[PATCH v4 2/2] panic, kexec: Make __crash_kexec() NMI safe

Valentin Schneider
2022-06-30T22:33:35Z

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:

6ce47fd961fa ("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

Fixes: 6ce47fd961fa ("rtmutex: Warn if trylock is called from hard/softirq context")
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
kernel/kexec.c | 11 ++++-------
kernel/kexec_core.c | 20 ++++++++++----------
kernel/kexec_file.c | 4 ++--
kernel/kexec_internal.h | 15 ++++++++++++++-
4 files changed, 30 insertions(+), 20 deletions(-)

diff --git a/kernel/kexec.c b/kernel/kexec.c
index b5e40f069768..cb8e6e6f983c 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -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;
}

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 16370926b21a..b03859a0fbaa 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -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;
@@ -964,7 +964,7 @@ 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.
*
@@ -972,7 +972,7 @@ 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;

@@ -981,7 +981,7 @@ 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);
@@ -1013,13 +1013,13 @@ 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;
}

@@ -1039,7 +1039,7 @@ 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) {
@@ -1078,7 +1078,7 @@ int crash_shrink_memory(unsigned long new_size)
insert_resource(&iomem_resource, ram_res);

unlock:
- mutex_unlock(&kexec_mutex);
+ kexec_unlock();
return ret;
}

@@ -1150,7 +1150,7 @@ int kernel_kexec(void)
{
int error = 0;

- if (!mutex_trylock(&kexec_mutex))
+ if (!kexec_trylock())
return -EBUSY;
if (!kexec_image) {
error = -EINVAL;
@@ -1226,7 +1226,7 @@ int kernel_kexec(void)
#endif

Unlock:
- mutex_unlock(&kexec_mutex);
+ kexec_unlock();
return error;
}

diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 145321a5e798..42b95bf58daf 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -334,7 +334,7 @@ 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;
@@ -406,7 +406,7 @@ 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;
}
diff --git a/kernel/kexec_internal.h b/kernel/kexec_internal.h
index 48aaf2ac0d0d..74da1409cd14 100644
--- a/kernel/kexec_internal.h
+++ b/kernel/kexec_internal.h
@@ -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>
--
2.31.1