[PATCH v2] panic, kexec: Make __crash_kexec() NMI safe

[Valentin Schneider <vschneid@redhat.com>]

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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