ucount: Make get_ucount a safe get_user replacement

When the ucount code was refactored to create get_ucount it was missed
that some of the contexts in which a rlimit is kept elevated can be
the only reference to the user/ucount in the system.

Ordinary ucount references exist in places that also have a reference
to the user namspace, but in POSIX message queues, the SysV shm code,
and the SIGPENDING code there is no independent user namespace
reference.

Inspection of the the user_namespace show no instance of circular
references between struct ucounts and the user_namespace.  So
hold a reference from struct ucount to i's user_namespace to
resolve this problem.

Link: https://lore.kernel.org/lkml/YZV7Z+yXbsx9p3JN@fixkernel.com/
Reported-by: Qian Cai <quic_qiancai@quicinc.com>
Reported-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Reviewed-by: Mathias Krause <minipli@grsecurity.net>
Reviewed-by: Alexey Gladkov <legion@kernel.org>
Fixes: d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of ucounts")
Fixes: 6e52a9f0532f ("Reimplement RLIMIT_MSGQUEUE on top of ucounts")
Fixes: d7c9e99aee48 ("Reimplement RLIMIT_MEMLOCK on top of ucounts")
Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 7b32c356ebc5cacba2e82b8f40cceb59a9161e8c..65b597431c861aa4a949e4914f073118aa7b0567 100644 (file)
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -190,6 +190,7 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
                        kfree(new);
                } else {
                        hlist_add_head(&new->node, hashent);
+                       get_user_ns(new->ns);
                        spin_unlock_irq(&ucounts_lock);
                        return new;
                }
@@ -210,6 +211,7 @@ void put_ucounts(struct ucounts *ucounts)
        if (atomic_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, flags)) {
                hlist_del_init(&ucounts->node);
                spin_unlock_irqrestore(&ucounts_lock, flags);
+               put_user_ns(ucounts->ns);
                kfree(ucounts);
        }
 }