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SubjectRe: [PATCH -tip 18/32] kernel/entry: Add support for core-wide protection of kernel-mode
On Tue, Nov 17, 2020 at 06:19:48PM -0500, Joel Fernandes (Google) wrote:
> Core-scheduling prevents hyperthreads in usermode from attacking each
> other, but it does not do anything about one of the hyperthreads
> entering the kernel for any reason. This leaves the door open for MDS
> and L1TF attacks with concurrent execution sequences between
> hyperthreads.
>
> This patch therefore adds support for protecting all syscall and IRQ
> kernel mode entries. Care is taken to track the outermost usermode exit
> and entry using per-cpu counters. In cases where one of the hyperthreads
> enter the kernel, no additional IPIs are sent. Further, IPIs are avoided
> when not needed - example: idle and non-cookie HTs do not need to be
> forced into kernel mode.
>
> More information about attacks:
> For MDS, it is possible for syscalls, IRQ and softirq handlers to leak
> data to either host or guest attackers. For L1TF, it is possible to leak
> to guest attackers. There is no possible mitigation involving flushing
> of buffers to avoid this since the execution of attacker and victims
> happen concurrently on 2 or more HTs.

> .../admin-guide/kernel-parameters.txt | 11 +
> include/linux/entry-common.h | 12 +-
> include/linux/sched.h | 12 +
> kernel/entry/common.c | 28 +-
> kernel/sched/core.c | 241 ++++++++++++++++++
> kernel/sched/sched.h | 3 +
> 6 files changed, 304 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index bd1a5b87a5e2..b185c6ed4aba 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4678,6 +4678,17 @@
>
> sbni= [NET] Granch SBNI12 leased line adapter
>
> + sched_core_protect_kernel=
> + [SCHED_CORE] Pause SMT siblings of a core running in
> + user mode, if at least one of the siblings of the core
> + is running in kernel mode. This is to guarantee that
> + kernel data is not leaked to tasks which are not trusted
> + by the kernel. A value of 0 disables protection, 1
> + enables protection. The default is 1. Note that protection
> + depends on the arch defining the _TIF_UNSAFE_RET flag.
> + Further, for protecting VMEXIT, arch needs to call
> + KVM entry/exit hooks.
> +
> sched_debug [KNL] Enables verbose scheduler debug messages.
>
> schedstats= [KNL,X86] Enable or disable scheduled statistics.

So I don't like the parameter name, it's too long. Also I don't like it
because its a boolean.

You're adding syscall,irq,kvm under a single knob where they're all due
to different flavours of broken. Different hardware might want/need
different combinations.

Hardware without MDS but with L1TF wouldn't need the syscall hook, but
you're not givng a choice here. And this is generic code, you can't
assume stuff like this.

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