linux-kernel - Re: [tip: x86/urgent] lkdtm: Add a DOUBLE

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Date:   Wed, 27 Nov 2019 19:48:37 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        x86 <x86@...nel.org>
Subject: Re: [tip: x86/urgent] lkdtm: Add a DOUBLE_FAULT crash type on x86


* Kees Cook <keescook@...omium.org> wrote:

> On Wed, Nov 27, 2019 at 08:19:30AM -0000, tip-bot2 for Andy Lutomirski wrote:
> > The following commit has been merged into the x86/urgent branch of tip:
> > 
> > Commit-ID:     b09511c253e5c739a60952b97c071a93e92b2e88
> > Gitweb:        https://git.kernel.org/tip/b09511c253e5c739a60952b97c071a93e92b2e88
> > Author:        Andy Lutomirski <luto@...nel.org>
> > AuthorDate:    Sun, 24 Nov 2019 21:18:04 -08:00
> > Committer:     Ingo Molnar <mingo@...nel.org>
> > CommitterDate: Tue, 26 Nov 2019 21:53:34 +01:00
> > 
> > lkdtm: Add a DOUBLE_FAULT crash type on x86
> > 
> > The DOUBLE_FAULT crash does INT $8, which is a decent approximation
> > of a double fault.  This is useful for testing the double fault
> > handling.  Use it like:
> > 
> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > ---
> >  drivers/misc/lkdtm/bugs.c  | 39 +++++++++++++++++++++++++++++++++++++-
> >  drivers/misc/lkdtm/core.c  |  3 +++-
> >  drivers/misc/lkdtm/lkdtm.h |  3 +++-
> >  3 files changed, 45 insertions(+)
> > 
> > diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
> > index 7284a22..a4fdad0 100644
> > --- a/drivers/misc/lkdtm/bugs.c
> > +++ b/drivers/misc/lkdtm/bugs.c
> > @@ -12,6 +12,10 @@
> >  #include <linux/sched/task_stack.h>
> >  #include <linux/uaccess.h>
> >  
> > +#ifdef CONFIG_X86_32
> > +#include <asm/desc.h>
> > +#endif
> > +
> >  struct lkdtm_list {
> >  	struct list_head node;
> >  };
> > @@ -337,3 +341,38 @@ void lkdtm_UNSET_SMEP(void)
> >  	pr_err("FAIL: this test is x86_64-only\n");
> >  #endif
> >  }
> > +
> > +#ifdef CONFIG_X86_32
> > +void lkdtm_DOUBLE_FAULT(void)
> > +{
> > +	/*
> > +	 * Trigger #DF by setting the stack limit to zero.  This clobbers
> > +	 * a GDT TLS slot, which is okay because the current task will die
> > +	 * anyway due to the double fault.
> > +	 */
> > +	struct desc_struct d = {
> > +		.type = 3,	/* expand-up, writable, accessed data */
> > +		.p = 1,		/* present */
> > +		.d = 1,		/* 32-bit */
> > +		.g = 0,		/* limit in bytes */
> > +		.s = 1,		/* not system */
> > +	};
> > +
> > +	local_irq_disable();
> > +	write_gdt_entry(get_cpu_gdt_rw(smp_processor_id()),
> > +			GDT_ENTRY_TLS_MIN, &d, DESCTYPE_S);
> > +
> > +	/*
> > +	 * Put our zero-limit segment in SS and then trigger a fault.  The
> > +	 * 4-byte access to (%esp) will fault with #SS, and the attempt to
> > +	 * deliver the fault will recursively cause #SS and result in #DF.
> > +	 * This whole process happens while NMIs and MCEs are blocked by the
> > +	 * MOV SS window.  This is nice because an NMI with an invalid SS
> > +	 * would also double-fault, resulting in the NMI or MCE being lost.
> > +	 */
> > +	asm volatile ("movw %0, %%ss; addl $0, (%%esp)" ::
> > +		      "r" ((unsigned short)(GDT_ENTRY_TLS_MIN << 3)));
> > +
> > +	panic("tried to double fault but didn't die\n");
> 
> I'll modify this in some later patches, but I prefer the #ifdef inside
> the function so that all tests are visible on all
> architectures/configurations. And it should not panic on a test failure,
> it should continue (see the others) with something like:
> 
> 	pr_err("FAIL: did not double fault!\n");
> 
> E.g. an external system monitor would see the double-fault and the panic as
> both causing a system reboot, but only the double-fault should do that.
> 
> > +}
> > +#endif
> > diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> > index cbc4c90..ee0d6e7 100644
> > --- a/drivers/misc/lkdtm/core.c
> > +++ b/drivers/misc/lkdtm/core.c
> > @@ -171,6 +171,9 @@ static const struct crashtype crashtypes[] = {
> >  	CRASHTYPE(USERCOPY_KERNEL_DS),
> >  	CRASHTYPE(STACKLEAK_ERASING),
> >  	CRASHTYPE(CFI_FORWARD_PROTO),
> > +#ifdef CONFIG_X86_32
> > +	CRASHTYPE(DOUBLE_FAULT),
> > +#endif
> 
> And then ifdefs aren't needed here either.
> 
> >  };
> >  
> >  
> > diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> > index ab446e0..c56d23e 100644
> > --- a/drivers/misc/lkdtm/lkdtm.h
> > +++ b/drivers/misc/lkdtm/lkdtm.h
> > @@ -28,6 +28,9 @@ void lkdtm_CORRUPT_USER_DS(void);
> >  void lkdtm_STACK_GUARD_PAGE_LEADING(void);
> >  void lkdtm_STACK_GUARD_PAGE_TRAILING(void);
> >  void lkdtm_UNSET_SMEP(void);
> > +#ifdef CONFIG_X86_32
> > +void lkdtm_DOUBLE_FAULT(void);
> > +#endif
> 
> Same.

If you think I can apply your fixes on top of x86/urgent, so that it goes 
upstream in a clean fashion?

I can also rebase and remove it for the time being - your call!

Thanks,

	Ingo