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([2a01:e0a:f0e:9070:527b:9dff:feef:3874]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49989ae96c1sm25906085e9.12.2026.08.14.05.36.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 14 Aug 2026 05:36:44 -0700 (PDT) Message-ID: <0ce5e496-a067-48fd-80d2-94fe9981d505@redhat.com> Date: Fri, 14 Aug 2026 14:36:42 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/4] KVM: arm64: vgic-its: Don't dereference a NULL collection on ITT save Content-Language: en-US To: Fuad Tabba , Marc Zyngier , Oliver Upton Cc: Joey Gouly , Steffen Eiden , Suzuki K Poulose , Zenghui Yu , Will Deacon , Sascha Bischoff , Sebastian Ene , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20260807104102.2410744-1-fuad.tabba@linux.dev> <20260807104102.2410744-2-fuad.tabba@linux.dev> From: Eric Auger In-Reply-To: <20260807104102.2410744-2-fuad.tabba@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Fuad, On 8/7/26 12:40 PM, Fuad Tabba wrote: > MAPC with V=0 drops ite->collection but leaves the ITE on the device's > ITT list, and vgic_its_save_ite() dereferences it unconditionally. A > guest that issues MAPD, MAPTI and then MAPC(V=0) therefore oopses the > host when the VMM issues KVM_DEV_ARM_ITS_SAVE_TABLES to migrate it. Thank you for fixing this bug! > That sequence is UNPREDICTABLE per the architecture, but KVM already > handles the resulting state in the translate, MOVI and DISCARD paths. Maybe: "its_is_collection_mapped() already checks the collection is non null in translate, movi and discard" > > Save a zeroed entry, which vgic_its_restore_ite() reads back as > invalid. Skipping the ITE instead would leave the ITT slot holding > whatever is in guest memory, and restore rejects an entry naming a > collection the restored collection table does not have. > > Fixes: eff484e0298da ("KVM: arm64: vgic-its: ITT save and restore") > Cc: stable@vger.kernel.org > Signed-off-by: Fuad Tabba > --- > arch/arm64/kvm/vgic/vgic-its.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c > index 36ab3e4929154..ed281fbf008b9 100644 > --- a/arch/arm64/kvm/vgic/vgic-its.c > +++ b/arch/arm64/kvm/vgic/vgic-its.c > @@ -2119,6 +2119,14 @@ static int vgic_its_save_ite(struct vgic_its *its, struct its_device *dev, > u32 next_offset; > u64 val; > > + /* > + * MAPC with V=0 keeps the ITEs mapped but drops their collection, > + * and with it the ICID. Save a zeroed entry, which the restore path > + * reads back as invalid. in https://lore.kernel.org/all/CA+EHjTymdanCKoQO3ATmUZ=7Ph2wv4ES8SfkSsTgmOYAYBWayQ@mail.gmail.com/ you also mentionned this can also happen on clearing GITS_BASER.Valid clear. So maybe don't specifically focus on MAPC in in the comment. > + */ > + if (!ite->collection) > + return vgic_its_write_entry_lock(its, gpa, 0ULL, ite); I would rather squash 4/4 here to emphasize we are unlinking that entry from the scan chain. Besides I also wonder if we should directly remove those entries from the &device->itt_head on MAPC and GITS_BASER.Valid reset. Thanks Eric> next_offset = compute_next_eventid_offset(&dev->itt_head, ite); > val = ((u64)next_offset << KVM_ITS_ITE_NEXT_SHIFT) | > ((u64)ite->irq->intid << KVM_ITS_ITE_PINTID_SHIFT) |