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Date:   Sun, 10 May 2020 10:59:21 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Rafael Aquini <aquini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kexec@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        dyoung@...hat.com, corbet@....net, mcgrof@...nel.org,
        keescook@...omium.org, akpm@...ux-foundation.org, cai@....pw,
        rdunlap@...radead.org, tytso@....edu, bunk@...nel.org,
        torvalds@...ux-foundation.org, gregkh@...uxfoundation.org,
        labbott@...hat.com, jeffm@...e.com, jikos@...nel.org, jeyu@...e.de,
        tiwai@...e.de, AnDavis@...e.com, rpalethorpe@...e.de
Subject: Re: [PATCH v3] kernel: add panic_on_taint

On 05/09/20 at 09:57am, Rafael Aquini wrote:
> Analogously to the introduction of panic_on_warn, this patch
> introduces a kernel option named panic_on_taint in order to
> provide a simple and generic way to stop execution and catch
> a coredump when the kernel gets tainted by any given taint flag.
> 
> This is useful for debugging sessions as it avoids rebuilding
> the kernel to explicitly add calls to panic() or BUG() into
> code sites that introduce the taint flags of interest.
> Another, perhaps less frequent, use for this option would be
> as a mean for assuring a security policy (in paranoid mode)
> case where no single taint is allowed for the running system.
> 
> Suggested-by: Qian Cai <cai@....pw>
> Signed-off-by: Rafael Aquini <aquini@...hat.com>
> ---
> Changelog:
> * v2: get rid of unnecessary/misguided compiler hints		(Luis)
> * v2: enhance documentation text for the new kernel parameter	(Randy)
> * v3: drop sysctl interface, keep it only as a kernel parameter (Luis)
> 
>  Documentation/admin-guide/kdump/kdump.rst     | 10 +++++
>  .../admin-guide/kernel-parameters.txt         | 15 +++++++
>  include/linux/kernel.h                        |  2 +
>  kernel/panic.c                                | 40 +++++++++++++++++++
>  kernel/sysctl.c                               |  9 ++++-
>  5 files changed, 75 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
> index ac7e131d2935..de3cf6d377cc 100644
> --- a/Documentation/admin-guide/kdump/kdump.rst
> +++ b/Documentation/admin-guide/kdump/kdump.rst
> @@ -521,6 +521,16 @@ will cause a kdump to occur at the panic() call.  In cases where a user wants
>  to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
>  to achieve the same behaviour.
>  
> +Trigger Kdump on add_taint()
> +============================
> +
> +The kernel parameter, panic_on_taint, calls panic() from within add_taint(),
> +whenever the value set in this bitmask matches with the bit flag being set
> +by add_taint(). This will cause a kdump to occur at the panic() call.
> +In cases where a user wants to specify this during runtime,
> +/proc/sys/kernel/panic_on_taint can be set to a respective bitmask value
> +to achieve the same behaviour.
> +
>  Contact
>  =======
>  
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 7bc83f3d9bdf..4a69fe49a70d 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3404,6 +3404,21 @@
>  	panic_on_warn	panic() instead of WARN().  Useful to cause kdump
>  			on a WARN().
>  
> +	panic_on_taint=	[KNL] conditionally panic() in add_taint()
> +			Format: <str>
			Changed it as 'Format: <string>' to be
consistent with the existing other options?
> +			Specifies, as a string, the TAINT flag set that will
> +			compose a bitmask for calling panic() when the kernel
> +			gets tainted.
> +			See Documentation/admin-guide/tainted-kernels.rst for
> +			details on the taint flags that users can pick to
> +			compose the bitmask to assign to panic_on_taint.
> +			When the string is prefixed with a '-' the bitmask
> +			set in panic_on_taint will be mutually exclusive
> +			with the sysctl knob kernel.tainted, and any attempt
> +			to write to that sysctl will fail with -EINVAL for
> +			any taint value that masks with the flags set for
> +			this option.
> +
>  	crash_kexec_post_notifiers
>  			Run kdump after running panic-notifiers and dumping
>  			kmsg. This only for the users who doubt kdump always
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 9b7a8d74a9d6..66bc102cb59a 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -528,6 +528,8 @@ extern int panic_on_oops;
>  extern int panic_on_unrecovered_nmi;
>  extern int panic_on_io_nmi;
>  extern int panic_on_warn;
> +extern unsigned long panic_on_taint;
> +extern bool panic_on_taint_exclusive;
>  extern int sysctl_panic_on_rcu_stall;
>  extern int sysctl_panic_on_stackoverflow;
>  
> diff --git a/kernel/panic.c b/kernel/panic.c
> index b69ee9e76cb2..65c62f8a1de8 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -25,6 +25,7 @@
>  #include <linux/kexec.h>
>  #include <linux/sched.h>
>  #include <linux/sysrq.h>
> +#include <linux/ctype.h>
>  #include <linux/init.h>
>  #include <linux/nmi.h>
>  #include <linux/console.h>
> @@ -44,6 +45,8 @@ static int pause_on_oops_flag;
>  static DEFINE_SPINLOCK(pause_on_oops_lock);
>  bool crash_kexec_post_notifiers;
>  int panic_on_warn __read_mostly;
> +unsigned long panic_on_taint;
> +bool panic_on_taint_exclusive = false;
>  
>  int panic_timeout = CONFIG_PANIC_TIMEOUT;
>  EXPORT_SYMBOL_GPL(panic_timeout);
> @@ -434,6 +437,11 @@ void add_taint(unsigned flag, enum lockdep_ok lockdep_ok)
>  		pr_warn("Disabling lock debugging due to kernel taint\n");
>  
>  	set_bit(flag, &tainted_mask);
> +
> +	if (tainted_mask & panic_on_taint) {
> +		panic_on_taint = 0;

This panic_on_taint resetting is redundant? It will trigger crash, do we
need care if it's 0 or not?

> +		panic("panic_on_taint set ...");
> +	}
>  }
>  EXPORT_SYMBOL(add_taint);
>  
> @@ -686,3 +694,35 @@ static int __init oops_setup(char *s)
>  	return 0;
>  }
>  early_param("oops", oops_setup);
> +
> +static int __init panic_on_taint_setup(char *s)
> +{
> +	/* we just ignore panic_on_taint if passed without flags */
> +	if (!s)
> +		goto out;
> +
> +	for (; *s; s++) {
> +		int i;
> +
> +		if (*s == '-') {
> +			panic_on_taint_exclusive = true;
> +			continue;
> +		}
> +
> +		for (i = 0; i < TAINT_FLAGS_COUNT; i++) {
> +			if (toupper(*s) == taint_flags[i].c_true) {
> +				set_bit(i, &panic_on_taint);
> +				break;
> +			}
> +		}

Read admin-guide/tainted-kernels.rst, but still do not get what 'G' means.
If I specify 'panic_on_taint="G"' or 'panic_on_taint="-G"' in cmdline,
what is expected for this customer behaviour?

Except of above minor nitpicks, this patch looks good to me, thanks.

Reviewed-by: Baoquan He <bhe@...hat.com>

Thanks
Baoquan

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