Re: [PATCH] Introduce the pkill_on_warn boot parameter

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Sep 29, 2021 at 10:01:33PM +0300, Alexander Popov wrote:
> On 29.09.2021 21:58, Alexander Popov wrote:
> > Currently, the Linux kernel provides two types of reaction to kernel
> > warnings:
> >  1. Do nothing (by default),
> >  2. Call panic() if panic_on_warn is set. That's a very strong reaction,
> >     so panic_on_warn is usually disabled on production systems.
> > 
> > From a safety point of view, the Linux kernel misses a middle way of
> > handling kernel warnings:
> >  - The kernel should stop the activity that provokes a warning,
> >  - But the kernel should avoid complete denial of service.
> > 
> > From a security point of view, kernel warning messages provide a lot of
> > useful information for attackers. Many GNU/Linux distributions allow
> > unprivileged users to read the kernel log, so attackers use kernel
> > warning infoleak in vulnerability exploits. See the examples:
> >   https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
> >   https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html
> > 
> > Let's introduce the pkill_on_warn boot parameter.
> > If this parameter is set, the kernel kills all threads in a process
> > that provoked a kernel warning. This behavior is reasonable from a safety
> > point of view described above. It is also useful for kernel security
> > hardening because the system kills an exploit process that hits a
> > kernel warning.
> > 
> > Signed-off-by: Alexander Popov <alex.popov@linux.com>
> 
> This patch was tested using CONFIG_LKDTM.
> The kernel kills a process that performs this:
>   echo WARNING > /sys/kernel/debug/provoke-crash/DIRECT
> 
> If you are fine with this approach, I will prepare a patch adding the
> pkill_on_warn sysctl.

I suspect that you need a list of kthreads for which you are better
off just invoking panic().  RCU's various kthreads, for but one set
of examples.

							Thanx, Paul

> Best regards,
> Alexander
> 
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt | 4 ++++
> >  kernel/panic.c                                  | 5 +++++
> >  2 files changed, 9 insertions(+)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 91ba391f9b32..86c748907666 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4112,6 +4112,10 @@
> >  	pirq=		[SMP,APIC] Manual mp-table setup
> >  			See Documentation/x86/i386/IO-APIC.rst.
> >  
> > +	pkill_on_warn=	Kill all threads in a process that provoked a
> > +			kernel warning.
> > +			Format: { "0" | "1" }
> > +
> >  	plip=		[PPT,NET] Parallel port network link
> >  			Format: { parport<nr> | timid | 0 }
> >  			See also Documentation/admin-guide/parport.rst.
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index cefd7d82366f..47b728bfb1d3 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -53,6 +53,7 @@ static int pause_on_oops_flag;
> >  static DEFINE_SPINLOCK(pause_on_oops_lock);
> >  bool crash_kexec_post_notifiers;
> >  int panic_on_warn __read_mostly;
> > +int pkill_on_warn __read_mostly;
> >  unsigned long panic_on_taint;
> >  bool panic_on_taint_nousertaint = false;
> >  
> > @@ -610,6 +611,9 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
> >  
> >  	print_oops_end_marker();
> >  
> > +	if (pkill_on_warn && system_state >= SYSTEM_RUNNING)
> > +		do_group_exit(SIGKILL);
> > +
> >  	/* Just a warning, don't kill lockdep. */
> >  	add_taint(taint, LOCKDEP_STILL_OK);
> >  }
> > @@ -694,6 +698,7 @@ core_param(panic, panic_timeout, int, 0644);
> >  core_param(panic_print, panic_print, ulong, 0644);
> >  core_param(pause_on_oops, pause_on_oops, int, 0644);
> >  core_param(panic_on_warn, panic_on_warn, int, 0644);
> > +core_param(pkill_on_warn, pkill_on_warn, int, 0644);
> >  core_param(crash_kexec_post_notifiers, crash_kexec_post_notifiers, bool, 0644);
> >  
> >  static int __init oops_setup(char *s)
> > 
>