From: Alexander Potapenko <glider@google.com>
To: Jann Horn <jannh@google.com>
Cc: Marco Elver <elver@google.com>,
Andrew Morton <akpm@linux-foundation.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>,
Catalin Marinas <catalin.marinas@arm.com>,
Christoph Lameter <cl@linux.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
David Rientjes <rientjes@google.com>,
Dmitry Vyukov <dvyukov@google.com>,
Eric Dumazet <edumazet@google.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Hillf Danton <hdanton@sina.com>, Ingo Molnar <mingo@redhat.com>,
Jonathan Cameron <Jonathan.Cameron@huawei.com>,
Jonathan Corbet <corbet@lwn.net>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
joern@purestorage.com, Kees Cook <keescook@chromium.org>,
Mark Rutland <mark.rutland@arm.com>,
Pekka Enberg <penberg@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
SeongJae Park <sjpark@amazon.com>,
Thomas Gleixner <tglx@linutronix.de>,
Vlastimil Babka <vbabka@suse.cz>, Will Deacon <will@kernel.org>,
"the arch/x86 maintainers" <x86@kernel.org>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
kernel list <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 v6 7/9] kfence, Documentation: add KFENCE documentation
Date: Fri, 30 Oct 2020 10:59:48 +0100 [thread overview]
Message-ID: <CAG_fn=Xq+E5s_2rVBm-cM4Bvfyn9Ar9fTHWtxeFFZkcAUBwHiQ@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez2ak7mWSSJJ3Zxd+cK1c5uZVqeF2zZ9HLtmXEoiG5=m-Q@mail.gmail.com>
On Fri, Oct 30, 2020 at 3:50 AM Jann Horn <jannh@google.com> wrote:
>
> On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@google.com> wrote:
> > Add KFENCE documentation in dev-tools/kfence.rst, and add to index.
> [...]
> > +The KFENCE memory pool is of fixed size, and if the pool is exhausted, no
> > +further KFENCE allocations occur. With ``CONFIG_KFENCE_NUM_OBJECTS`` (default
> > +255), the number of available guarded objects can be controlled. Each object
> > +requires 2 pages, one for the object itself and the other one used as a guard
> > +page; object pages are interleaved with guard pages, and every object page is
> > +therefore surrounded by two guard pages.
> > +
> > +The total memory dedicated to the KFENCE memory pool can be computed as::
> > +
> > + ( #objects + 1 ) * 2 * PAGE_SIZE
>
> Plus memory overhead from shattered hugepages. With the default object
> count, on x86, we allocate 2MiB of memory pool, but if we have to
> shatter a 2MiB hugepage for that, we may cause the allocation of one
> extra page table, or 4KiB. Of course that's pretty much negligible.
> But on arm64 it's worse, because there we have to disable hugepages in
> the linear map completely. So on a device with 4GiB memory, we might
> end up with something on the order of 4GiB/2MiB * 0x1000 bytes = 8MiB
> of extra L1 page tables that wouldn't have been needed otherwise -
> significantly more than the default memory pool size.
Note that with CONFIG_RODATA_FULL_DEFAULT_ENABLED (which is on by
default now) these hugepages are already disabled (see patch 3/9)
> If the memory overhead is documented, this detail should probably be
> documented, too.
But, yes, documenting that also makes sense.
> > +Using the default config, and assuming a page size of 4 KiB, results in
> > +dedicating 2 MiB to the KFENCE memory pool.
> [...]
> > +For such errors, the address where the corruption as well as the invalidly
>
> nit: "the address where the corruption occurred" or "the address of
> the corruption"
>
> > +written bytes (offset from the address) are shown; in this representation, '.'
> > +denote untouched bytes. In the example above ``0xac`` is the value written to
> > +the invalid address at offset 0, and the remaining '.' denote that no following
> > +bytes have been touched. Note that, real values are only shown for
> > +``CONFIG_DEBUG_KERNEL=y`` builds; to avoid information disclosure for non-debug
> > +builds, '!' is used instead to denote invalidly written bytes.
> [...]
> > +KFENCE objects each reside on a dedicated page, at either the left or right
> > +page boundaries selected at random. The pages to the left and right of the
> > +object page are "guard pages", whose attributes are changed to a protected
> > +state, and cause page faults on any attempted access. Such page faults are then
> > +intercepted by KFENCE, which handles the fault gracefully by reporting an
> > +out-of-bounds access.
>
> ... and marking the page as accessible so that the faulting code can
> continue (wrongly) executing.
>
>
> [...]
> > +Interface
> > +---------
> > +
> > +The following describes the functions which are used by allocators as well page
>
> nit: "as well as"?
>
>
>
> > +handling code to set up and deal with KFENCE allocations.
--
Alexander Potapenko
Software Engineer
Google Germany GmbH
Erika-Mann-Straße, 33
80636 München
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
next prev parent reply other threads:[~2020-10-30 10:00 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-29 13:16 [PATCH v6 0/9] KFENCE: A low-overhead sampling-based memory safety error detector Marco Elver
2020-10-29 13:16 ` [PATCH v6 1/9] mm: add Kernel Electric-Fence infrastructure Marco Elver
2020-10-30 2:49 ` Jann Horn
2020-10-30 19:16 ` Marco Elver
2020-10-29 13:16 ` [PATCH v6 2/9] x86, kfence: enable KFENCE for x86 Marco Elver
2020-10-30 2:49 ` Jann Horn
2020-10-30 13:00 ` Marco Elver
2020-10-30 15:22 ` Jann Horn
2020-10-29 13:16 ` [PATCH v6 3/9] arm64, kfence: enable KFENCE for ARM64 Marco Elver
2020-10-30 2:49 ` Jann Horn
2020-10-30 16:00 ` Mark Rutland
2020-10-30 15:47 ` Mark Rutland
2020-10-30 15:54 ` Marco Elver
2020-10-29 13:16 ` [PATCH v6 4/9] mm, kfence: insert KFENCE hooks for SLAB Marco Elver
2020-10-30 2:49 ` Jann Horn
2020-10-30 15:41 ` Marco Elver
2020-10-29 13:16 ` [PATCH v6 5/9] mm, kfence: insert KFENCE hooks for SLUB Marco Elver
2020-10-30 2:49 ` Jann Horn
2020-10-29 13:16 ` [PATCH v6 6/9] kfence, kasan: make KFENCE compatible with KASAN Marco Elver
2020-10-30 2:49 ` Jann Horn
2020-10-30 13:46 ` Marco Elver
2020-10-30 15:08 ` Jann Horn
2020-10-30 15:19 ` Marco Elver
2020-10-29 13:16 ` [PATCH v6 7/9] kfence, Documentation: add KFENCE documentation Marco Elver
2020-10-30 2:49 ` Jann Horn
2020-10-30 9:59 ` Alexander Potapenko [this message]
2020-10-29 13:16 ` [PATCH v6 8/9] kfence: add test suite Marco Elver
2020-10-30 2:49 ` Jann Horn
2020-10-30 10:50 ` Marco Elver
2020-10-29 13:16 ` [PATCH v6 9/9] MAINTAINERS: Add entry for KFENCE Marco Elver
2020-10-30 2:50 ` Jann Horn
2020-10-30 2:49 ` [PATCH v6 0/9] KFENCE: A low-overhead sampling-based memory safety error detector Jann Horn
2020-10-30 10:56 ` Marco Elver
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