[GIT PULL] ucount rlimit fixes for v5.17-rc2

["Eric W. Biederman" <ebiederm@xmission.com>]

Linus,

Please pull the ucount-rlimit-fixes-for-v5.17-rc2 branch from the git tree:

  git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git ucount-rlimit-fixes-for-v5.17-rc2
  HEAD: f9d87929d451d3e649699d0f1d74f71f77ad38f5 ucount:  Make get_ucount a safe get_user replacement


From: "Eric W. Biederman" <ebiederm@xmission.com>
Date: Mon, 24 Jan 2022 12:46:50 -0600
Subject: [PATCH] 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 it'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>
---
 kernel/ucount.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 7b32c356ebc5..65b597431c86 100644
--- 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);
 	}
 }
-- 
2.29.2

Eric