From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp-190f.mail.infomaniak.ch (smtp-190f.mail.infomaniak.ch [185.125.25.15]) (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 0AA06194C75 for ; Fri, 9 Aug 2024 13:37:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.125.25.15 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723210652; cv=none; b=mvlMDoIchASb+WUsbt+XRD9R8rE6Flvv7PwN7ZuRRs9g7hyGwr6LfgOnR/JP5nZ88lq5IASaLGwWk4qvQlbgt9qddAhW3STwYYlHGMrwlNrNESjLCcHLI0l3fAVFtMiv3uUGdg6lKF6Zq+rnfvJGfL3AwSbT1FION0pPsjBEOAQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723210652; c=relaxed/simple; bh=CV2KsuTzZFM3tb3jpFAEOVmdFeFl9DGCIr23w73TUBc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=XJoDCs6pcY0eAswiYZcXiuHraHejalIe54CIFhXKzB1vaDuu2QacgDfZindYS3Tadpe0PbAgPlaYMHuVB7xWYG3Y9ipjhbc5CRsElPUr6OoVrqo5ICM8R8/bDTd298xM1EjQojPtZTqSCkxXPPSpvT/X+1yo6YNCwqnklnK4MDk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net; spf=pass smtp.mailfrom=digikod.net; dkim=pass (1024-bit key) header.d=digikod.net header.i=@digikod.net header.b=OpYdACcN; arc=none smtp.client-ip=185.125.25.15 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=digikod.net Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=digikod.net Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=digikod.net header.i=@digikod.net header.b="OpYdACcN" Received: from smtp-3-0000.mail.infomaniak.ch (smtp-3-0000.mail.infomaniak.ch [10.4.36.107]) by smtp-4-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 4WgQ2G15hFzsbq; Fri, 9 Aug 2024 15:37:26 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=digikod.net; s=20191114; t=1723210646; bh=8yuEdFJ5d9KshgHjOdDGSrL9W7g0PXB4OxTMQzj+pOw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=OpYdACcNA4gLC3N3pH0KoWMirAjjPj2EBVJfEGi4dfMGog77vLpqZuSTrW72bVb3M +EQ3x26RAlAsnbvc1YpYxMtM/93AA3EBwXiEFROh8BxkZ5/30nqycipm6+yqoJkswN Bts5A/dMbwc93yssOu8etR1rmfOSZV1LvaBfXbOU= Received: from unknown by smtp-3-0000.mail.infomaniak.ch (Postfix) with ESMTPA id 4WgQ2F4g09z7Hk; Fri, 9 Aug 2024 15:37:25 +0200 (CEST) Date: Fri, 9 Aug 2024 15:37:20 +0200 From: =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= To: Jann Horn Cc: Tahera Fahimi , outreachy@lists.linux.dev, gnoack@google.com, paul@paul-moore.com, jmorris@namei.org, serge@hallyn.com, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, bjorn3_gh@protonmail.com, netdev@vger.kernel.org Subject: Re: [PATCH v2 1/4] Landlock: Add signal control Message-ID: <20240809.fee1eiPohphu@digikod.net> References: <49557e48c1904d2966b8aa563215d2e1733dad95.1722966592.git.fahimitahera@gmail.com> <20240807.Yee4al2lahCo@digikod.net> <20240808.kaiyaeZoo1ha@digikod.net> <20240809.eejeekoo4Quo@digikod.net> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Infomaniak-Routing: alpha On Fri, Aug 09, 2024 at 02:44:06PM +0200, Jann Horn wrote: > On Fri, Aug 9, 2024 at 12:59 PM Mickaël Salaün wrote: > > On Thu, Aug 08, 2024 at 04:42:23PM +0200, Jann Horn wrote: > > > On Thu, Aug 8, 2024 at 4:09 PM Mickaël Salaün wrote: > > > > On Thu, Aug 08, 2024 at 03:10:54AM +0200, Jann Horn wrote: > > > > > On Thu, Aug 8, 2024 at 1:36 AM Tahera Fahimi wrote: > > > > > > On Wed, Aug 07, 2024 at 08:16:47PM +0200, Mickaël Salaün wrote: > > > > > > > On Tue, Aug 06, 2024 at 11:55:27PM +0200, Jann Horn wrote: > > > > > > > > On Tue, Aug 6, 2024 at 8:56 PM Jann Horn wrote: > > > > > > > > > On Tue, Aug 6, 2024 at 8:11 PM Tahera Fahimi wrote: > > > > > > > > > > Currently, a sandbox process is not restricted to send a signal > > > > > > > > > > (e.g. SIGKILL) to a process outside of the sandbox environment. > > > > > > > > > > Ability to sending a signal for a sandboxed process should be > > > > > > > > > > scoped the same way abstract unix sockets are scoped. Therefore, > > > > > > > > > > we extend "scoped" field in a ruleset with > > > > > > > > > > "LANDLOCK_SCOPED_SIGNAL" to specify that a ruleset will deny > > > > > > > > > > sending any signal from within a sandbox process to its > > > > > > > > > > parent(i.e. any parent sandbox or non-sandboxed procsses). > > > > > > > > [...] > > > > > > > > > > + if (is_scoped) > > > > > > > > > > + return 0; > > > > > > > > > > + > > > > > > > > > > + return -EPERM; > > > > > > > > > > +} > > > > > > > > > > + > > > > > > > > > > +static int hook_file_send_sigiotask(struct task_struct *tsk, > > > > > > > > > > + struct fown_struct *fown, int signum) > > > > > > > > > > > > > > I was wondering if we should handle this case, but I guess it makes > > > > > > > sense to have a consistent policy for all kind of user-triggerable > > > > > > > signals. > > > > > > > > > > > > > > > > > +{ > > > > > > > > > > + bool is_scoped; > > > > > > > > > > + const struct landlock_ruleset *dom, *target_dom; > > > > > > > > > > + struct task_struct *result = get_pid_task(fown->pid, fown->pid_type); > > > > > > > > > > > > > > > > > > I'm not an expert on how the fowner stuff works, but I think this will > > > > > > > > > probably give you "result = NULL" if the file owner PID has already > > > > > > > > > exited, and then the following landlock_get_task_domain() would > > > > > > > > > probably crash? But I'm not entirely sure about how this works. > > > > > > > > > > > > > > > > > > I think the intended way to use this hook would be to instead use the > > > > > > > > > "file_set_fowner" hook to record the owning domain (though the setup > > > > > > > > > for that is going to be kind of a pain...), see the Smack and SELinux > > > > > > > > > definitions of that hook. Or alternatively maybe it would be even > > > > > > > > > nicer to change the fown_struct to record a cred* instead of a uid and > > > > > > > > > euid and then use the domain from those credentials for this hook... > > > > > > > > > I'm not sure which of those would be easier. > > > > > > > > > > > > > > > > (For what it's worth, I think the first option would probably be > > > > > > > > easier to implement and ship for now, since you can basically copy > > > > > > > > what Smack and SELinux are already doing in their implementations of > > > > > > > > these hooks. I think the second option would theoretically result in > > > > > > > > nicer code, but it might require a bit more work, and you'd have to > > > > > > > > include the maintainers of the file locking code in the review of such > > > > > > > > refactoring and have them approve those changes. So if you want to get > > > > > > > > this patchset into the kernel quickly, the first option might be > > > > > > > > better for now?) > > > > > > > > > > > > > > > > > > > > > > I agree, let's extend landlock_file_security with a new "fown" pointer > > > > > > > to a Landlock domain. We'll need to call landlock_get_ruleset() in > > > > > > > hook_file_send_sigiotask(), and landlock_put_ruleset() in a new > > > > > > > hook_file_free_security(). > > > > > > I think we should add a new hook (hook_file_set_owner()) to initialize > > > > > > the "fown" pointer and call landlock_get_ruleset() in that? > > > > > > > > > > Yeah. Initialize the pointer in the file_set_fowner hook, and read the > > > > > pointer in the file_send_sigiotask hook. > > > > > > > > > > Note that in the file_set_fowner hook, you'll probably need to use > > > > > both landlock_get_ruleset() (to take a reference on the ruleset you're > > > > > storing in the fown pointer) and landlock_put_ruleset() (to drop the > > > > > reference to the ruleset that the fown pointer was pointing to > > > > > before). And you'll need to use some kind of lock to protect the fown > > > > > pointer - either by adding an appropriate lock next to your fown > > > > > pointer or by using some appropriate existing lock in "struct file". > > > > > Probably it's cleanest to have your own lock for this? (This lock will > > > > > have to be something like a spinlock, not a mutex, since you need to > > > > > be able to acquire it in the file_set_fowner hook, which runs inside > > > > > an RCU read-side critical section, where sleeping is forbidden - > > > > > acquiring a mutex can sleep and therefore is forbidden in this > > > > > context, acquiring a spinlock can't sleep.) > > > > > > > > Yes, I think this should work for file_set_fowner: > > > > > > > > struct landlock_ruleset *prev_dom, *new_dom; > > > > > > > > new_dom = landlock_get_current_domain(); > > > > landlock_get_ruleset(new_dom); > > > > > > > > /* Cf. f_modown() */ > > > > write_lock_irq(&filp->f_owner.lock); > > > > prev_dom = rcu_replace_pointer(&landlock_file(file)->fown_domain, > > > > new_dom, lockdep_is_held(&filp->f_owner.lock)); > > > > write_unlock_irq(&filp->f_owner.lock); > > > > > > > > landlock_put_ruleset_rcu(prev_dom); > > > > > > > > > > > > With landlock_put_ruleset_rcu() define with this: > > > > > > > > diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c > > > > index a93bdbf52fff..897116205520 100644 > > > > --- a/security/landlock/ruleset.c > > > > +++ b/security/landlock/ruleset.c > > > > @@ -524,6 +524,20 @@ void landlock_put_ruleset_deferred(struct landlock_ruleset *const ruleset) > > > > } > > > > } > > > > > > > > +static void free_ruleset_rcu(struct rcu_head *const head) > > > > +{ > > > > + struct landlock_ruleset *ruleset; > > > > + > > > > + ruleset = container_of(head, struct landlock_ruleset, rcu); > > > > + free_ruleset(ruleset); > > > > +} > > > > > > free_ruleset() can block but RCU callbacks aren't allowed to block, > > > that's why landlock_put_ruleset_deferred() exists. > > > > Yes, but landlock_put_ruleset_deferred() doesn't wait for RCU read-side > > critical sections. > > Ah, I phrased that badly - I didn't mean to suggest that you should > use landlock_put_ruleset_deferred() as a replacement for call_rcu(). > > [...] > > > So if you want to use RCU lifetime for this, I think you'll have to > > > turn landlock_put_ruleset() and landlock_put_ruleset_deferred() into > > > one common function that always, when reaching refcount 0, schedules > > > an RCU callback which then schedules a work_struct which then does > > > free_ruleset(). > > > > > > I think that would be a little ugly, and it would look nicer to just > > > use normal locking in the file_send_sigiotask hook? > > > > I don't see how we can do that without delaying the free_ruleset() call > > to after the RCU read-side critical section in f_setown(). > > It should work if you used landlock_put_ruleset_deferred() instead of > landlock_put_ruleset(). Calling landlock_put_ruleset_deferred() in hook_file_set_fowner() or replacing all landlock_put_ruleset() calls? The deferred work queue is not guarantee to run after all concurrent RCU read-side critical sections right? Calling synchronize_rcu() in free_ruleset_work() should give this guarantee, but it's not nice. We could add a boolean in landlock_ruleset to only call synchronize_rcu() when required (i.e. called from file_set_fowner). > > > What about calling refcount_dec_and_test() in free_ruleset_rcu()? That > > would almost always queue this call but it looks safe. > > Every queued RCU invocation needs to have its own rcu_head - I think > the approach you're suggesting could end up queuing the same rcu_head > multiple times? Right > > > An alternative might be to call synchronize_rcu() in free_ruleset(), but > > it's a big ugly too.