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Regression with CET: [PATCH v1] x86/mm: avoid inadvertently degrading a TLB flush to local only
- To: David Vrabel <dvrabel@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Wed, 27 Apr 2022 18:03:42 +0000
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, David Vrabel <dvrabel@xxxxxxxxxxxx>
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- Thread-topic: Regression with CET: [PATCH v1] x86/mm: avoid inadvertently degrading a TLB flush to local only
On 19/04/2022 16:03, David Vrabel wrote:
> From: David Vrabel <dvrabel@xxxxxxxxxxxx>
>
> If the direct map is incorrectly modified with interrupts disabled,
> the required TLB flushes are degraded to flushing the local CPU only.
>
> This could lead to very hard to diagnose problems as different CPUs will
> end up with different views of memory. Although, no such issues have yet
> been identified.
>
> Change the check in the flush_area() macro to look at system_state
> instead. This defers the switch from local to all later in the boot
> (see xen/arch/x86/setup.c:__start_xen()). This is fine because
> additional PCPUs are not brought up until after the system state is
> SYS_STATE_smp_boot.
>
> Signed-off-by: David Vrabel <dvrabel@xxxxxxxxxxxx>
This explodes on CET systems:
(XEN) Assertion 'local_irq_is_enabled()' failed at arch/x86/smp.c:265
(XEN) ----[ Xen-4.17.0-10.24-d x86_64 debug=y Not tainted ]----
(XEN) CPU: 0
(XEN) RIP: e008:[<ffff82d040345300>] flush_area_mask+0x40/0x13e
<snip>
(XEN) Xen call trace:
(XEN) [<ffff82d040345300>] R flush_area_mask+0x40/0x13e
(XEN) [<ffff82d040338a40>] F modify_xen_mappings+0xc5/0x958
(XEN) [<ffff82d0404474f9>] F
arch/x86/alternative.c#_alternative_instructions+0xb7/0xb9
(XEN) [<ffff82d0404476cc>] F alternative_branches+0xf/0x12
(XEN) [<ffff82d04044e37d>] F __start_xen+0x1ef4/0x2776
(XEN) [<ffff82d040203344>] F __high_start+0x94/0xa0
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Assertion 'local_irq_is_enabled()' failed at arch/x86/smp.c:265
(XEN) ****************************************
(XEN)
We really did want a local-only flush here, because we specifically
intended to make self-modifying changes before bringing secondary CPUs up.
~Andrew
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