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Fri, 03 Jul 2026 11:02:45 +0000 From: =?UTF-8?q?Carlos=20L=C3=B3pez?= To: kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org Cc: maz@kernel.org, oupton@kernel.org, joey.gouly@arm.com, seiden@linux.ibm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, =?UTF-8?q?Carlos=20L=C3=B3pez?= Subject: [PATCH v3 1/2] KVM: arm64: vgic: Fix race between LPI release and re-registration Date: Fri, 3 Jul 2026 13:01:29 +0200 Message-ID: <20260703110130.2609872-3-clopez@suse.de> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260703110130.2609872-2-clopez@suse.de> References: <20260703110130.2609872-2-clopez@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Action: no action X-Spamd-Result: default: False [-3.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; MID_CONTAINS_FROM(1.00)[]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; MIME_TRACE(0.00)[0:+]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCPT_COUNT_TWELVE(0.00)[12]; ARC_NA(0.00)[]; FROM_HAS_DN(0.00)[]; RCVD_TLS_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; DNSWL_BLOCKED(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RCVD_VIA_SMTP_AUTH(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; DKIM_TRACE(0.00)[suse.de:+]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email,suse.de:dkim,suse.de:mid,imap1.dmz-prg2.suse.org:helo,imap1.dmz-prg2.suse.org:rdns] X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Queue-Id: C223B74889 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260703_040247_729584_284FB2CD X-CRM114-Status: GOOD ( 18.27 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Fix a potential race between decrementing an LPI's reference count and evicting that structure from the LPI xarray. LPI structures are maintained in the VGIC LPI xarray (dist->lpi_xa). When the reference count of an LPI structure drops to zero, vgic_release_lpi_locked() removes the structure from the xarray and frees it under the xarray lock. However, the release of an LPI can race with a concurrent LPI re-registration with the same INTID via vgic_add_lpi() on another CPU, since the reference count drop and the xarray eviction are not performed in a single atomic step. This can happen e.g. if the guest issues a DISCARD while the LPI is still referenced from a vCPU's active-pending list (ap_list), and the same INTID is re-mapped via MAPTI. Particularly, vgic_release_lpi_locked() is called from two distinct paths: direct release via vgic_put_irq(), and deferred release via vgic_release_deleted_lpis(). During direct release, the issue can result in deleting a newly registered LPI from the xarray: CPU0 (Releasing LPI) CPU1 (Adding new LPI) ==================== ===================== vgic_put_irq() __vgic_put_irq() refcount_dec_and_test() vgic_add_lpi() xa_lock_irqsave() old_irq = xa_load(.., intid) vgic_try_get_irq_ref(old_irq) == false new IRQ inserted --> __xa_store(.., intid, ..) xa_unlock_irqrestore() xa_lock_irqsave(); vgic_release_lpi_locked() __xa_erase(.., irq->intid) <-- BUG: new IRQ is erased kfree_rcu(old_irq) During the deferred release path, the old IRQ can be leaked: CPU0 (Releasing LPI) CPU1 (Adding new LPI) ==================== ===================== vgic_put_irq_norelease() __vgic_put_irq() refcount_dec_and_test() irq->pending_release = true vgic_add_lpi() xa_lock_irqsave() old_irq = xa_load(.., intid) vgic_try_get_irq_ref(oldirq) == false BUG: old IRQ overwritten --> __xa_store(.., intid, ..) xa_unlock_irqrestore() vgic_release_deleted_lpis() xa_lock_irqsave() xa_for_each() { .. } <-- old IRQ with pending_release = true is gone, so it cannot be released To fix the direct release path, move the reference count drop inside the xarray lock, making sure that vgic_add_lpi() never encounters the to-be-released LPI. To fix the deferred release path, since the refcount drop must happen under a raw spinlock, the same solution does not work. Instead, update vgic_add_lpi(), so that if it evicts a non-NULL refcount=0 LPI from the xarray, it takes on the responsibility of releasing it. If this happens, vgic_release_deleted_lpis() will iterate the xarray normally and will simply not find the already released structure. Reported-by: Claude:claude-opus-4-6 Fixes: 3a08a6ca7c37 ("KVM: arm64: vgic-v3: Use bare refcount for VGIC LPIs") Fixes: d54594accf73 ("KVM: arm64: vgic-v3: Erase LPIs from xarray outside of raw spinlocks") Signed-off-by: Carlos López --- arch/arm64/kvm/vgic/vgic-its.c | 10 +++++++++- arch/arm64/kvm/vgic/vgic.c | 10 ++++++---- 2 files changed, 15 insertions(+), 5 deletions(-) diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c index 67d107e9a77d..577286069368 100644 --- a/arch/arm64/kvm/vgic/vgic-its.c +++ b/arch/arm64/kvm/vgic/vgic-its.c @@ -116,7 +116,15 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid, kfree(irq); irq = oldirq; } else { - ret = xa_err(__xa_store(&dist->lpi_xa, intid, irq, 0)); + /* + * The entry is either empty or contains a dead LPI (refcount=0) + * from the deferred release path, pending cleanup by + * vgic_release_deleted_lpis(). Evict and free it if present. + */ + oldirq = __xa_store(&dist->lpi_xa, intid, irq, 0); + ret = xa_err(oldirq); + if (!ret && oldirq) + kfree_rcu(oldirq, rcu); } xa_unlock_irqrestore(&dist->lpi_xa, flags); diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c index 5a4768d8cd4f..cc09e0c45b46 100644 --- a/arch/arm64/kvm/vgic/vgic.c +++ b/arch/arm64/kvm/vgic/vgic.c @@ -167,12 +167,14 @@ void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq) guard(spinlock_irqsave)(&dist->lpi_xa.xa_lock); } - if (!__vgic_put_irq(kvm, irq)) + if (!irq_is_lpi(kvm, irq->intid)) return; - xa_lock_irqsave(&dist->lpi_xa, flags); - vgic_release_lpi_locked(dist, irq); - xa_unlock_irqrestore(&dist->lpi_xa, flags); + if (refcount_dec_and_lock_irqsave(&irq->refcount, + &dist->lpi_xa.xa_lock, &flags)) { + vgic_release_lpi_locked(dist, irq); + xa_unlock_irqrestore(&dist->lpi_xa, flags); + } } static void vgic_release_deleted_lpis(struct kvm *kvm) -- 2.51.0