[PATCH] panic: Ensure preemption is disabled during panic()

[Will Deacon <will@kernel.org>]

Calling 'panic()' on a kernel with CONFIG_PREEMPT=y can leave the
calling CPU in an infinite loop, but with interrupts and preemption
enabled. From this state, userspace can continue to be scheduled,
despite the system being "dead" as far as the kernel is concerned. This
is easily reproducible on arm64 when booting with "nosmp" on the command
line; a couple of shell scripts print out a periodic "Ping" message
whilst another triggers a crash by writing to /proc/sysrq-trigger:

  | sysrq: Trigger a crash
  | Kernel panic - not syncing: sysrq triggered crash
  | CPU: 0 PID: 1 Comm: init Not tainted 5.2.15 #1
  | Hardware name: linux,dummy-virt (DT)
  | Call trace:
  |  dump_backtrace+0x0/0x148
  |  show_stack+0x14/0x20
  |  dump_stack+0xa0/0xc4
  |  panic+0x140/0x32c
  |  sysrq_handle_reboot+0x0/0x20
  |  __handle_sysrq+0x124/0x190
  |  write_sysrq_trigger+0x64/0x88
  |  proc_reg_write+0x60/0xa8
  |  __vfs_write+0x18/0x40
  |  vfs_write+0xa4/0x1b8
  |  ksys_write+0x64/0xf0
  |  __arm64_sys_write+0x14/0x20
  |  el0_svc_common.constprop.0+0xb0/0x168
  |  el0_svc_handler+0x28/0x78
  |  el0_svc+0x8/0xc
  | Kernel Offset: disabled
  | CPU features: 0x0002,24002004
  | Memory Limit: none
  | ---[ end Kernel panic - not syncing: sysrq triggered crash ]---
  |  Ping 2!
  |  Ping 1!
  |  Ping 1!
  |  Ping 2!

The issue can also be triggered on x86 kernels if CONFIG_SMP=n, otherwise
local interrupts are disabled in 'smp_send_stop()'.

Disable preemption in 'panic()' before re-enabling interrupts.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/BX1W47JXPMR8.58IYW53H6M5N@dragonstone
Reported-by: Xogium <contact@xogium.me>
Signed-off-by: Will Deacon <will@kernel.org>
---
 kernel/panic.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/panic.c b/kernel/panic.c
index 47e8ebccc22b..f470a038b05b 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -180,6 +180,7 @@ void panic(const char *fmt, ...)
 	 * after setting panic_cpu) from invoking panic() again.
 	 */
 	local_irq_disable();
+	preempt_disable_notrace();
 
 	/*
 	 * It's possible to come here directly from a panic-assertion and
-- 
2.23.0.444.g18eeb5a265-goog