Re: [PATCH RFC 01/10] mm: add Kernel Electric-Fence infrastructure

[Dmitry Vyukov]

On Thu, Sep 10, 2020 at 5:06 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> > On Mon, Sep 7, 2020 at 3:41 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> > > +config KFENCE_NUM_OBJECTS
> > > +       int "Number of guarded objects available"
> > > +       default 255
> > > +       range 1 65535
> > > +       help
> > > +         The number of guarded objects available. For each KFENCE object, 2
> > > +         pages are required; with one containing the object and two adjacent
> > > +         ones used as guard pages.
> >
> > Hi Marco,
> >
> > Wonder if you tested build/boot with KFENCE_NUM_OBJECTS=65535? Can a
> > compiler create such a large object?
>
> Indeed, I get a "ld: kernel image bigger than KERNEL_IMAGE_SIZE".
> Let's lower it to something more reasonable.
>
> The main reason to have the limit is to constrain random configs and
> avoid the inevitable error reports.
>
> > > +config KFENCE_FAULT_INJECTION
> > > +       int "Fault injection for stress testing"
> > > +       default 0
> > > +       depends on EXPERT
> > > +       help
> > > +         The inverse probability with which to randomly protect KFENCE object
> > > +         pages, resulting in spurious use-after-frees. The main purpose of
> > > +         this option is to stress-test KFENCE with concurrent error reports
> > > +         and allocations/frees. A value of 0 disables fault injection.
> >
> > I would name this differently. "FAULT_INJECTION" is already taken for
> > a different thing, so it's a bit confusing.
> > KFENCE_DEBUG_SOMETHING may be a better name.
> > It would also be good to make it very clear in the short description
> > that this is for testing of KFENCE itself. When I configure syzbot I
> > routinely can't figure out if various DEBUG configs detect user
> > errors, or enable additional unit tests, or something else.
>
> Makes sense, we'll change the name.
>
> > Maybe it should depend on DEBUG_KERNEL as well?
>
> EXPERT selects DEBUG_KERNEL, so depending on DEBUG_KERNEL doesn't make sense.
>
> > > +/*
> > > + * Get the canary byte pattern for @addr. Use a pattern that varies based on the
> > > + * lower 3 bits of the address, to detect memory corruptions with higher
> > > + * probability, where similar constants are used.
> > > + */
> > > +#define KFENCE_CANARY_PATTERN(addr) ((u8)0xaa ^ (u8)((unsigned long)addr & 0x7))
> >
> > (addr) in macro body
>
> Done for v2.
>
> > > +       seq_con_printf(seq,
> > > +                      "kfence-#%zd [0x" PTR_FMT "-0x" PTR_FMT
> >
> > PTR_FMT is only used in this file, should it be declared in report.c?
>
> It's also used by the test.
>
> > Please post example reports somewhere. It's hard to figure out all
> > details of the reporting/formatting.
>
> They can be seen in Documentation added later in the series (also
> viewable here: https://github.com/google/kasan/blob/kfence/Documentation/dev-tools/kfence.rst)


Looking at the first report. I got impression we are trying to skip
__kfence frames, but this includes it:

kfence-#17 [0xffffffffb672f000-0xffffffffb672f01f, size=32,
cache=kmalloc-32] allocated in:
   __kfence_alloc+0x42d/0x4c0
   __kmalloc+0x133/0x200

Is it working as intended?