Re: [PATCH v2] KVM: arm64: Trap guest MPAM accesses whenever MPAM is implemented

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Guests can bypass EL2 traps and directly access physical MPAM registers if the host has asymmetric CPUs or boots with `arm64.nompam`.
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commit e23ed2fd702c6c43a8b21feffe356931e0f7eb5a
Author: Fuad Tabba <fuad.tabba@linux.dev>

KVM: arm64: Trap guest MPAM accesses whenever MPAM is implemented

This commit modifies the EL2 MPAM trap activation to gate on the sanitized
ID registers rather than the MPAMEN bit. This ensures that guest MPAM
accesses are trapped whenever MPAM registers are implemented, even if
firmware hasn't enabled MPAM.

> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index 1ce7130e25490..8941335724f6b 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
[ ... ]
> @@ -298,7 +298,7 @@ static inline void  __activate_traps_mpam(struct kvm_vcpu *vcpu)
>  	u64 clr = MPAM2_EL2_EnMPAMSM;
>  	u64 set = MPAM2_EL2_TRAPMPAM0EL1 | MPAM2_EL2_TRAPMPAM1EL1;
>  
> -	if (!system_supports_mpam())
> +	if (!system_supports_mpam_sysregs())

[Severity: High]
This is a pre-existing issue, but does gating this early return on the
sanitized system-wide ID registers allow the guest direct access to
physical MPAM registers in asymmetric or overridden configurations?

During early boot, head.S (in finalise_el2_state) reads the physical
CPU ID registers and clears the EL2 MPAM traps. If the system has
asymmetric MPAM CPUs, or if the kernel is booted with arm64.nompam,
system_supports_mpam_sysregs() evaluates to false.

When that happens, __activate_traps_mpam() takes this early return and
skips setting TRAPMPAM0EL1 and TRAPMPAM1EL1. Since the hardware traps
were already cleared by head.S (or left clear by firmware), the guest's
MPAM register accesses execute natively without trapping to EL2. This
could allow an untrusted guest to manipulate host cache and memory
bandwidth partitioning.

This patch swaps one system-wide capability (ARM64_MPAM) for another
(ARM64_MPAM_SYSREGS), which inherits the exact same bypass. Should KVM
base its trap activation on the physical ID registers to safely isolate
the guest instead?

>  		return;
>  
>  	/* trap guest access to MPAMIDR_EL1 */

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Sashiko AI review · https://sashiko.dev/#/patchset/20260908145651.2828597-1-fuad.tabba@linux.dev?part=1