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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, "chengjian (D)" <cj.chengjian@huawei.com>,
        Paul Moore <paul@paul-moore.com>,
        John Johansen <john.johansen@canonical.com>,
        James Morris <james.morris@microsoft.com>,
        Casey Schaufler <casey@schaufler-ca.com>
Subject: [PATCH 5.1 036/122] proc: prevent changes to overridden credentials
Date:   Thu, 23 May 2019 21:05:58 +0200
Message-Id: <20190523181709.625861225@linuxfoundation.org>
In-Reply-To: <20190523181705.091418060@linuxfoundation.org>
References: <20190523181705.091418060@linuxfoundation.org>
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From: Paul Moore <paul@paul-moore.com>

commit 35a196bef449b5824033865b963ed9a43fb8c730 upstream.

Prevent userspace from changing the the /proc/PID/attr values if the
task's credentials are currently overriden.  This not only makes sense
conceptually, it also prevents some really bizarre error cases caused
when trying to commit credentials to a task with overridden
credentials.

Cc: <stable@vger.kernel.org>
Reported-by: "chengjian (D)" <cj.chengjian@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Acked-by: James Morris <james.morris@microsoft.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/proc/base.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2540,6 +2540,11 @@ static ssize_t proc_pid_attr_write(struc
 		rcu_read_unlock();
 		return -EACCES;
 	}
+	/* Prevent changes to overridden credentials. */
+	if (current_cred() != current_real_cred()) {
+		rcu_read_unlock();
+		return -EBUSY;
+	}
 	rcu_read_unlock();
 
 	if (count > PAGE_SIZE)