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    Subject[PATCH 5.8 102/148] watch_queue: Limit the number of watches a user can hold
    Date
    From: David Howells <dhowells@redhat.com>

    [ Upstream commit 29e44f4535faa71a70827af3639b5e6762d8f02a ]

    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>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 917d88edb7b9d..a8ec3b6093fcb 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 f74020f6bd9d5..0ef8f65bd2d71 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);
    --
    2.25.1


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