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

From: David Howells
Date: Fri Jul 24 2020 - 18:17:36 EST

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

(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

Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>

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;
+ atomic_t nr_watches; /* The number of watches this user currently has */

/* 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);

+ atomic_dec(&watch->cred->user->nr_watches);

@@ -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;
+ }