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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:6ce47fd961
("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:6ce47fd961
("rtmutex: Warn if trylock is called from hard/softirq context") Signed-off-by: Valentin 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: Andrew Morton <akpm@linux-foundation.org>
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@ -93,13 +93,10 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
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/*
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* Because we write directly to the reserved memory region when loading
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* crash kernels we need a mutex here to prevent multiple crash kernels
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* from attempting to load simultaneously, and to prevent a crash kernel
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* from loading over the top of a in use crash kernel.
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*
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* KISS: always take the mutex.
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* crash kernels we need a serialization here to prevent multiple crash
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* kernels from attempting to load simultaneously.
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*/
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if (!mutex_trylock(&kexec_mutex))
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if (!kexec_trylock())
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return -EBUSY;
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if (flags & KEXEC_ON_CRASH) {
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@ -165,7 +162,7 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
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kimage_free(image);
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out_unlock:
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mutex_unlock(&kexec_mutex);
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kexec_unlock();
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return ret;
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}
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@ -46,7 +46,7 @@
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#include <crypto/hash.h>
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#include "kexec_internal.h"
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DEFINE_MUTEX(kexec_mutex);
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atomic_t __kexec_lock = ATOMIC_INIT(0);
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/* Per cpu memory for storing cpu states in case of system crash. */
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note_buf_t __percpu *crash_notes;
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@ -959,7 +959,7 @@ late_initcall(kexec_core_sysctl_init);
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*/
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void __noclone __crash_kexec(struct pt_regs *regs)
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{
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/* Take the kexec_mutex here to prevent sys_kexec_load
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/* Take the kexec_lock here to prevent sys_kexec_load
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* running on one cpu from replacing the crash kernel
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* we are using after a panic on a different cpu.
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*
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@ -967,7 +967,7 @@ void __noclone __crash_kexec(struct pt_regs *regs)
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* of memory the xchg(&kexec_crash_image) would be
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* sufficient. But since I reuse the memory...
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*/
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if (mutex_trylock(&kexec_mutex)) {
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if (kexec_trylock()) {
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if (kexec_crash_image) {
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struct pt_regs fixed_regs;
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@ -976,7 +976,7 @@ void __noclone __crash_kexec(struct pt_regs *regs)
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machine_crash_shutdown(&fixed_regs);
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machine_kexec(kexec_crash_image);
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}
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mutex_unlock(&kexec_mutex);
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kexec_unlock();
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}
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}
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STACK_FRAME_NON_STANDARD(__crash_kexec);
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@ -1008,13 +1008,13 @@ ssize_t crash_get_memory_size(void)
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{
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ssize_t size = 0;
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if (!mutex_trylock(&kexec_mutex))
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if (!kexec_trylock())
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return -EBUSY;
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if (crashk_res.end != crashk_res.start)
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size = resource_size(&crashk_res);
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mutex_unlock(&kexec_mutex);
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kexec_unlock();
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return size;
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}
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@ -1025,7 +1025,7 @@ int crash_shrink_memory(unsigned long new_size)
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unsigned long old_size;
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struct resource *ram_res;
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if (!mutex_trylock(&kexec_mutex))
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if (!kexec_trylock())
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return -EBUSY;
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if (kexec_crash_image) {
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@ -1064,7 +1064,7 @@ int crash_shrink_memory(unsigned long new_size)
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insert_resource(&iomem_resource, ram_res);
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unlock:
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mutex_unlock(&kexec_mutex);
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kexec_unlock();
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return ret;
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}
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@ -1136,7 +1136,7 @@ int kernel_kexec(void)
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{
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int error = 0;
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if (!mutex_trylock(&kexec_mutex))
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if (!kexec_trylock())
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return -EBUSY;
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if (!kexec_image) {
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error = -EINVAL;
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@ -1212,6 +1212,6 @@ int kernel_kexec(void)
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#endif
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Unlock:
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mutex_unlock(&kexec_mutex);
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kexec_unlock();
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return error;
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}
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@ -339,7 +339,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
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image = NULL;
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if (!mutex_trylock(&kexec_mutex))
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if (!kexec_trylock())
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return -EBUSY;
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dest_image = &kexec_image;
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@ -411,7 +411,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
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if ((flags & KEXEC_FILE_ON_CRASH) && kexec_crash_image)
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arch_kexec_protect_crashkres();
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mutex_unlock(&kexec_mutex);
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kexec_unlock();
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kimage_free(image);
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return ret;
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}
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@ -13,7 +13,20 @@ void kimage_terminate(struct kimage *image);
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int kimage_is_destination_range(struct kimage *image,
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unsigned long start, unsigned long end);
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extern struct mutex kexec_mutex;
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/*
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* Whatever is used to serialize accesses to the kexec_crash_image needs to be
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* NMI safe, as __crash_kexec() can happen during nmi_panic(), so here we use a
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* "simple" atomic variable that is acquired with a cmpxchg().
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*/
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extern atomic_t __kexec_lock;
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static inline bool kexec_trylock(void)
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{
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return atomic_cmpxchg_acquire(&__kexec_lock, 0, 1) == 0;
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}
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static inline void kexec_unlock(void)
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{
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atomic_set_release(&__kexec_lock, 0);
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}
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#ifdef CONFIG_KEXEC_FILE
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#include <linux/purgatory.h>
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