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watchdog: Fix potential dereferencing of null pointer
[ Upstream commit 6f733cb ] A reboot notifier, which stops the WDT by calling the stop hook without any check, would be registered when we set WDOG_STOP_ON_REBOOT flag. Howerer we allow the WDT driver to omit the stop hook since commit "d0684c8a93549" ("watchdog: Make stop function optional") and provide a module parameter for user that controls the WDOG_STOP_ON_REBOOT flag in commit 9232c80 ("watchdog: Add stop_on_reboot parameter to control reboot policy"). Together that commits make user potential to insert a watchdog driver that don't provide a stop hook but with the stop_on_reboot parameter set, then dereferencing of null pointer occurs on system reboot. Check the stop hook before registering the reboot notifier to fix the issue. Fixes: d0684c8 ("watchdog: Make stop function optional") Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com> Reviewed-by: Guenter Roeck <linux@roeck-us.net> Link: https://lore.kernel.org/r/20201109130512.28121-1-wangwensheng4@huawei.com Signed-off-by: Guenter Roeck <linux@roeck-us.net> Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org> Signed-off-by: Sasha Levin <sashal@kernel.org>
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