From: Matthew Garrett <mjg59@srcf.ucam.org>
To: Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc: jmorris@namei.org, linux-security-module@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
David Howells <dhowells@redhat.com>,
Kees Cook <keescook@chromium.org>,
x86@kernel.org
Subject: Re: [PATCH V40 12/29] x86: Lock down IO port access when the kernel is locked down
Date: Wed, 5 Jan 2022 10:14:37 +0000 [thread overview]
Message-ID: <20220105101437.GA32516@srcf.ucam.org> (raw)
In-Reply-To: <CAAd53p6VcAo0MVMWerTag42cWFE2ifzdQ=AFmGd9a=2gFjgv5A@mail.gmail.com>
On Wed, Jan 05, 2022 at 06:05:26PM +0800, Kai-Heng Feng wrote:
> On Wed, Jan 5, 2022 at 3:20 PM Matthew Garrett <mjg59@srcf.ucam.org> wrote:
> >
> > On Wed, Jan 05, 2022 at 02:57:57PM +0800, Kai-Heng Feng wrote:
> >
> > > The affected system from the customer has SecureBoot enabled (and
> > > hence lockdown), and the kernel upgrade surprisingly broke ioperm()
> > > usage.
> >
> > Which kernel was being used that was signed but didn't implement
> > lockdown? That sounds, uh, bad.
>
> It was upgraded from older distro release. Older kernels don't have lockdown.
But have a signed bootloader? Which releases?
> > There's two main choices:
> >
> > 1) Disable secure boot on the system in question - if there's a need to
> > run userland that can do arbitrary port IO then secure boot isn't
> > providing any meaningful security benefit in any case.
>
> How so?
> Other security features are still incredible valuable, we don't want
> to toss them out just because someone has to use ioperm().
Because having the ability to do port io allows you to tamper with the
running kernel and disable all the other security boundaries, making
them pointless. Many PCI devices have a port IO side channel into MMIO
BARs for use in early boot, so if an attacker can fill that BAR as they
wish and then modify the BAR to map it into the kernel address space
(and fix up the bridges appropriately), or if the port IO interface can
be used to trigger DMA, the outcomes are pretty bad. The point of
lockdown is to disable every plausible interface for userland (even uid
0) to have access to any interfaces that would let them insert modified
code into ring 0 - port IO is definitely one of those interfaces. An
attacker could just take a kernel that allows ioperm(), add an initramfs
containing their payload, boot, hotpatch the kernel to disable lockdown,
and then kexec into their backdoored payload.
> >
> > 2) Implement a kernel driver that abstracts the hardware access away
> > from userland, and ensures that all the accesses are performed in a safe
> > way.
> >
> > Doing port IO from userland is almost always a terrible idea - it
> > usually involves indexed accesses (you write an address to one port and
> > then write or read data from another), and if two processes are trying
> > to do this simultaneously (either because SMP or because one process
> > gets preempted after writing the address but before accessing the data
> > register), and in that case you can end up with accesses to the wrong
> > register as a result. You really want this sort of thing to be mediated
> > by the kernel, both from a safety perspective and to ensure appropriate
> > synchronisation.
>
> Agree, let me start a discussion with them.
Sounds good.
next prev parent reply other threads:[~2022-01-05 10:14 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-20 0:17 [PATCH V40 00/29] Add kernel lockdown functionality Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 01/29] security: Support early LSMs Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 02/29] security: Add a "locked down" LSM hook Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 03/29] security: Add a static lockdown policy LSM Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 04/29] lockdown: Enforce module signatures if the kernel is locked down Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 05/29] lockdown: Restrict /dev/{mem,kmem,port} when " Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 06/29] kexec_load: Disable at runtime if " Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 07/29] lockdown: Copy secure_boot flag in boot params across kexec reboot Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 08/29] kexec_file: split KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE Matthew Garrett
2019-08-30 14:26 ` Philipp Rudo
2019-08-20 0:17 ` [PATCH V40 09/29] kexec_file: Restrict at runtime if the kernel is locked down Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 10/29] hibernate: Disable when " Matthew Garrett
2019-08-20 21:43 ` Rafael J. Wysocki
2019-08-25 9:51 ` Pavel Machek
2019-08-20 0:17 ` [PATCH V40 11/29] PCI: Lock down BAR access " Matthew Garrett
2019-08-20 19:45 ` Bjorn Helgaas
2019-08-20 21:04 ` Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 12/29] x86: Lock down IO port " Matthew Garrett
2022-01-05 6:25 ` Kai-Heng Feng
2022-01-05 6:48 ` Matthew Garrett
2022-01-05 6:57 ` Kai-Heng Feng
2022-01-05 7:20 ` Matthew Garrett
2022-01-05 10:05 ` Kai-Heng Feng
2022-01-05 10:14 ` Matthew Garrett [this message]
2019-08-20 0:17 ` [PATCH V40 13/29] x86/msr: Restrict MSR " Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 14/29] ACPI: Limit access to custom_method " Matthew Garrett
2019-08-20 22:07 ` Rafael J. Wysocki
2019-08-20 0:17 ` [PATCH V40 15/29] acpi: Ignore acpi_rsdp kernel param when the kernel has been " Matthew Garrett
2019-08-20 22:08 ` Rafael J. Wysocki
2019-08-20 0:17 ` [PATCH V40 16/29] acpi: Disable ACPI table override if the kernel is " Matthew Garrett
2019-08-20 22:08 ` Rafael J. Wysocki
2019-08-20 0:17 ` [PATCH V40 17/29] lockdown: Prohibit PCMCIA CIS storage when " Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 18/29] lockdown: Lock down TIOCSSERIAL Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 19/29] lockdown: Lock down module params that specify hardware parameters (eg. ioport) Matthew Garrett
2019-08-20 16:39 ` Jessica Yu
2019-08-20 0:17 ` [PATCH V40 20/29] x86/mmiotrace: Lock down the testmmiotrace module Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 21/29] lockdown: Lock down /proc/kcore Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 22/29] lockdown: Lock down tracing and perf kprobes when in confidentiality mode Matthew Garrett
2019-08-20 0:17 ` [PATCH V40 23/29] bpf: Restrict bpf when kernel lockdown is " Matthew Garrett
2019-08-20 0:18 ` [PATCH V40 24/29] lockdown: Lock down perf when " Matthew Garrett
2019-08-20 0:18 ` [PATCH V40 25/29] kexec: Allow kexec_file() with appropriate IMA policy when locked down Matthew Garrett
2019-08-20 0:18 ` [PATCH V40 26/29] debugfs: Restrict debugfs when the kernel is " Matthew Garrett
2019-08-20 0:18 ` [PATCH V40 27/29] tracefs: Restrict tracefs " Matthew Garrett
2019-08-20 0:18 ` [PATCH V40 28/29] efi: Restrict efivar_ssdt_load " Matthew Garrett
2019-08-20 0:18 ` [PATCH V40 29/29] lockdown: Print current->comm in restriction messages Matthew Garrett
2019-08-20 6:45 ` [PATCH V40 00/29] Add kernel lockdown functionality James Morris
2019-08-30 16:28 ` [PATCH V40 03/29] security: Add a static lockdown policy LSM David Howells
2019-09-04 16:51 ` Matthew Garrett
2019-09-10 10:06 ` Matthew Garrett
2019-08-30 16:31 ` [PATCH V40 04/29] lockdown: Enforce module signatures if the kernel is locked down David Howells
2019-09-04 16:57 ` Matthew Garrett
2019-08-30 16:32 ` [PATCH V40 23/29] bpf: Restrict bpf when kernel lockdown is in confidentiality mode David Howells
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