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Subject[PATCH v2] panic, kexec: Make __crash_kexec() NMI safe
Date
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 it and
use an atomic variable to serialize reads vs writes of
kexec_crash_image.

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>
---
v1 -> v2
++++++++

o Changed from Peterson-like synchronization to simpler atomic_cmpxchg
(Petr)
o Slightly reworded changelog
o Added Fixes: tag. Technically should be up to since kexec can happen
in an NMI, but that isn't such a clear target
---
include/linux/kexec.h | 1 +
kernel/kexec.c | 16 ++++++++++++----
kernel/kexec_core.c | 36 +++++++++++++++++++-----------------
kernel/kexec_file.c | 11 +++++++++++
4 files changed, 43 insertions(+), 21 deletions(-)

diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index ce6536f1d269..5849a15ae3dd 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -369,6 +369,7 @@ extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);

extern struct kimage *kexec_image;
extern struct kimage *kexec_crash_image;
+extern atomic_t crash_kexec_lock;
extern int kexec_load_disabled;

#ifndef kexec_flush_icache_page
diff --git a/kernel/kexec.c b/kernel/kexec.c
index b5e40f069768..73e0df2c608f 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -94,14 +94,20 @@ 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.
+ * from attempting to load simultaneously.
*/
if (!mutex_trylock(&kexec_mutex))
return -EBUSY;

+ /*
+ * Prevent loading a new crash kernel while one is in use.
+ * See associated comment in __crash_kexec().
+ */
+ if (atomic_cmpxchg_acquire(&crash_kexec_lock, 0, 1)) {
+ ret = -EBUSY;
+ goto out_unlock_mutex;
+ }
+
if (flags & KEXEC_ON_CRASH) {
dest_image = &kexec_crash_image;
if (kexec_crash_image)
@@ -165,6 +171,8 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,

kimage_free(image);
out_unlock:
+ atomic_set_release(&crash_kexec_lock, 0);
+out_unlock_mutex:
mutex_unlock(&kexec_mutex);
return ret;
}
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 4d34c78334ce..f957109a266c 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -933,6 +933,7 @@ int kimage_load_segment(struct kimage *image,

struct kimage *kexec_image;
struct kimage *kexec_crash_image;
+atomic_t crash_kexec_lock = ATOMIC_INIT(0);
int kexec_load_disabled;
#ifdef CONFIG_SYSCTL
static struct ctl_table kexec_core_sysctls[] = {
@@ -964,25 +965,26 @@ late_initcall(kexec_core_sysctl_init);
*/
void __noclone __crash_kexec(struct pt_regs *regs)
{
- /* Take the kexec_mutex 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.
- *
- * If the crash kernel was not located in a fixed area
- * of memory the xchg(&kexec_crash_image) would be
- * sufficient. But since I reuse the memory...
+ /*
+ * This should be taking kexec_mutex before doing anything with the
+ * kexec_crash_image, but this code can be run in NMI context which
+ * means we can't even trylock. This is circumvented by using an
+ * atomic variable that is *also* used by the codepaths that take
+ * the mutex to modify kexec_crash_image.
*/
- if (mutex_trylock(&kexec_mutex)) {
- if (kexec_crash_image) {
- struct pt_regs fixed_regs;
-
- crash_setup_regs(&fixed_regs, regs);
- crash_save_vmcoreinfo();
- machine_crash_shutdown(&fixed_regs);
- machine_kexec(kexec_crash_image);
- }
- mutex_unlock(&kexec_mutex);
+ if (atomic_cmpxchg_acquire(&crash_kexec_lock, 0, 1))
+ return;
+
+ if (kexec_crash_image) {
+ struct pt_regs fixed_regs;
+
+ crash_setup_regs(&fixed_regs, regs);
+ crash_save_vmcoreinfo();
+ machine_crash_shutdown(&fixed_regs);
+ machine_kexec(kexec_crash_image);
}
+
+ atomic_set_release(&crash_kexec_lock, 0);
}
STACK_FRAME_NON_STANDARD(__crash_kexec);

diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 145321a5e798..3faec031cfc9 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -337,6 +337,15 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
if (!mutex_trylock(&kexec_mutex))
return -EBUSY;

+ /*
+ * Prevent loading a new crash kernel while one is in use.
+ * See associated comment in __crash_kexec().
+ */
+ if (atomic_cmpxchg_acquire(&crash_kexec_lock, 0, 1)) {
+ ret = -EBUSY;
+ goto out_mutex_unlock;
+ }
+
dest_image = &kexec_image;
if (flags & KEXEC_FILE_ON_CRASH) {
dest_image = &kexec_crash_image;
@@ -406,6 +415,8 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
if ((flags & KEXEC_FILE_ON_CRASH) && kexec_crash_image)
arch_kexec_protect_crashkres();

+ atomic_set_release(&crash_kexec_lock, 0);
+out_mutex_unlock:
mutex_unlock(&kexec_mutex);
kimage_free(image);
return ret;
--
2.31.1
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