linux-kernel - Re: [PATCH] panic: Ensure preemption is disabled during panic()

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Date:   Wed, 2 Oct 2019 14:45:58 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        contact@...ium.me, Russell King <linux@...linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>, stable@...r.kernel.org
Subject: Re: [PATCH] panic: Ensure preemption is disabled during panic()

On Wed,  2 Oct 2019 13:35:38 +0100 Will Deacon <will@...nel.org> wrote:

> 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.
> 
> ...
>
> --- 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

We still do a lot of stuff (kexec, kgdb, etc) after this
preempt_disable() and I worry that something in there will now trigger
a might_sleep() warning as a result?