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Message-ID: <ac989387-3359-f8da-23f9-f5f6deca4db8@linux.com>
Date: Fri, 12 Nov 2021 21:52:42 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: Jonathan Corbet <corbet@....net>,
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Subject: Re: [PATCH v2 0/2] Introduce the pkill_on_warn parameter
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(-)
>
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