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

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Date:   Thu, 10 Sep 2020 16:57:54 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mark Rutland <mark.rutland@....com>,
        Pekka Enberg <penberg@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>, Qian Cai <cai@....pw>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH RFC 01/10] mm: add Kernel Electric-Fence infrastructure

On Mon, Sep 7, 2020 at 3:41 PM Marco Elver <elver@...gle.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?


> +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.
Maybe it should depend on DEBUG_KERNEL as well?

> +/*
> + * 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

> +       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?

Please post example reports somewhere. It's hard to figure out all
details of the reporting/formatting.