From: David Howells <dhowells@redhat.com>
To: torvalds@linux-foundation.org
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
dhowells@redhat.com, keyrings@vger.kernel.org,
linux-security-module@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: [PATCH] watch_queue: Limit the number of watches a user can hold
Date: Mon, 17 Aug 2020 11:07:28 +0100 [thread overview]
Message-ID: <159765884811.1436382.16387370517871969276.stgit@warthog.procyon.org.uk> (raw)
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@linux-foundation.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.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);
next reply other threads:[~2020-08-17 10:07 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-17 10:07 David Howells [this message]
-- strict thread matches above, loose matches on Subject: below --
2020-07-24 22:17 [PATCH] watch_queue: Limit the number of watches a user can hold David Howells
2020-07-25 3:25 ` Jarkko Sakkinen
2020-07-30 10:46 ` David Howells
2020-07-30 17:19 ` David Howells
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