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Date:   Wed, 6 May 2020 20:12:33 -0400
From:   Rafael Aquini <aquini@...hat.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kexec@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        dyoung@...hat.com, bhe@...hat.com, corbet@....net,
        keescook@...omium.org, akpm@...ux-foundation.org, cai@....pw
Subject: Re: [PATCH] kernel: add panic_on_taint

On Wed, May 06, 2020 at 11:24:48PM +0000, Luis Chamberlain wrote:
> On Wed, May 06, 2020 at 06:28:15PM -0400, 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>
> > ---
> >  Documentation/admin-guide/kdump/kdump.rst     | 10 ++++++
> >  .../admin-guide/kernel-parameters.txt         |  3 ++
> >  Documentation/admin-guide/sysctl/kernel.rst   | 36 +++++++++++++++++++
> >  include/linux/kernel.h                        |  1 +
> >  kernel/panic.c                                |  7 ++++
> >  kernel/sysctl.c                               |  7 ++++
> >  6 files changed, 64 insertions(+)
> > 
> > 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..75c02c1841b2 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -3404,6 +3404,9 @@
> >  	panic_on_warn	panic() instead of WARN().  Useful to cause kdump
> >  			on a WARN().
> >  
> > +	panic_on_taint	panic() when the kernel gets tainted, if the taint
> > +			flag being set matches with the assigned bitmask.
> > +
> >  	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/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> > index 0d427fd10941..5b880102f2e3 100644
> > --- a/Documentation/admin-guide/sysctl/kernel.rst
> > +++ b/Documentation/admin-guide/sysctl/kernel.rst
> > @@ -658,6 +658,42 @@ a kernel rebuild when attempting to kdump at the location of a WARN().
> >  = ================================================
> >  
> >  
> > +panic_on_taint
> > +==============
> > +
> > +Bitmask for calling panic() in the add_taint() path.
> > +This is useful to avoid a kernel rebuild when attempting to
> > +kdump at the insertion of any specific TAINT flags.
> > +When set to 0 (default) add_taint() default behavior is maintained.
> > +
> > +====== ============================
> > +bit  0 TAINT_PROPRIETARY_MODULE
> > +bit  1 TAINT_FORCED_MODULE
> > +bit  2 TAINT_CPU_OUT_OF_SPEC
> > +bit  3 TAINT_FORCED_RMMOD
> > +bit  4 TAINT_MACHINE_CHECK
> > +bit  5 TAINT_BAD_PAGE
> > +bit  6 TAINT_USER
> > +bit  7 TAINT_DIE
> > +bit  8 TAINT_OVERRIDDEN_ACPI_TABLE
> > +bit  9 TAINT_WARN
> > +bit 10 TAINT_CRAP
> > +bit 11 TAINT_FIRMWARE_WORKAROUND
> > +bit 12 TAINT_OOT_MODULE
> > +bit 13 TAINT_UNSIGNED_MODULE
> > +bit 14 TAINT_SOFTLOCKUP
> > +bit 15 TAINT_LIVEPATCH
> > +bit 16 TAINT_AUX
> > +bit 17 TAINT_RANDSTRUCT
> > +bit 18 TAINT_FLAGS_COUNT
> > +====== ============================
> > +
> > +So, for example, to panic if the kernel gets tainted due to
> > +occurrences of bad pages and/or machine check errors, a user can::
> > +
> > +  echo 48 > /proc/sys/kernel/panic_on_taint
> > +
> > +
> >  panic_print
> >  ===========
> >  
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 9b7a8d74a9d6..518b9fd381c2 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -528,6 +528,7 @@ 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 int sysctl_panic_on_rcu_stall;
> >  extern int sysctl_panic_on_stackoverflow;
> >  
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index b69ee9e76cb2..e2d4771ab911 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -44,6 +44,7 @@ static int pause_on_oops_flag;
> >  static DEFINE_SPINLOCK(pause_on_oops_lock);
> >  bool crash_kexec_post_notifiers;
> >  int panic_on_warn __read_mostly;
> > +unsigned long panic_on_taint __read_mostly;
> 
> What justification do we have for using __read_mostly here?
> See patch I just sent out, hope that helps.
>

Given the rationale on the hint usage (from your re-sent patch)
this one should not be hinted. I'll get rid of the hint.

 
> >  int panic_timeout = CONFIG_PANIC_TIMEOUT;
> >  EXPORT_SYMBOL_GPL(panic_timeout);
> > @@ -434,6 +435,11 @@ 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 (unlikely(tainted_mask & panic_on_taint)) {
> 
> unlikely() is telling the merit may not be that strong?
> 
> > +		panic_on_taint = 0;
> > +		panic("panic_on_taint set ...");
> > +	}
> >  }
> >  EXPORT_SYMBOL(add_taint);
> >  
> > @@ -675,6 +681,7 @@ 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(panic_on_taint, panic_on_taint, ulong, 0644);
> >  core_param(crash_kexec_post_notifiers, crash_kexec_post_notifiers, bool, 0644);
> >  
> >  static int __init oops_setup(char *s)
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index 8a176d8727a3..b80ab660d727 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -1217,6 +1217,13 @@ static struct ctl_table kern_table[] = {
> >  		.extra1		= SYSCTL_ZERO,
> >  		.extra2		= SYSCTL_ONE,
> >  	},
> > +	{
> > +		.procname	= "panic_on_taint",
> > +		.data		= &panic_on_taint,
> > +		.maxlen		= sizeof(unsigned long),
> > +		.mode		= 0644,
> > +		.proc_handler	= proc_doulongvec_minmax,
> 
> proc_doulongvec_minmax supports a min and max, do we want to
> set it so that we have a sanity check for values used? To see
> an example, refer to the file-max entry.
>
It didn't seem necessary to declare the range limits here, as
albeit he current set of taint flags would cause tainted_mask
to strecth all the way from 0 (none set) to ULONG_MAX (all set), 
that's its valid range given the usage. 
That's why I didn't declare the extra values for range-checking.
I can do it, though, if you rather have it that way.


 
> That would allow for example to error our if a value was
> tried but it is a taint flag which we don't support on an older
> kernel.
> 
> You know what would be *really* useful as well, is a way to
> cat out our current taint, and perhaps another that spits it
> out in English. This can allow scripts to check that for
> validity, instead of scraping kernel logs.
> 
> For instance, I would love to easily just check if TAIN_WARN
> was hit on some tests I am working on, but I don't want to scrape
> the kernel log for this, as I think this is overkill.
> 
I can definitely take a look into these suggestions for a later 
patch, as I think they're nice but they don't look as a deal-breaker 
for the simple feature being proposed here.

Thanks for your feedback!
-- Rafael

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