* Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxx>
> > AuthorDate: Sun, 24 Nov 2019 21:18:04 -08:00
> > Committer: Ingo Molnar <mingo@xxxxxxxxxx>
> > 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@xxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> > ---
> > 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
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