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watch_queue: Limit the number of watches a user can hold

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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: c73be61ced ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
David Howells 2020-08-17 11:07:28 +01:00 committed by Linus Torvalds
parent 9123e3a74e
commit 29e44f4535
2 changed files with 11 additions and 0 deletions
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3
include/linux/sched/user.h
View File

@ -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;

8
kernel/watch_queue.c
View File

@ -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);