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Mon, 02 Nov 2026 01:47:56 -0800 (PST) Date: Mon, 2 Nov 2026 10:47:49 +0100 X-Archived-By: Mime-Version: 1.0 X-Mailer: git-send-email 2.56.0.317.g454f8302e1-goog Message-ID: <20261102094748.2638468-2-ardb+git@google.com> Subject: [PATCH] KVM: arm64: Don't livelock when AT can't resolve a fault IPA From: Ard Biesheuvel To: kvmarm@lists.linux.dev Cc: linux-arm-kernel@lists.infradead.org, Ard Biesheuvel , Marc Zyngier , Oliver Upton , Fuad Tabba , Joey Gouly , Steffen Eiden , Suzuki K Poulose , Zenghui Yu Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261102_014758_778882_7037B378 X-CRM114-Status: GOOD ( 22.39 ) X-Spam-Score: -9.5 (---------) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-26.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIMWL_WL_MED,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT, USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 From: Ard Biesheuvel When a stage-2 permission fault doesn't report its IPA in HPFAR_EL2, __get_fault_info() recovers it with an AT S1E1R/S1E1A on FAR_EL2. If that fails, kvm_hyp_handle_memory_fault() goes straight back to the guest, on the assumption that the replayed access will either succeed or take a fault that the slow path can handle. That assumption doesn't hold when the AT fails because its stage-1 walk takes a stage-2 fault on one of the guest's page-table pages, while the guest still holds a cached translation for the faulting VA. The guest doesn't walk its page tables, so it never takes the S1PTW fault that would let the slow path fix things up. It takes the same permission fault again, the AT fails again, and the vCPU livelocks in the world switch. This was observed on Apple M2, which has no hardware Access flag management at stage 2, and no DIC, so execute permission is granted lazily and the first instruction fetch from a page takes a permission fault. With DAMON_STAT enabled (CONFIG_DAMON_STAT_ENABLED_DEFAULT=y), kdamond keeps sampling guest memory, and damon_pmdp_mkold() ages a whole PMD at a time through mmu_notifier_clear_young(). With THP-backed guest memory, kvm_age_gfn() therefore clears the stage-2 AF of 32MB of guest pages at once, without TLB invalidation. When that covers both freshly loaded code and the page-table page that maps it, the first instruction fetch from that code livelocks, and the vCPU only ever recovers when its thread happens to be moved to another CPU. In all 52 instrumented occurrences, the AT reported a stage-2 Access flag fault on a stage-1 table walk (PAR_EL1.{S,PTW} = {1,1}, FST = Access flag fault, level 3), while an AT S12E1R on the same VA succeeded. With DAMON_STAT disabled, the AT didn't fail once in 210 boots. Nothing in this is specific to nested virtualisation, nor to instruction fetches: a write to a read-only memslot hit it too. A stage-1 fault can lead to the same livelock. A guest that ages its own page tables without invalidating the TLB, as MGLRU does through ptep_test_and_clear_young(), can keep using a cached translation while the descriptor in memory has its Access flag clear. If such an access takes a stage-2 permission fault (lazy execute permission, or dirty logging), the AT reports a stage-1 Access flag fault (PAR_EL1.S = 0), which the guest never takes either, as it doesn't walk its page tables. This hasn't been observed, but it is why the invalidation below is done whenever the AT fails, rather than only when PAR_EL1.PTW reports a stage-2 fault on the walk. A guest that merely races with its own page-table updates isn't harmed by it: at worst, it takes a translation fault that it was allowed to take anyway. Invalidate this VMID's TLB entries on the local CPU before replaying the access, so that the guest has to walk its page tables and takes the fault that the AT ran into: either a stage-2 fault on the page-table page, with a valid HPFAR_EL2, which the slow path resolves, or a stage-1 fault, which the guest resolves itself. In all instrumented cases, the next exit was an S1PTW Access flag fault on the page-table page. This only happens when the AT fails, so the cost is negligible. With this change, an EL2 UEFI guest that livelocked in 27 out of 210 boots didn't hang once in 420 boots. Signed-off-by: Ard Biesheuvel --- Cc: Marc Zyngier Cc: Oliver Upton Cc: Fuad Tabba Cc: Joey Gouly Cc: Steffen Eiden Cc: Suzuki K Poulose Cc: Zenghui Yu arch/arm64/kvm/hyp/include/hyp/switch.h | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h index e0c55644224c..4ba61b71d511 100644 --- a/arch/arm64/kvm/hyp/include/hyp/switch.h +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h @@ -843,8 +843,21 @@ static inline bool kvm_hyp_handle_cp15_32(struct kvm_vcpu *vcpu, u64 *exit_code) static inline bool kvm_hyp_handle_memory_fault(struct kvm_vcpu *vcpu, u64 *exit_code) { - if (!__populate_fault_info(vcpu)) + if (!__populate_fault_info(vcpu)) { + /* + * The AT couldn't resolve the IPA, typically because its + * stage-1 walk took a stage-2 fault, such as an Access flag + * fault on an aged page-table page. If the guest still has a + * cached translation for the VA, it won't take that fault + * itself, and would replay the same permission fault forever. + * Drop this VMID's TLB entries on this CPU, so that the replay + * walks the page tables and faults with a valid HPFAR_EL2. + */ + __tlbi(vmalls12e1); + dsb(nsh); + isb(); return true; + } return false; } -- 2.56.0.317.g454f8302e1-goog