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 CD473395D90; Fri, 10 Apr 2026 11:12:41 +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=1775819561; cv=none; b=LJuKXtPxcDn5BrfCjmqym+DKUwKkSEU5GeisImZ0qV/rV7EX2yV6uvJuesUoOkPRmzICpOUytIoNRrli6OA+95ColKLctcRi7lR4hhojG1nMaiU9PBuX5hz8ytzB00Q6C75sgQyxfUTbTK5CLYO+lsYnIbEhNJmm8lOTW2Zo5rA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775819561; c=relaxed/simple; bh=wM2A+17g/Ua5mDDboh62uYwjkJFHo1LNxa0bdAUoYyU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ihPShliJfpqesws9F/RfhkYs3EMjc5fEl0M0opUj2bwdkpxHapkwLtjOJgD2G8cPyLnqSABoxK1o9fOdFlA1S6Ui+gfYcX35SeCj/iaxc66raA2OEqJ8rV7dQwOAKQbUSDBzNsL5tclaqaMVGC+nm8QPIERlBOaUAjQXokjvOh0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=M6+BkjPn; 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="M6+BkjPn" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ADE4CC19421; Fri, 10 Apr 2026 11:12:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775819561; bh=wM2A+17g/Ua5mDDboh62uYwjkJFHo1LNxa0bdAUoYyU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=M6+BkjPn9JJSeBobaJAw39DbhZtFmFjFtsLkNJGdQDqNS99hyg8pV6Y4OglTbAOK2 Kj3aSvpgbSzKVrDJLytge3h4zr4cgQHDamzsyT+Hz5RrMOk3cdnnqcLwOwl+CIkIsU QgZDe/7rQODR/mMufQw6vNe7FfegPC5GokpbJk32hwu50IAMizJE5mUK/OyxfpX4IZ gmKmtPjIKexBKb5hjmO/1eJck8RPguMuLaIEVIPCGEejFnGmmyOaX2y3oPiUFrSzE+ Ek60KaZMn1Au0r5fZJh41xsOMCfjeBPLoztmC36Uu0uZ1Md752vsvBQYhMzrhV1tRY qiKw7qShMOqFQ== Date: Fri, 10 Apr 2026 13:12:36 +0200 From: Christian Brauner To: Alexey Gladkov Cc: Dan Klishch , Al Viro , "Eric W . Biederman" , Kees Cook , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v8 4/5] proc: Relax check of mount visibility Message-ID: <20260410-textstellen-anrollen-17acbf930c3a@brauner> References: <0943f113592a25bee341aae25d1cea088791054f.1770979341.git.legion@kernel.org> <20260217-helikopter-parkplatz-859f8f7c0054@brauner> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260217-helikopter-parkplatz-859f8f7c0054@brauner> On Tue, Feb 17, 2026 at 12:59:54PM +0100, Christian Brauner wrote: > On Fri, Feb 13, 2026 at 11:44:29AM +0100, Alexey Gladkov wrote: > > When /proc is mounted with the subset=pid option, all system files from > > the root of the file system are not accessible in userspace. Only > > dynamic information about processes is available, which cannot be > > hidden with overmount. > > > > For this reason, checking for full visibility is not relevant if > > mounting is performed with the subset=pid option. > > > > Signed-off-by: Alexey Gladkov > > --- > > fs/namespace.c | 29 ++++++++++++++++------------- > > fs/proc/root.c | 17 ++++++++++------- > > include/linux/fs/super_types.h | 2 ++ > > 3 files changed, 28 insertions(+), 20 deletions(-) > > > > diff --git a/fs/namespace.c b/fs/namespace.c > > index c58674a20cad..7daa86315c05 100644 > > --- a/fs/namespace.c > > +++ b/fs/namespace.c > > @@ -6116,7 +6116,8 @@ static bool mnt_already_visible(struct mnt_namespace *ns, > > /* This mount is not fully visible if it's root directory > > * is not the root directory of the filesystem. > > */ > > - if (mnt->mnt.mnt_root != mnt->mnt.mnt_sb->s_root) > > + if (!(sb->s_iflags & SB_I_USERNS_ALLOW_REVEALING) && > > + mnt->mnt.mnt_root != mnt->mnt.mnt_sb->s_root) > > continue; > > > > /* A local view of the mount flags */ > > @@ -6136,18 +6137,20 @@ static bool mnt_already_visible(struct mnt_namespace *ns, > > ((mnt_flags & MNT_ATIME_MASK) != (new_flags & MNT_ATIME_MASK))) > > continue; > > There are a few things that I find problematic here. > > Even before your change the mount flags of the first fully visible > procfs mount would be picked up. If the caller was unlucky they could > stumble upon the most restricted procfs mount in the mount namespace > rbtree. Leading to weird scenarios where a user cannot write to the > procfs instance they just mounted but could to another one that is also > in their namespace. > > The other thing is that with this change specifically: > > if (!(sb->s_iflags & SB_I_USERNS_ALLOW_REVEALING) && > mnt->mnt.mnt_root != mnt->mnt.mnt_sb->s_root) > > we start caring about mount options of even partially exposed procfs > mounts. IOW, if someone had a bind-mount of e.g., /proc/pressure > somewhere that got inherited via CLONE_NEWNS then we suddenly take the > mount options of that into account for a new /proc//* only instance. > I think we should continue caring only about procfs mounts that are > visible from their root. > > The the other problem is that it is really annoying that we walk all > mounts in a mount namespace just to find procfs and sysfs mounts in > there. Currently a lot of workloads still do the CLONE_NEWNS dance > meaning they inherit all the crap from the host and then proceed to > setup their new rootfs. Busy container workloads that can be a lot. > > So let's just be honest about it and treat procfs and sysfs as the > snowflakes that they have become and record their instances in a > separate per mount namespace hlist as in the (untested) patch below [1]. > > Also SB_I_USERNS_ALLOW_REVEALING seems unnecessary. The only time we > care about that flag is when we setup a new superblock. So this could > easily be a struct fs_context bitfield that just exists for the duration > of the creation of the new superblock and mount. So maybe pass that down > to mount_too_revealing() and further down into the actual helper. > > [1]: > >From 4bbd41e7a3ef91667dd334f31b1b6bf8caec0599 Mon Sep 17 00:00:00 2001 > From: Christian Brauner > Date: Tue, 17 Feb 2026 12:02:34 +0100 > Subject: [PATCH] namespace: record fully visible mounts in list > > Instead of wading through all the mounts in the mount namespace rbtree > to find fully visible procfs and sysfs mounts, be honest about them > being special cruft and record them in a separate per-mount namespace > list. If you rework this I would expect to take it for v7.3. It's a bit late now...