Re: [PATCH v2 0/2] Introduce the pkill_on_warn parameter

[Alexander Popov <alex.popov@linux.com>]

On 28.10.2021 02:32, Alexander Popov wrote:
> Hello! This is the v2 of pkill_on_warn.
> Changes from v1 and tricks for testing are described below.

Hello everyone!
Friendly ping for your feedback.

Thanks.
Alexander

> Rationale
> =========
> 
> 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/2021/02/09/CVE-2021-26708.html
> https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
> https://googleprojectzero.blogspot.com/2018/09/a-cache-invalidation-bug-in-linux.html
> 
> Let's introduce the pkill_on_warn sysctl.
> 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.
> 
> Moreover, bugs usually don't come alone, and a kernel warning may be
> followed by memory corruption or other bad effects. So pkill_on_warn allows
> the kernel to stop the process when the first signs of wrong behavior
> are detected.
> 
> 
> Changes from v1
> ===============
> 
> 1) Introduce do_pkill_on_warn() and call it in all warning handling paths.
> 
> 2) Do refactoring without functional changes in a separate patch.
> 
> 3) Avoid killing init and kthreads.
> 
> 4) Use do_send_sig_info() instead of do_group_exit().
> 
> 5) Introduce sysctl instead of using core_param().
> 
> 
> Tricks for testing
> ==================
> 
> 1) This patch series was tested on x86_64 using CONFIG_LKDTM.
> The kernel kills a process that performs this:
>    echo WARNING > /sys/kernel/debug/provoke-crash/DIRECT
> 
> 2) The warn_slowpath_fmt() path was tested using this trick:
> diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> index 84b87538a15d..3106c203ebb6 100644
> --- a/arch/x86/include/asm/bug.h
> +++ b/arch/x86/include/asm/bug.h
> @@ -73,7 +73,7 @@ do {                                                          \
>    * were to trigger, we'd rather wreck the machine in an attempt to get the
>    * message out than not know about it.
>    */
> -#define __WARN_FLAGS(flags)                                    \
> +#define ___WARN_FLAGS(flags)                                   \
>   do {                                                           \
>          instrumentation_begin();                                \
>          _BUG_FLAGS(ASM_UD2, BUGFLAG_WARNING|(flags));           \
> 
> 3) Testing pkill_on_warn with kthreads was done using this trick:
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index bce848e50512..13c56f472681 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2133,6 +2133,8 @@ static int __noreturn rcu_gp_kthread(void *unused)
>                  WRITE_ONCE(rcu_state.gp_state, RCU_GP_CLEANUP);
>                  rcu_gp_cleanup();
>                  WRITE_ONCE(rcu_state.gp_state, RCU_GP_CLEANED);
> +
> +               WARN_ONCE(1, "hello from kthread\n");
>          }
>   }
> 
> 4) Changing drivers/misc/lkdtm/bugs.c:lkdtm_WARNING() allowed me
> to test all warning flavours:
>   - WARN_ON()
>   - WARN()
>   - WARN_TAINT()
>   - WARN_ON_ONCE()
>   - WARN_ONCE()
>   - WARN_TAINT_ONCE()
> 
> Thanks!
> 
> Alexander Popov (2):
>    bug: do refactoring allowing to add a warning handling action
>    sysctl: introduce kernel.pkill_on_warn
> 
>   Documentation/admin-guide/sysctl/kernel.rst | 14 ++++++++
>   include/asm-generic/bug.h                   | 37 +++++++++++++++------
>   include/linux/panic.h                       |  3 ++
>   kernel/panic.c                              | 22 +++++++++++-
>   kernel/sysctl.c                             |  9 +++++
>   lib/bug.c                                   | 22 ++++++++----
>   6 files changed, 90 insertions(+), 17 deletions(-)
>