linux-kernel - [PATCH] watch_queue: Limit the number of watches a user can hold

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Message-ID: <159765884811.1436382.16387370517871969276.stgit@warthog.procyon.org.uk>
Date:   Mon, 17 Aug 2020 11:07:28 +0100
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        dhowells@...hat.com, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] watch_queue: Limit the number of watches a user can hold

Impose a limit on the number of watches that a user can hold so that they
can't use this mechanism to fill up all the available memory.

This is done by putting a counter in user_struct that's incremented when a
watch is allocated and decreased when it is released.  If the number
exceeds the RLIMIT_NOFILE limit, the watch is rejected with EAGAIN.

This can be tested by the following means:

 (1) Create a watch queue and attach it to fd 5 in the program given - in
     this case, bash:

	keyctl watch_session /tmp/nlog /tmp/gclog 5 bash

 (2) In the shell, set the maximum number of files to, say, 99:

	ulimit -n 99

 (3) Add 200 keyrings:

	for ((i=0; i<200; i++)); do keyctl newring a$i @s || break; done

 (4) Try to watch all of the keyrings:

	for ((i=0; i<200; i++)); do echo $i; keyctl watch_add 5 %:a$i || break; done

     This should fail when the number of watches belonging to the user hits
     99.

 (5) Remove all the keyrings and all of those watches should go away:

	for ((i=0; i<200; i++)); do keyctl unlink %:a$i; done

 (6) Kill off the watch queue by exiting the shell spawned by
     watch_session.

Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: David Howells <dhowells@...hat.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
---

 include/linux/sched/user.h |    3 +++
 kernel/watch_queue.c       |    8 ++++++++
 2 files changed, 11 insertions(+)

diff --git a/include/linux/sched/user.h b/include/linux/sched/user.h
index 917d88edb7b9..a8ec3b6093fc 100644
--- a/include/linux/sched/user.h
+++ b/include/linux/sched/user.h
@@ -36,6 +36,9 @@ struct user_struct {
     defined(CONFIG_NET) || defined(CONFIG_IO_URING)
 	atomic_long_t locked_vm;
 #endif
+#ifdef CONFIG_WATCH_QUEUE
+	atomic_t nr_watches;	/* The number of watches this user currently has */
+#endif
 
 	/* Miscellaneous per-user rate limit */
 	struct ratelimit_state ratelimit;
diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index f74020f6bd9d..0ef8f65bd2d7 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -393,6 +393,7 @@ static void free_watch(struct rcu_head *rcu)
 	struct watch *watch = container_of(rcu, struct watch, rcu);
 
 	put_watch_queue(rcu_access_pointer(watch->queue));
+	atomic_dec(&watch->cred->user->nr_watches);
 	put_cred(watch->cred);
 }
 
@@ -452,6 +453,13 @@ int add_watch_to_object(struct watch *watch, struct watch_list *wlist)
 	watch->cred = get_current_cred();
 	rcu_assign_pointer(watch->watch_list, wlist);
 
+	if (atomic_inc_return(&watch->cred->user->nr_watches) >
+	    task_rlimit(current, RLIMIT_NOFILE)) {
+		atomic_dec(&watch->cred->user->nr_watches);
+		put_cred(watch->cred);
+		return -EAGAIN;
+	}
+
 	spin_lock_bh(&wqueue->lock);
 	kref_get(&wqueue->usage);
 	kref_get(&watch->usage);