kernel: rh_features: fix reading empty feature list from /proc

JIRA: https://issues.redhat.com/browse/RHEL-122981

This patch is a backport of the following RHEL-only commit:
commit 08d81cb85b7ed466994e0dc13faec55f62e60e17
Author: Ricardo Robaina <rrobaina@redhat.com>
Date:   Fri May 10 20:31:21 2024 -0300

    kernel: rh_features: fix reading empty feature list from /proc

    JIRA: https://issues.redhat.com/browse/RHEL-32987

    This patch is a backport of the following upstream commit:
    commit ffe4913e104bd29ee17af7e18c79dea05d6fc4df
    Author: Jiri Benc <jbenc@redhat.com>
    Date:   Tue Jun 30 09:50:35 2020 -0400

        [kernel] kernel: rh_features: fix reading empty feature list from /proc

        Message-id: <2554b7608fb3e30847cf1f7c10e565666663036c.1593510618.git.jbenc@redhat.com>
        Patchwork-id: 321700
        Patchwork-instance: patchwork
        O-Subject: [RHEL8.3 net] kernel: rh_features: fix reading empty feature list from /proc
        Bugzilla: 1843064
        CVE: CVE-2020-10774
        RH-Acked-by: Ivan Vecera <ivecera@redhat.com>
        RH-Acked-by: Frantisek Hrbata <fhrbata@redhat.com>
        RH-Acked-by: Hangbin Liu <haliu@redhat.com>

        Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1843064
        CVE: CVE-2020-10774
        Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=29033587
        Upstream status: RHEL only
        Tested: using slub_debug=P

        This fixes a RHEL only feature. The recent move of the rh_features file to
        /proc/sys/kernel introduced a bug: when the feature list is empty,
        uninitialized data is returned on read. Set the initial string len to zero.

        Fixes: b42078ac845a ("[kernel] rh_features: move rh_features entry to sys/kernel")
        Signed-off-by: Jiri Benc <jbenc@redhat.com>
        Signed-off-by: Frantisek Hrbata <fhrbata@redhat.com>

    Signed-off-by: Ricardo Robaina <rrobaina@redhat.com>

Signed-off-by: Ricardo Robaina <rrobaina@redhat.com>

Author: rprobaina

kernel/rh_features.c

@@ -80,6 +80,7 @@ static int rh_features_show(struct ctl_table *ctl, int write,
 	tbl.data = kmalloc(tbl.maxlen, GFP_KERNEL);
 	if (!tbl.data)
 		return -ENOMEM;
+	((char *)tbl.data)[0] = '\0';
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(feat, &rh_feature_list, list) {