Re: FAILED: patch "[PATCH] x86/speculation: Avoid force-disabling IBPB based on STIBP" failed to apply to 4.19-stable tree

From: Sasha Levin <sashal@kernel.org>
To: gregkh@linuxfoundation.org
Cc: asteinhauser@google.com, tglx@linutronix.de, stable@vger.kernel.org
Subject: Re: FAILED: patch "[PATCH] x86/speculation: Avoid force-disabling IBPB based on STIBP" failed to apply to 4.19-stable tree
Date: Mon, 15 Jun 2020 14:03:18 -0400	[thread overview]
Message-ID: <20200615180318.GG5492@sasha-vm> (raw)
In-Reply-To: <159222800515682@kroah.com>

On Mon, Jun 15, 2020 at 03:33:25PM +0200, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 4.19-stable tree.
>If someone wants it applied there, or to any other stable or longterm
>tree, then please email the backport, including the original git commit
>id to <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From 21998a351512eba4ed5969006f0c55882d995ada Mon Sep 17 00:00:00 2001
>From: Anthony Steinhauser <asteinhauser@google.com>
>Date: Tue, 19 May 2020 06:40:42 -0700
>Subject: [PATCH] x86/speculation: Avoid force-disabling IBPB based on STIBP
> and enhanced IBRS.
>
>When STIBP is unavailable or enhanced IBRS is available, Linux
>force-disables the IBPB mitigation of Spectre-BTB even when simultaneous
>multithreading is disabled. While attempts to enable IBPB using
>prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, ...) fail with
>EPERM, the seccomp syscall (or its prctl(PR_SET_SECCOMP, ...) equivalent)
>which are used e.g. by Chromium or OpenSSH succeed with no errors but the
>application remains silently vulnerable to cross-process Spectre v2 attacks
>(classical BTB poisoning). At the same time the SYSFS reporting
>(/sys/devices/system/cpu/vulnerabilities/spectre_v2) displays that IBPB is
>conditionally enabled when in fact it is unconditionally disabled.
>
>STIBP is useful only when SMT is enabled. When SMT is disabled and STIBP is
>unavailable, it makes no sense to force-disable also IBPB, because IBPB
>protects against cross-process Spectre-BTB attacks regardless of the SMT
>state. At the same time since missing STIBP was only observed on AMD CPUs,
>AMD does not recommend using STIBP, but recommends using IBPB, so disabling
>IBPB because of missing STIBP goes directly against AMD's advice:
>https://developer.amd.com/wp-content/resources/Architecture_Guidelines_Update_Indirect_Branch_Control.pdf
>
>Similarly, enhanced IBRS is designed to protect cross-core BTB poisoning
>and BTB-poisoning attacks from user space against kernel (and
>BTB-poisoning attacks from guest against hypervisor), it is not designed
>to prevent cross-process (or cross-VM) BTB poisoning between processes (or
>VMs) running on the same core. Therefore, even with enhanced IBRS it is
>necessary to flush the BTB during context-switches, so there is no reason
>to force disable IBPB when enhanced IBRS is available.
>
>Enable the prctl control of IBPB even when STIBP is unavailable or enhanced
>IBRS is available.
>
>Fixes: 7cc765a67d8e ("x86/speculation: Enable prctl mode for spectre_v2_user")
>Signed-off-by: Anthony Steinhauser <asteinhauser@google.com>
>Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>Cc: stable@vger.kernel.org

I took these two additional patches for 4.19-4.4:

aa77bfb354c4 ("x86/speculation: Change misspelled STIPB to STIBP")
20c3a2c33e9f ("x86/speculation: Add support for STIBP always-on preferred mode")

With tiny massaging on 4.9 and 4.4.

-- 
Thanks,
Sasha