drm/nouveau/clk: Fix an incorrect NULL check on list iterator

stable inclusion
from stable-v5.10.121
commit 97a9ec86ccb4e336ecde46db42b59b2ff7e0d719
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5L6CQ

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=97a9ec86ccb4e336ecde46db42b59b2ff7e0d719



--------------------------------

commit 1c3b2a27 upstream.

The bug is here:
	if (nvkm_cstate_valid(clk, cstate, max_volt, clk->temp))
		return cstate;

The list iterator value 'cstate' will *always* be set and non-NULL
by list_for_each_entry_from_reverse(), so it is incorrect to assume
that the iterator value will be unchanged if the list is empty or no
element is found (In fact, it will be a bogus pointer to an invalid
structure object containing the HEAD). Also it missed a NULL check
at callsite and may lead to invalid memory access after that.

To fix this bug, just return 'encoder' when found, otherwise return
NULL. And add the NULL check.

Cc: stable@vger.kernel.org
Fixes: 1f7f3d91 ("drm/nouveau/clk: Respect voltage limits in nvkm_cstate_prog")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220327075824.11806-1-xiam0nd.tong@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>