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([2406:3003:2007:2131:15e3:dec5:94ba:4da7]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-359f05ee4a7sm50316a91.4.2026.03.09.09.02.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Mar 2026 09:02:10 -0700 (PDT) Message-ID: Date: Tue, 10 Mar 2026 00:02:07 +0800 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] KVM: pfncache: Fix uhva validity check in kvm_gpc_is_valid_len() To: Sean Christopherson Cc: Paolo Bonzini , syzbot+cde12433b6c56f55d9ed@syzkaller.appspotmail.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260309075629.24569-2-phind.uet@gmail.com> Content-Language: en-US From: Phi Nguyen In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 3/9/2026 10:39 PM, Sean Christopherson wrote: > On Mon, Mar 09, 2026, phind.uet@gmail.com wrote: >> From: Nguyen Dinh Phi >> >> In kvm_gpc_is_valid_len(), if the GPA is an error GPA, the function uses >> uhva to calculate the page offset. However, if uhva is invalid, its value >> can still be page-aligned (for example, PAGE_OFFSET) and this function will >> still return true. > > The HVA really shouldn't be invalid in the first place. Ideally, Xen code wouldn't > call kvm_gpc_refresh() on an inactive cache, but I suspect we'd end up with TOCTOU > flaws even if we tried to add checks. > > The next best thing would be to explicitly check if the gpc is active. That should > preserve the WARN if KVM tries to pass in a garbage address to __kvm_gpc_activate(). > > diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c > index 728d2c1b488a..8372d1712471 100644 > --- a/virt/kvm/pfncache.c > +++ b/virt/kvm/pfncache.c > @@ -369,6 +369,9 @@ int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len) > > guard(mutex)(&gpc->refresh_lock); > > + if (!gpc->active) > + return -EINVAL; > + > if (!kvm_gpc_is_valid_len(gpc->gpa, gpc->uhva, len)) > return -EINVAL; In this reproducer, userspace invokes KVM_XEN_HVM_EVTCHN_SEND without first configuring the cache. As a result, kvm_xen_set_evtchn_fast() returns -EWOULDBLOCK when kvm_gpc_check() fails. The -EWOULDBLOCK error then causes kvm_xen_set_evtchn() to fall back to calling kvm_gpc_refresh(). IMO, if the cache is not active, kvm_xen_set_evtchn_fast() should return -EINVAL instead. It may be better to check the active state of the GPC in kvm_xen_set_evtchn_fast() rather than kvm_gpc_refresh()? Br, Phi