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From: Mark Rutland <mark.rutland@arm.com>
To: Kees Cook <keescook@chromium.org>
Cc: Kristina Martsenko <kristina.martsenko@arm.com>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Adam Wallis <awallis@codeaurora.org>,
	Amit Kachhap <Amit.Kachhap@arm.com>,
	Andrew Jones <drjones@redhat.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christoffer Dall <christoffer.dall@arm.com>,
	Dave P Martin <Dave.Martin@arm.com>,
	Jacob Bramley <jacob.bramley@arm.com>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
	"Suzuki K . Poulose" <suzuki.poulose@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arch <linux-arch@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 00/17] ARMv8.3 pointer authentication support
Date: Wed, 14 Nov 2018 21:47:02 +0000
Message-ID: <20181114214701.gdnrznakwtm76jlt@blommer> (raw)
In-Reply-To: <CAGXu5jJMfCz4qFY8y1Uf7BdP961ZeBotFVUb5owaksxnz26akg@mail.gmail.com>

On Tue, Nov 13, 2018 at 05:09:00PM -0600, Kees Cook wrote:
> On Tue, Nov 13, 2018 at 10:17 AM, Kristina Martsenko
> <kristina.martsenko@arm.com> wrote:
> > When the PAC authentication fails, it doesn't actually generate an
> > exception, it just flips a bit in the high-order bits of the pointer,
> > making the pointer invalid. Then when the pointer is dereferenced (e.g.
> > as a function return address), it generates the usual type of exception
> > for an invalid address.
> 
> Ah! Okay, thanks. I missed that detail. :)
> 
> What area of memory ends up being addressable with such bit flips?
> (i.e. is the kernel making sure nothing executable ends up there?)
> 
> > So when a function return fails in user mode, the exception is handled
> > in __do_user_fault and a forced SIGSEGV is delivered to the task. When a
> > function return fails in kernel mode, the exception is handled in
> > __do_kernel_fault and the task is killed.
> >
> > This is different from stack protector as we don't panic the kernel, we
> > just kill the task. It would be difficult to panic as we don't have a
> > reliable way of knowing that the exception was caused by a PAC
> > authentication failure (we just have an invalid pointer with a specific
> > bit flipped). We also don't print out any PAC-related warning.
> 
> There are other "guesses" in __do_kernel_fault(), I think? Could a
> "PAC mismatch?" warning be included in the Oops if execution fails in
> the address range that PAC failures would resolve into?

I'd personally prefer that we didn't try to guess if a fault is due to a failed
AUT*, even for logging.

Presently, it's not possible to distinguish between a fault resulting from a
failed AUT* and a fault which happens to have hte same bits/clear, so there are
false positives. The architecture may also change the precise details of the
faulting address, and we'd have false negatives in that case.

Given that, I think suggesting that a fault is due to a failed AUT* is liable
to make things more confusing.

Thanks,
Mark.

  parent reply index

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-05  8:47 Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 01/17] arm64: add pointer authentication register bits Kristina Martsenko
2018-10-11 16:28   ` Will Deacon
2018-10-12  8:53     ` Mark Rutland
2018-10-12  8:56       ` Will Deacon
2018-10-12  9:50         ` Mark Rutland
2018-10-05  8:47 ` [PATCH v5 02/17] arm64/kvm: consistently handle host HCR_EL2 flags Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 03/17] arm64/kvm: hide ptrauth from guests Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 04/17] arm64: Don't trap host pointer auth use to EL2 Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 05/17] arm64/cpufeature: detect pointer authentication Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 06/17] asm-generic: mm_hooks: allow hooks to be overridden individually Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 07/17] arm64: add basic pointer authentication support Kristina Martsenko
2018-10-11 16:00   ` Suzuki K Poulose
2018-10-19 11:15   ` Catalin Marinas
2018-10-19 11:24     ` Will Deacon
2018-10-19 15:36       ` Kees Cook
2018-10-19 15:49         ` Will Deacon
2018-10-19 16:05           ` Kees Cook
2018-10-19 16:16             ` Will Deacon
2018-10-19 15:54         ` Mark Rutland
2018-10-19 16:49       ` Cyrill Gorcunov
2018-11-14 18:11       ` Will Deacon
2018-11-15 10:25         ` Dave Martin
2018-10-23  8:36     ` Ramana Radhakrishnan
2018-10-23 10:20       ` Will Deacon
2018-10-05  8:47 ` [PATCH v5 08/17] arm64: expose user PAC bit positions via ptrace Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 09/17] arm64: perf: strip PAC when unwinding userspace Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 10/17] arm64: enable pointer authentication Kristina Martsenko
2018-10-05  8:47 ` [PATCH v5 11/17] arm64: docs: document " Kristina Martsenko
2018-10-05  9:04   ` Ramana Radhakrishnan
2018-10-16 16:14     ` Kristina Martsenko
2018-10-19 11:35       ` Catalin Marinas
2018-10-19 11:47         ` Marc Zyngier
2018-10-19 12:22         ` Will Deacon
2018-10-19 14:42         ` Kristina Martsenko
2018-10-19 15:10           ` Catalin Marinas
2018-10-19 17:45             ` Will Deacon
2018-11-02  6:02               ` Jon Masters
2018-10-24 10:56         ` Ramana Radhakrishnan
2018-10-15 22:35   ` Kees Cook
2018-11-02  9:46     ` Ramana Radhakrishnan
2018-10-05  8:47 ` [RFC 12/17] arm64: move ptrauth keys to thread_info Kristina Martsenko
2018-10-19 11:38   ` Catalin Marinas
2018-10-05  8:47 ` [RFC 13/17] arm64: install user ptrauth keys at kernel exit time Kristina Martsenko
2018-10-05  8:47 ` [RFC 14/17] arm64: unwind: strip PAC from kernel addresses Kristina Martsenko
2018-10-05  8:47 ` [RFC 15/17] arm64: enable ptrauth earlier Kristina Martsenko
2018-10-06 12:51   ` Amit Kachhap
2018-10-05  8:47 ` [RFC 16/17] arm64: initialize and switch ptrauth kernel keys Kristina Martsenko
2018-10-06 12:56   ` Amit Kachhap
2018-10-05  8:47 ` [RFC 17/17] arm64: compile the kernel with ptrauth -msign-return-address Kristina Martsenko
2018-10-05  9:01   ` Ramana Radhakrishnan
2018-10-11 14:00     ` Kristina Martsenko
2018-10-11 14:23   ` Vladimir Murzin
2018-10-15 22:38     ` Kees Cook
2018-10-15 22:42 ` [PATCH 00/17] ARMv8.3 pointer authentication support Kees Cook
2018-11-13 16:17   ` Kristina Martsenko
2018-11-13 23:09     ` Kees Cook
2018-11-14 15:54       ` Kristina Martsenko
2018-11-14 21:47       ` Mark Rutland [this message]
2018-11-14 22:48         ` Kees Cook
2018-10-19 12:36 ` Will Deacon
2018-10-23  8:39   ` Ramana Radhakrishnan

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