[PATCH v6 0/6] panic: fix panic_force_cpu= redirect races and NMI bypass

[PATCH v6 0/6] panic: fix panic_force_cpu= redirect races and NMI bypass

[Bradley Morgan]

The panic_force_cpu= parameter redirects a panic to a specific CPU so
the crash kernel runs there. The redirect code in panic_try_force_cpu()
had two races and an NMI bypass, all found by Sashiko. This series
closes them and kills one more hack that the rework turned up.

Patch 1: fix the redirect CPU race.

The redirect is gated by an atomic cmpxchg on panic_redirect_cpu, so
only one CPU sends the redirect IPI. The cmpxchg loser used to return
false and fall through into vpanic(), where it could win
panic_try_start() and run crash_kexec on the wrong CPU before the
target ever received the IPI. The loser has to stop. It cannot just
return true, though, because panic_try_force_cpu() can be called twice
on the same CPU (a nested NMI during the message formatting, before
the IPI is sent), and a blind stop on that second call would abandon
the panic with no IPI sent. The loser now returns true to stop, unless
it is reentering on the same CPU (old_cpu == this_cpu), in which case
it returns false and falls through.

Patch 1 also fixes the panic_in_progress() guard and moves the
panic_try_force_cpu() doc comment update for the new return values
here, as requested by Petr.

The race, two non target CPUs A and B (target is C):

             cpu A                          cpu B
          ----------                     ----------
    panic_try_force_cpu()                panic_try_force_cpu()
        cmpxchg wins                         cmpxchg fails
        IPI -> C                             return false  <- old BUG
        return true                      panic_try_start() wins
    panic_smp_self_stop()                 __crash_kexec() on B
    (A stops)                             (target C bypassed)

Patch 2: flatten nmi_panic control flow.

A behavior preserving cleanup. panic() is noreturn, so the else after
it is dropped and the body flattened, ready for patch 5 to add the
redirect step without piling more onto the if else chain.

Patch 3: fix va_list reuse in panic_try_force_cpu().

vsnprintf() consumes the caller's va_list. When the redirect fails,
vpanic() reuses it for the panic message, which is undefined behavior.
Fixed with va_copy(). Already reviewed by Petr, unchanged since v5.

Patch 4: restore variable arguments to nmi_panic().

nmi_panic() used to take variable arguments until commit ebc41f20d77f
("panic: change nmi_panic from macro to function") flattened it to a
final message string. Bring them back and format with vpanic()
directly, so the next patch can hand the arguments to both
panic_try_force_cpu() and vpanic() without any pointer passing.
Every existing caller passes a plain string literal, so nothing
changes for them.

Patch 5: allow force_cpu redirect from an NMI.

A panic from an NMI used to bypass the redirect entirely. nmi_panic()
called panic_try_start() first, which claims panic_cpu, so by the time
panic() reached panic_try_force_cpu() the panic_in_progress() check saw
panic_cpu set, returned false, and never sent the redirect IPI. The
crash kernel ran on the CPU that took the NMI instead of the requested
one.

nmi_panic() now calls panic_try_force_cpu() before claiming panic_cpu,
as suggested by Petr. The requested CPU claims panic_cpu itself when
it runs panic(), so panic_cpu does not need to be handed off.

Patch 6: kill the "buffer unavailable" redirect fallback.

The redirect buffer is kmalloc'ed in a late_initcall with no error
check. Until then the redirect hands the target CPU the message
"Redirected panic (buffer unavailable)", so the crash kernel knows
that it panicked but not why. The whole boot is that window. Make it
a static 1KB buffer and kill the initcall.

Bradley Morgan (6):
  panic: fix redirect CPU race in panic_try_force_cpu()
  panic: flatten nmi_panic control flow
  panic: fix va_list reuse in panic_try_force_cpu()
  panic: restore variable arguments to nmi_panic()
  panic: allow force_cpu redirect from an NMI
  panic: kill the "buffer unavailable" redirect fallback

 include/linux/panic.h |  3 +-
 kernel/panic.c        | 79 ++++++++++++++++++++++++-------------------
 2 files changed, 47 insertions(+), 35 deletions(-)

--
2.47.3

---
Changes since v5 (all from Petr's review of 4/4):
 - Dropped the va_list pointer and the NULL means final message
   convention. panic_try_force_cpu() takes a plain va_list again.
 - Dropped the va_copy() in vpanic(). panic_try_force_cpu() already
   copies before formatting, so the caller's arguments are reusable.
 - New patch 4 restores the variable arguments of nmi_panic() that
   ebc41f20d77f removed (solution B), as its own patch.
 - The doc comment change for the new return values moved into
   patch 1.
 - Patch 5 is now just the redirect block and the comment update,
   as sketched in Petr's review.
 - New patch 6 kills the redirect buffer hack that the rework turned
   up.

[PATCH v6 1/6] panic: fix redirect CPU race in panic_try_force_cpu()

[Bradley Morgan]

The cmpxchg() in panic_try_force_cpu() makes sure that only one CPU
tries to redirect panic() to the requested CPU. It is similar to the
cmpxchg() in panic_try_start() which makes sure that only one CPU does
the panic(). In both situations, only the winner of cmpxchg() should
proceed further. Other CPUs should go offline.

There is a bug because the cmpxchg loser returns false and falls through
into vpanic(). Two non-target CPUs A and B panic, the requested CPU is C:

             cpu A                          cpu B
          ----------                     ----------
    panic()                              panic()
    vpanic()                             vpanic()
    panic_try_force_cpu()                panic_try_force_cpu()
        cmpxchg wins                        cmpxchg fails
        redirect = A                        old_cpu = A
        IPI -> C                            return false      <- BUG
        return true                     panic_try_start() wins
    panic_smp_self_stop()                __crash_kexec() on B
    (A stops)                            (target C bypassed)

The loser must stop, not fall through. It cannot just return true,
though. A CPU that already won the redirect cmpxchg can reenter
panic_try_force_cpu() on the same CPU, for example a nested NMI during
the message formatting, before the IPI is sent:

             cpu A (1st)                  cpu A (nested)
          ----------                     ----------
    panic()
    vpanic()
    panic_try_force_cpu()
        cmpxchg wins (redirect = A)
        vsnprintf(msg) ...
            <-- NMI, nested panic -->
                                     panic()
                                     vpanic()
                                     panic_try_force_cpu()
                                         cmpxchg fails
                                         old_cpu == A (this CPU)
                                         return true   <- would halt
                                     panic_smp_self_stop()
                                     (IPI never sent, panic abandoned)

Check old_cpu against this_cpu so a second call from the same CPU
returns false and falls through to panic_try_start() instead.

Also fix the panic_in_progress() check. We must not redirect when
panic_cpu is already assigned. Return true to stop when the panic is on
another CPU, false to proceed when it is this one.

Update the panic_try_force_cpu() doc comment for the new return value
semantics.

Fixes: 2e171ab29f91 ("panic: add panic_force_cpu= parameter to redirect panic to a specific CPU")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260705164123.18746-1-include@grrlz.net
Closes: https://sashiko.dev/#/patchset/20260707172252.4842-1-include@grrlz.net
Cc: stable@vger.kernel.org
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 kernel/panic.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index 213725b612aa..010b331658b6 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -371,8 +371,9 @@ int __weak panic_smp_redirect_cpu(int target_cpu, void *msg)
  * for the crash kernel to function correctly. This function redirects
  * panic handling to the CPU specified via the panic_force_cpu= boot parameter.
  *
- * Returns false if panic should proceed on current CPU.
- * Returns true if panic was redirected.
+ * Returns true when this CPU must stop: the panic was redirected or is
+ * already running on another CPU.
+ * Returns false when panic() should proceed on this CPU.
  */
 __printf(1, 0)
 static bool panic_try_force_cpu(const char *fmt, va_list args)
@@ -396,16 +397,20 @@ static bool panic_try_force_cpu(const char *fmt, va_list args)
 		return false;
 	}
 
-	/* Another panic already in progress */
+	/*
+	 * Don't redirect when a panic is already in progress. Stop this
+	 * CPU when it's another one, proceed when it's this one.
+	 */
 	if (panic_in_progress())
-		return false;
+		return panic_on_other_cpu();
 
 	/*
-	 * Only one CPU can do the redirect. Use atomic cmpxchg to ensure
-	 * we don't race with another CPU also trying to redirect.
+	 * Only one CPU can do the redirection. Others should go offline.
+	 * Continue with panic() when we already tried the redirection
+	 * from this CPU before, for example via nmi_panic().
 	 */
 	if (!atomic_try_cmpxchg(&panic_redirect_cpu, &old_cpu, this_cpu))
-		return false;
+		return old_cpu != this_cpu;
 
 	/*
 	 * Use dynamically allocated buffer if available, otherwise
-- 
2.47.3

[PATCH v6 2/6] panic: flatten nmi_panic control flow

[Bradley Morgan]

panic() is __noreturn, so the else after panic_try_start() is dead.
Drop it so the force_cpu path can be added cleanly on top.

Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 kernel/panic.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index 010b331658b6..e1b443150ba0 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -517,7 +517,8 @@ void nmi_panic(struct pt_regs *regs, const char *msg)
 {
 	if (panic_try_start())
 		panic("%s", msg);
-	else if (panic_on_other_cpu())
+
+	if (panic_on_other_cpu())
 		nmi_panic_self_stop(regs);
 }
 EXPORT_SYMBOL(nmi_panic);
-- 
2.47.3

[PATCH v6 3/6] panic: fix va_list reuse in panic_try_force_cpu()

[Bradley Morgan]

vsnprintf() consumes the caller's va_list. When the redirect fails,
vpanic() reuses it for the panic message, which is undefined
behavior. Use va_copy().

Fixes: 2e171ab29f91 ("panic: add panic_force_cpu= parameter to redirect panic to a specific CPU")
Cc: stable@vger.kernel.org
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 kernel/panic.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index e1b443150ba0..6b5728c3c9ce 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -417,7 +417,12 @@ static bool panic_try_force_cpu(const char *fmt, va_list args)
 	 * fall back to static message for early boot panics or allocation failure.
 	 */
 	if (panic_force_buf) {
-		vsnprintf(panic_force_buf, PANIC_MSG_BUFSZ, fmt, args);
+		va_list ap;
+
+		/* Do not consume args, the caller reuses it if we fail */
+		va_copy(ap, args);
+		vsnprintf(panic_force_buf, PANIC_MSG_BUFSZ, fmt, ap);
+		va_end(ap);
 		msg = panic_force_buf;
 	} else {
 		msg = "Redirected panic (buffer unavailable)";
-- 
2.47.3

[PATCH v6 4/6] panic: restore variable arguments to nmi_panic()

[Bradley Morgan]

nmi_panic() used to accept variable arguments until commit
ebc41f20d77f ("panic: change nmi_panic from macro to function")
flattened it to a final message string. vpanic() did not exist back
then, so the function had to format through panic("%s", msg).

Bring the variable arguments back and format with vpanic() directly.
The next patch makes nmi_panic() try the panic_force_cpu= redirect
before claiming panic_cpu, which needs the arguments twice: once to
format the message for the redirected CPU and once for vpanic() when
no redirect happens. Passing a final string would lose that.

Every existing caller passes a plain string literal with no format
specifiers, so nothing changes for them.

Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 include/linux/panic.h |  3 ++-
 kernel/panic.c        | 11 +++++++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/panic.h b/include/linux/panic.h
index f1dd417e54b2..a9128bf3c168 100644
--- a/include/linux/panic.h
+++ b/include/linux/panic.h
@@ -13,7 +13,8 @@ __printf(1, 2)
 void panic(const char *fmt, ...) __noreturn __cold;
 __printf(1, 0)
 void vpanic(const char *fmt, va_list args) __noreturn __cold;
-void nmi_panic(struct pt_regs *regs, const char *msg);
+__printf(2, 3)
+void nmi_panic(struct pt_regs *regs, const char *fmt, ...);
 void check_panic_on_warn(const char *origin);
 extern void oops_enter(void);
 extern void oops_exit(void);
diff --git a/kernel/panic.c b/kernel/panic.c
index 6b5728c3c9ce..bc142485faa4 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -518,13 +518,20 @@ EXPORT_SYMBOL(panic_on_other_cpu);
  * nmi_panic_self_stop() which can provide architecture dependent code such
  * as saving register state for crash dump.
  */
-void nmi_panic(struct pt_regs *regs, const char *msg)
+__printf(2, 3)
+void nmi_panic(struct pt_regs *regs, const char *fmt, ...)
 {
+	va_list args;
+
+	va_start(args, fmt);
+
 	if (panic_try_start())
-		panic("%s", msg);
+		vpanic(fmt, args);
 
 	if (panic_on_other_cpu())
 		nmi_panic_self_stop(regs);
+
+	va_end(args);
 }
 EXPORT_SYMBOL(nmi_panic);
 
-- 
2.47.3

[PATCH v6 5/6] panic: allow force_cpu redirect from an NMI

[Bradley Morgan]

nmi_panic() claims panic_cpu via panic_try_start() before calling
panic(). When the panic later reaches panic_try_force_cpu(), the
panic_in_progress() check sees panic_cpu set and refuses to redirect.
The crash kernel runs on the CPU that took the NMI instead of the CPU
requested with panic_force_cpu=:

  nmi_panic()
    panic_try_start()              wins, panic_cpu = X
    panic("%s", msg)
      vpanic()
        panic_try_force_cpu()
          panic_in_progress()      true, panic_cpu is X
          return false             redirect bypassed
        panic_try_start()          already won
        __crash_kexec()            on X, not the requested CPU

Try the redirect before claiming panic_cpu instead, as suggested by
Petr Mladek. nmi_panic() now calls panic_try_force_cpu() first and
claims panic_cpu only when no redirect happened. The requested CPU
claims panic_cpu itself when it runs panic(), so panic_cpu does not
need to be handed off. panic_try_force_cpu() copies the arguments
before formatting (patch 3), so nmi_panic() can pass them to vpanic()
again when no redirect happens.

The redirect IPI is sent with smp_call_function_single_async(), which
is not guaranteed to work from NMI context. Treat it as best effort.
It is worth the risk because the redirection is only used when the
crash kernel would not work on the panicking CPU anyway.

Keep returning when the panic is already running on this CPU. A
nested NMI, for example with unknown_nmi_panic while this CPU is
inside panic(), must return and let the interrupted panic() continue
instead of parking the CPU in nmi_panic_self_stop().

Mark the redirecting CPU offline before stopping it, like vpanic()
does, so that panic_other_cpus_shutdown() on the target CPU does not
wait for it.

Fixes: 2e171ab29f91 ("panic: add panic_force_cpu= parameter to redirect panic to a specific CPU")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260708164312.19044-1-include@grrlz.net
Cc: stable@vger.kernel.org
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 kernel/panic.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index bc142485faa4..356c03f2361c 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -513,10 +513,11 @@ bool panic_on_other_cpu(void)
 EXPORT_SYMBOL(panic_on_other_cpu);
 
 /*
- * A variant of panic() called from NMI context. We return if we've already
- * panicked on this CPU. If another CPU already panicked, loop in
- * nmi_panic_self_stop() which can provide architecture dependent code such
- * as saving register state for crash dump.
+ * A variant of panic() called from NMI context. The panic is first
+ * redirected to the CPU requested via panic_force_cpu=, when configured.
+ * We return if we've already panicked on this CPU. If another CPU already
+ * panicked, loop in nmi_panic_self_stop() which can provide architecture
+ * dependent code such as saving register state for crash dump.
  */
 __printf(2, 3)
 void nmi_panic(struct pt_regs *regs, const char *fmt, ...)
@@ -525,6 +526,16 @@ void nmi_panic(struct pt_regs *regs, const char *fmt, ...)
 
 	va_start(args, fmt);
 
+	/* Try to redirect to the requested CPU before claiming panic_cpu. */
+	if (panic_try_force_cpu(fmt, args)) {
+		/*
+		 * Mark ourselves offline so panic_other_cpus_shutdown() won't
+		 * wait for us on architectures that check num_online_cpus().
+		 */
+		set_cpu_online(raw_smp_processor_id(), false);
+		nmi_panic_self_stop(regs);
+	}
+
 	if (panic_try_start())
 		vpanic(fmt, args);
 
-- 
2.47.3

[PATCH v6 6/6] panic: kill the "buffer unavailable" redirect fallback

[Bradley Morgan]

The redirect buffer is a disgusting terrible hack. panic_force_buf is
kmalloc'ed in a late_initcall, the return value is not even checked,
and until then the redirect delivers this as the panic message:

  Redirected panic (buffer unavailable)

The whole point of the redirect is to hand the panic message to the
target CPU, so the crash kernel boots knowing it panicked and not why.
And that window is the entire boot, from the early_param to the
late_initcall, which is exactly when you most want the message. A
failed kmalloc just keeps it broken forever, silently.

Make it a static 1KB buffer and kill the initcall. The cost is 1KB of
.bss in SMP crash dump builds, and it is only ever touched when
panic_force_cpu= is set anyway.

Fixes: 2e171ab29f91 ("panic: add panic_force_cpu= parameter to redirect panic to a specific CPU")
Cc: stable@vger.kernel.org
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 kernel/panic.c | 32 +++++++-------------------------
 1 file changed, 7 insertions(+), 25 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index 356c03f2361c..b41a7efc0931 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -307,7 +307,7 @@ atomic_t panic_cpu = ATOMIC_INIT(PANIC_CPU_INVALID);
 atomic_t panic_redirect_cpu = ATOMIC_INIT(PANIC_CPU_INVALID);
 
 #if defined(CONFIG_SMP) && defined(CONFIG_CRASH_DUMP)
-static char *panic_force_buf;
+static char panic_force_buf[PANIC_MSG_BUFSZ];
 
 static int __init panic_force_cpu_setup(char *str)
 {
@@ -326,17 +326,6 @@ static int __init panic_force_cpu_setup(char *str)
 }
 early_param("panic_force_cpu", panic_force_cpu_setup);
 
-static int __init panic_force_cpu_late_init(void)
-{
-	if (panic_force_cpu < 0)
-		return 0;
-
-	panic_force_buf = kmalloc(PANIC_MSG_BUFSZ, GFP_KERNEL);
-
-	return 0;
-}
-late_initcall(panic_force_cpu_late_init);
-
 static void do_panic_on_target_cpu(void *info)
 {
 	panic("%s", (char *)info);
@@ -381,6 +370,7 @@ static bool panic_try_force_cpu(const char *fmt, va_list args)
 	int this_cpu = raw_smp_processor_id();
 	int old_cpu = PANIC_CPU_INVALID;
 	const char *msg;
+	va_list ap;
 
 	/* Feature not enabled via boot parameter */
 	if (panic_force_cpu < 0)
@@ -413,20 +403,12 @@ static bool panic_try_force_cpu(const char *fmt, va_list args)
 		return old_cpu != this_cpu;
 
 	/*
-	 * Use dynamically allocated buffer if available, otherwise
-	 * fall back to static message for early boot panics or allocation failure.
+	 * Do not consume args, the caller reuses them if we fail.
 	 */
-	if (panic_force_buf) {
-		va_list ap;
-
-		/* Do not consume args, the caller reuses it if we fail */
-		va_copy(ap, args);
-		vsnprintf(panic_force_buf, PANIC_MSG_BUFSZ, fmt, ap);
-		va_end(ap);
-		msg = panic_force_buf;
-	} else {
-		msg = "Redirected panic (buffer unavailable)";
-	}
+	va_copy(ap, args);
+	vsnprintf(panic_force_buf, PANIC_MSG_BUFSZ, fmt, ap);
+	va_end(ap);
+	msg = panic_force_buf;
 
 	console_verbose();
 	bust_spinlocks(1);
-- 
2.47.3

Re: [PATCH v6 0/6] panic: fix panic_force_cpu= redirect races and NMI bypass

[Andrew Morton]

On Tue, 18 Aug 2026 16:38:00 +0000 Bradley Morgan <include@grrlz.net> wrote:

> The panic_force_cpu= parameter redirects a panic to a specific CPU so
> the crash kernel runs there. The redirect code in panic_try_force_cpu()
> had two races and an NMI bypass, all found by Sashiko. This series
> closes them and kills one more hack that the rework turned up.

Thanks.  Sashiko had more to say:
	https://sashiko.dev/#/patchset/20260818163806.17460-1-include@grrlz.net

In the first one it suggests va_copy(), but perhaps this would be
better addressed with va_end/va_start.

Re: [PATCH v6 0/6] panic: fix panic_force_cpu= redirect races and NMI bypass

[Bradley Morgan]

On 18 August 2026 19:41:58 BST, Andrew Morton <akpm@linux-foundation.org>
wrote:
>On Tue, 18 Aug 2026 16:38:00 +0000 Bradley Morgan <include@grrlz.net>
>wrote:
>
>> The panic_force_cpu= parameter redirects a panic to a specific CPU so
>> the crash kernel runs there. The redirect code in panic_try_force_cpu()
>> had two races and an NMI bypass, all found by Sashiko. This series
>> closes them and kills one more hack that the rework turned up.
>
>Thanks.  Sashiko had more to say:
>	https://sashiko.dev/#/patchset/20260818163806.17460-1-include@grrlz.net
>
>In the first one it suggests va_copy(), but perhaps this would be
>better addressed with va_end/va_start.
>

Sashiko said there was ANOTHER bug, I might just screammm, at this point,
some sort of superhero needs to come by and suggest the fix that sashiko
likes :sob:

Thanks!