Commit 05c62574 authored by Valentin Schneider's avatar Valentin Schneider Committed by Andrew Morton
Browse files

panic, kexec: make __crash_kexec() NMI safe 

Attempting to get a crash dump out of a debug PREEMPT_RT kernel via an NMI
panic() doesn't work.  The cause of that lies in the PREEMPT_RT definition
of mutex_trylock():

	if (IS_ENABLED(CONFIG_DEBUG_RT_MUTEXES) && WARN_ON_ONCE(!in_task()))
		return 0;

This prevents an nmi_panic() from executing the main body of
__crash_kexec() which does the actual kexec into the kdump kernel.  The
warning and return are explained by:

  6ce47fd9 ("rtmutex: Warn if trylock is called from hard/softirq context")
  [...]
  The reasons for this are:

      1) There is a potential deadlock in the slowpath

      2) Another cpu which blocks on the rtmutex will boost the task
	 which allegedly locked the rtmutex, but that cannot work
	 because the hard/softirq context borrows the task context.

Furthermore, grabbing the lock isn't NMI safe, so do away with kexec_mutex
and replace it with an atomic variable.  This is somewhat overzealous as
*some* callsites could keep using a mutex (e.g.  the sysfs-facing ones
like crash_shrink_memory()), but this has the benefit of involving a
single unified lock and preventing any future NMI-related surprises.

Tested by triggering NMI panics via:

  $ echo 1 > /proc/sys/kernel/panic_on_unrecovered_nmi
  $ echo 1 > /proc/sys/kernel/unknown_nmi_panic
  $ echo 1 > /proc/sys/kernel/panic

  $ ipmitool power diag

Link: https://lkml.kernel.org/r/20220630223258.4144112-3-vschneid@redhat.com


Fixes: 6ce47fd9 ("rtmutex: Warn if trylock is called from hard/softirq context")
Signed-off-by: default avatarValentin Schneider <vschneid@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Baoquan He <bhe@redhat.com>
Cc: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: Juri Lelli <jlelli@redhat.com>
Cc: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 7bb5da0d

kernel/kexec.c

+4 −7
Original line number Diff line number Diff line
@@ -93,13 +93,10 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments, 


	/*

	 * Because we write directly to the reserved memory region when loading

	 * crash kernels we need a mutex here to prevent multiple crash kernels

	 * from attempting to load simultaneously, and to prevent a crash kernel

	 * from loading over the top of a in use crash kernel.

	 *

	 * KISS: always take the mutex.

	 * crash kernels we need a serialization here to prevent multiple crash

	 * kernels from attempting to load simultaneously.

	 */

	if (!mutex_trylock(&kexec_mutex))

	if (!kexec_trylock())

		return -EBUSY;



	if (flags & KEXEC_ON_CRASH) {
@@ -165,7 +162,7 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments, 


	kimage_free(image);

out_unlock:

	mutex_unlock(&kexec_mutex);

	kexec_unlock();

	return ret;

}

kernel/kexec_core.c

+10 −10
Original line number Diff line number Diff line
@@ -46,7 +46,7 @@ 
#include <crypto/hash.h>

#include "kexec_internal.h"



DEFINE_MUTEX(kexec_mutex);

atomic_t __kexec_lock = ATOMIC_INIT(0);



/* Per cpu memory for storing cpu states in case of system crash. */

note_buf_t __percpu *crash_notes;
@@ -959,7 +959,7 @@ late_initcall(kexec_core_sysctl_init); 
 */

void __noclone __crash_kexec(struct pt_regs *regs)

{

	/* Take the kexec_mutex here to prevent sys_kexec_load

	/* Take the kexec_lock here to prevent sys_kexec_load

	 * running on one cpu from replacing the crash kernel

	 * we are using after a panic on a different cpu.

	 *
@@ -967,7 +967,7 @@ void __noclone __crash_kexec(struct pt_regs *regs) 
	 * of memory the xchg(&kexec_crash_image) would be

	 * sufficient.  But since I reuse the memory...

	 */

	if (mutex_trylock(&kexec_mutex)) {

	if (kexec_trylock()) {

		if (kexec_crash_image) {

			struct pt_regs fixed_regs;
@@ -976,7 +976,7 @@ void __noclone __crash_kexec(struct pt_regs *regs) 
			machine_crash_shutdown(&fixed_regs);

			machine_kexec(kexec_crash_image);

		}

		mutex_unlock(&kexec_mutex);

		kexec_unlock();

	}

}

STACK_FRAME_NON_STANDARD(__crash_kexec);
@@ -1008,13 +1008,13 @@ ssize_t crash_get_memory_size(void) 
{

	ssize_t size = 0;



	if (!mutex_trylock(&kexec_mutex))

	if (!kexec_trylock())

		return -EBUSY;



	if (crashk_res.end != crashk_res.start)

		size = resource_size(&crashk_res);



	mutex_unlock(&kexec_mutex);

	kexec_unlock();

	return size;

}
@@ -1025,7 +1025,7 @@ int crash_shrink_memory(unsigned long new_size) 
	unsigned long old_size;

	struct resource *ram_res;



	if (!mutex_trylock(&kexec_mutex))

	if (!kexec_trylock())

		return -EBUSY;



	if (kexec_crash_image) {
@@ -1064,7 +1064,7 @@ int crash_shrink_memory(unsigned long new_size) 
	insert_resource(&iomem_resource, ram_res);



unlock:

	mutex_unlock(&kexec_mutex);

	kexec_unlock();

	return ret;

}
@@ -1136,7 +1136,7 @@ int kernel_kexec(void) 
{

	int error = 0;



	if (!mutex_trylock(&kexec_mutex))

	if (!kexec_trylock())

		return -EBUSY;

	if (!kexec_image) {

		error = -EINVAL;
@@ -1212,6 +1212,6 @@ int kernel_kexec(void) 
#endif



 Unlock:

	mutex_unlock(&kexec_mutex);

	kexec_unlock();

	return error;

}

kernel/kexec_file.c

+2 −2
Original line number Diff line number Diff line
@@ -339,7 +339,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd, 


	image = NULL;



	if (!mutex_trylock(&kexec_mutex))

	if (!kexec_trylock())

		return -EBUSY;



	dest_image = &kexec_image;
@@ -411,7 +411,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd, 
	if ((flags & KEXEC_FILE_ON_CRASH) && kexec_crash_image)

		arch_kexec_protect_crashkres();



	mutex_unlock(&kexec_mutex);

	kexec_unlock();

	kimage_free(image);

	return ret;

}

kernel/kexec_internal.h

+14 −1
Original line number Diff line number Diff line
@@ -13,7 +13,20 @@ void kimage_terminate(struct kimage *image); 
int kimage_is_destination_range(struct kimage *image,

				unsigned long start, unsigned long end);



extern struct mutex kexec_mutex;

/*

 * Whatever is used to serialize accesses to the kexec_crash_image needs to be

 * NMI safe, as __crash_kexec() can happen during nmi_panic(), so here we use a

 * "simple" atomic variable that is acquired with a cmpxchg().

 */

extern atomic_t __kexec_lock;

static inline bool kexec_trylock(void)

{

	return atomic_cmpxchg_acquire(&__kexec_lock, 0, 1) == 0;

}

static inline void kexec_unlock(void)

{

	atomic_set_release(&__kexec_lock, 0);

}



#ifdef CONFIG_KEXEC_FILE

#include <linux/purgatory.h>