linux-kernel - [PATCH 5.15 015/171] ucount: Make get_ucount a safe get

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Date:   Mon, 31 Jan 2022 11:54:40 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Qian Cai <quic_qiancai@...cinc.com>,
        Mathias Krause <minipli@...ecurity.net>,
        Alexey Gladkov <legion@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 5.15 015/171] ucount:  Make get_ucount a safe get_user replacement

From: Eric W. Biederman <ebiederm@...ssion.com>

commit f9d87929d451d3e649699d0f1d74f71f77ad38f5 upstream.

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@...cinc.com>
Reported-by: Mathias Krause <minipli@...ecurity.net>
Tested-by: Mathias Krause <minipli@...ecurity.net>
Reviewed-by: Mathias Krause <minipli@...ecurity.net>
Reviewed-by: Alexey Gladkov <legion@...nel.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@...r.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/ucount.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -184,6 +184,7 @@ struct ucounts *alloc_ucounts(struct use
 			kfree(new);
 		} else {
 			hlist_add_head(&new->node, hashent);
+			get_user_ns(new->ns);
 			spin_unlock_irq(&ucounts_lock);
 			return new;
 		}
@@ -204,6 +205,7 @@ 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);
 	}
 }