Messages in this thread Patch in this message | | | Date | Thu, 01 Mar 2012 13:39:55 -0800 | From | Greg KH <> | Subject | [ 33/34] PM / Sleep: Fix freezer failures due to racy usermodehelper_is_disabled() |
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2.6.32-longterm review patch. If anyone has any objections, please let me know.
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From: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
[ Upstream commit b298d289c79211508f11cb50749b0d1d54eb244a ]
Commit a144c6a (PM: Print a warning if firmware is requested when tasks are frozen) introduced usermodehelper_is_disabled() to warn and exit immediately if firmware is requested when usermodehelpers are disabled.
However, it is racy. Consider the following scenario, currently used in drivers/base/firmware_class.c:
... if (usermodehelper_is_disabled()) goto out;
/* Do actual work */ ...
out: return err;
Nothing prevents someone from disabling usermodehelpers just after the check in the 'if' condition, which means that it is quite possible to try doing the "actual work" with usermodehelpers disabled, leading to undesirable consequences.
In particular, this race condition in _request_firmware() causes task freezing failures whenever suspend/hibernation is in progress because, it wrongly waits to get the firmware/microcode image from userspace when actually the usermodehelpers are disabled or userspace has been frozen. Some of the example scenarios that cause freezing failures due to this race are those that depend on userspace via request_firmware(), such as x86 microcode module initialization and microcode image reload.
Previous discussions about this issue can be found at: http://thread.gmane.org/gmane.linux.kernel/1198291/focus=1200591
This patch adds proper synchronization to fix this issue.
It is to be noted that this patchset fixes the freezing failures but doesn't remove the warnings. IOW, it does not attempt to add explicit synchronization to x86 microcode driver to avoid requesting microcode image at inopportune moments. Because, the warnings were introduced to highlight such cases, in the first place. And we need not silence the warnings, since we take care of the *real* problem (freezing failure) and hence, after that, the warnings are pretty harmless anyway.
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> ---
drivers/base/firmware_class.c | 3 +++ include/linux/kmod.h | 4 ++++ kernel/kmod.c | 24 +++++++++++++++++++++++- 3 files changed, 30 insertions(+), 1 deletion(-)
--- a/drivers/base/firmware_class.c +++ b/drivers/base/firmware_class.c @@ -508,6 +508,8 @@ _request_firmware(const struct firmware return 0; } + read_lock_usermodehelper(); + if (WARN_ON(usermodehelper_is_disabled())) { dev_err(device, "firmware: %s will not be loaded\n", name); retval = -EBUSY; @@ -551,6 +553,7 @@ error_kfree_fw: kfree(firmware); *firmware_p = NULL; out: + read_unlock_usermodehelper(); return retval; } --- a/include/linux/kmod.h +++ b/include/linux/kmod.h @@ -108,8 +108,12 @@ extern int call_usermodehelper_pipe(char extern int usermodehelper_disable(void); extern void usermodehelper_enable(void); extern bool usermodehelper_is_disabled(void); +extern void read_lock_usermodehelper(void); +extern void read_unlock_usermodehelper(void); #else static inline bool usermodehelper_is_disabled(void) { return false; } +static inline void read_lock_usermodehelper(void) {} +static inline void read_unlock_usermodehelper(void) {} #endif #endif /* __LINUX_KMOD_H__ */ --- a/kernel/kmod.c +++ b/kernel/kmod.c @@ -35,6 +35,7 @@ #include <linux/resource.h> #include <linux/notifier.h> #include <linux/suspend.h> +#include <linux/rwsem.h> #include <asm/uaccess.h> #include <trace/events/module.h> @@ -43,6 +44,8 @@ extern int max_threads; static struct workqueue_struct *khelper_wq; +static DECLARE_RWSEM(umhelper_sem); + #ifdef CONFIG_MODULES /* @@ -286,6 +289,7 @@ static void __call_usermodehelper(struct * If set, call_usermodehelper_exec() will exit immediately returning -EBUSY * (used for preventing user land processes from being created after the user * land has been frozen during a system-wide hibernation or suspend operation). + * Should always be manipulated under umhelper_sem acquired for write. */ static int usermodehelper_disabled; @@ -304,6 +308,18 @@ static DECLARE_WAIT_QUEUE_HEAD(running_h */ #define RUNNING_HELPERS_TIMEOUT (5 * HZ) +void read_lock_usermodehelper(void) +{ + down_read(&umhelper_sem); +} +EXPORT_SYMBOL_GPL(read_lock_usermodehelper); + +void read_unlock_usermodehelper(void) +{ + up_read(&umhelper_sem); +} +EXPORT_SYMBOL_GPL(read_unlock_usermodehelper); + /** * usermodehelper_disable - prevent new helpers from being started */ @@ -311,8 +327,10 @@ int usermodehelper_disable(void) { long retval; + down_write(&umhelper_sem); usermodehelper_disabled = 1; - smp_mb(); + up_write(&umhelper_sem); + /* * From now on call_usermodehelper_exec() won't start any new * helpers, so it is sufficient if running_helpers turns out to @@ -325,7 +343,9 @@ int usermodehelper_disable(void) if (retval) return 0; + down_write(&umhelper_sem); usermodehelper_disabled = 0; + up_write(&umhelper_sem); return -EAGAIN; } @@ -334,7 +354,9 @@ int usermodehelper_disable(void) */ void usermodehelper_enable(void) { + down_write(&umhelper_sem); usermodehelper_disabled = 0; + up_write(&umhelper_sem); } /**
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