Re: [RFC PATCH v2] do_exit(): panic() recursion detected

[Vladimir Kondratiev]

Please ignore version 1 of the patch - it was sent from wrong mail address.

To clarify the reason:

Situation where do_exit() re-entered, discovered by static code analysis.
For safety critical system, it is better to panic() when minimal chance 
of corruption detected. For this reason, we also panic on fatal signal 
delivery - patch for this not submitted yet.

On 12/7/20 2:44 PM, Vladimir Kondratiev wrote:
> Recursive do_exit() is symptom of compromised kernel integrity.
> For safety critical systems, it may be better to
> panic() in this case to minimize risk.
>
> Signed-off-by: Vladimir Kondratiev <vladimir.kondratiev@xxxxxxxxxxxxxxx>
> Change-Id: I42f45900a08c4282c511b05e9e6061360d07db60
> ---
>   Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
>   include/linux/kernel.h                          | 1 +
>   kernel/exit.c                                   | 7 +++++++
>   kernel/sysctl.c                                 | 9 +++++++++
>   4 files changed, 23 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 44fde25bb221..6e12a6804557 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3508,6 +3508,12 @@
>   			bit 4: print ftrace buffer
>   			bit 5: print all printk messages in buffer
>   
> +	panic_on_exit_recursion
> +			panic() when do_exit() recursion detected, rather then
> +			try to stay running whenever possible.
> +			Useful on safety critical systems; re-entry in do_exit
> +			is a symptom of compromised kernel integrity.
> +
>   	panic_on_taint=	Bitmask for conditionally calling panic() in add_taint()
>   			Format: <hex>[,nousertaint]
>   			Hexadecimal bitmask representing the set of TAINT flags
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 2f05e9128201..5afb20534cb2 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -539,6 +539,7 @@ extern int sysctl_panic_on_rcu_stall;
>   extern int sysctl_panic_on_stackoverflow;
>   
>   extern bool crash_kexec_post_notifiers;
> +extern int panic_on_exit_recursion;
>   
>   /*
>    * panic_cpu is used for synchronizing panic() and crash_kexec() execution. It
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 1f236ed375f8..162799a8b539 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -68,6 +68,9 @@
>   #include <asm/unistd.h>
>   #include <asm/mmu_context.h>
>   
> +int panic_on_exit_recursion __read_mostly;
> +core_param(panic_on_exit_recursion, panic_on_exit_recursion, int, 0644);
> +
>   static void __unhash_process(struct task_struct *p, bool group_dead)
>   {
>   	nr_threads--;
> @@ -757,6 +760,10 @@ void __noreturn do_exit(long code)
>   	 */
>   	if (unlikely(tsk->flags & PF_EXITING)) {
>   		pr_alert("Fixing recursive fault but reboot is needed!\n");
> +		if (panic_on_exit_recursion)
> +			panic("Recursive do_exit() detected in %s[%d]\n",
> +			      current->comm, task_pid_nr(current));
> +
>   		futex_exit_recursive(tsk);
>   		set_current_state(TASK_UNINTERRUPTIBLE);
>   		schedule();
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index afad085960b8..bb397fba2c42 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -2600,6 +2600,15 @@ static struct ctl_table kern_table[] = {
>   		.extra2		= &one_thousand,
>   	},
>   #endif
> +	{
> +		.procname	= "panic_on_exit_recursion",
> +		.data		= &panic_on_exit_recursion,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= SYSCTL_ZERO,
> +		.extra2		= SYSCTL_ONE,
> +	},
>   	{
>   		.procname	= "panic_on_warn",
>   		.data		= &panic_on_warn,