() and do_set_group() are operating on different aspects of the same thing - propagation graph. The latter asks for mounts involved to be mounted in namespace(s) the caller has CAP_SYS_ADMIN for. The former is a mess - originally it didn't even check that mount *is* mounted. That got fixed, but the resulting check turns out to be too strict for userland - in effect, we check that mount is in our namespace, having already checked that we have CAP_SYS_ADMIN there. What we really need (in both cases) is * we only touch mounts that are mounted. Hard requirement, data corruption if that's get violated. * we don't allow to mess with a namespace unless you already have enough permissions to do so (i.e. CAP_SYS_ADMIN in its userns). That's an equivalent of what do_set_group() does; let's extract that into a helper (may_change_propagation()) and use it in both do_set_group() and do_change_type(). Fixes: 12f147ddd6de "do_change_type(): refuse to operate on unmounted/not ours mounts" Signed-off-by: Al Viro --- diff --git a/fs/namespace.c b/fs/namespace.c index ddfd4457d338..e7d9b23f1e9e 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -2862,6 +2862,19 @@ static int graft_tree(struct mount *mnt, struct mount *p, struct mountpoint *mp) return attach_recursive_mnt(mnt, p, mp); } +static int may_change_propagation(const struct mount *m) +{ + struct mnt_namespace *ns = m->mnt_ns; + + // it must be mounted in some namespace + if (IS_ERR_OR_NULL(ns)) // is_mounted() + return -EINVAL; + // and the caller must be admin in userns of that namespace + if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN)) + return -EPERM; + return 0; +} + /* * Sanity check the flags to change_mnt_propagation. */ @@ -2898,10 +2911,10 @@ static int do_change_type(struct path *path, int ms_flags) return -EINVAL; namespace_lock(); - if (!check_mnt(mnt)) { - err = -EINVAL; + err = may_change_propagation(mnt); + if (err) goto out_unlock; - } + if (type == MS_SHARED) { err = invent_group_ids(mnt, recurse); if (err) @@ -3347,18 +3360,11 @@ static int do_set_group(struct path *from_path, struct path *to_path) namespace_lock(); - err = -EINVAL; - /* To and From must be mounted */ - if (!is_mounted(&from->mnt)) - goto out; - if (!is_mounted(&to->mnt)) - goto out; - - err = -EPERM; - /* We should be allowed to modify mount namespaces of both mounts */ - if (!ns_capable(from->mnt_ns->user_ns, CAP_SYS_ADMIN)) + err = may_change_propagation(from); + if (err) goto out; - if (!ns_capable(to->mnt_ns->user_ns, CAP_SYS_ADMIN)) + err = may_change_propagation(from); + if (err) goto out; err = -EINVAL;