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

[Marco Elver]

On Thu, 10 Sep 2020 at 17:48, Dmitry Vyukov <dvyukov at google.com> wrote:
>
> On Thu, Sep 10, 2020 at 5:06 PM Marco Elver <elver at google.com> wrote:
> > > On Mon, Sep 7, 2020 at 3:41 PM Marco Elver <elver at google.com> 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?

We're not skipping them for the allocation/free stacks. We can skip
the kfence+kmalloc frame as well.