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

From: Dmitry Vyukov <dvyukov@google.com>
To: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christoph Lameter <cl@linux.com>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Pekka Enberg <penberg@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Andrey Konovalov <andreyknvl@google.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eric Dumazet <edumazet@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Ingo Molnar <mingo@redhat.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Peter Zijlstra <peterz@infradead.org>, Qian Cai <cai@lca.pw>,
	Thomas Gleixner <tglx@linutronix.de>,
	Will Deacon <will@kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH RFC 01/10] mm: add Kernel Electric-Fence infrastructure
Date: Thu, 10 Sep 2020 16:57:54 +0200	[thread overview]
Message-ID: <CACT4Y+aBpeQYOWGrCoaJ=HAa0BsSekyL88kcLBTGwc--C+Ch0w@mail.gmail.com> (raw)
In-Reply-To: <20200907134055.2878499-2-elver@google.com>

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

> +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.