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Wed, 26 Nov 2025 02:29:51 -0800 (PST) Received: from [192.168.1.3] ([185.48.77.170]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4790add5b80sm37746505e9.6.2025.11.26.02.29.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Nov 2025 02:29:51 -0800 (PST) Message-ID: Date: Wed, 26 Nov 2025 10:29:50 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 2/9] KVM: arm64: Fix Trace Buffer trap polarity for protected VMs To: Marc Zyngier , Fuad Tabba Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, oliver.upton@linux.dev, will@kernel.org, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, vladimir.murzin@arm.com References: <20251118103807.707500-1-tabba@google.com> <20251118103807.707500-3-tabba@google.com> <868qftqehe.wl-maz@kernel.org> Content-Language: en-US From: James Clark In-Reply-To: <868qftqehe.wl-maz@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251126_022954_152794_AE45ED65 X-CRM114-Status: GOOD ( 17.51 ) 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 On 26/11/2025 10:23 am, Marc Zyngier wrote: > On Tue, 18 Nov 2025 10:37:59 +0000, > Fuad Tabba wrote: >> >> The E2TB bits in MDCR_EL2 control trapping of Trace Buffer system >> register accesses. These accesses are trapped to EL2 when the bits are >> clear. >> >> The trap initialization logic for protected VMs in pvm_init_traps_mdcr() >> had the polarity inverted. When a guest did not support the Trace Buffer >> feature, the code was setting E2TB. This incorrectly disabled the trap, >> potentially allowing a protected guest to access registers for a feature >> it was not given. >> >> Fix this by inverting the operation. >> >> Fixes: f50758260bff ("KVM: arm64: Group setting traps for protected VMs by control register") >> Signed-off-by: Fuad Tabba >> --- >> arch/arm64/kvm/hyp/nvhe/pkvm.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c >> index 8d06a246dfd1..f6f8996c4f97 100644 >> --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c >> +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c >> @@ -118,7 +118,7 @@ static void pvm_init_traps_mdcr(struct kvm_vcpu *vcpu) >> val |= MDCR_EL2_TTRF; >> >> if (!kvm_has_feat(kvm, ID_AA64DFR0_EL1, TraceBuffer, IMP)) >> - val |= MDCR_EL2_E2TB_MASK; >> + val &= ~MDCR_EL2_E2TB_MASK; > > This does not only change the trapping logic (bit 24). It also change > the ownership of the buffer (bit 25). I wonder whether you should do > something for that, maybe by clearing TRBLIMITR_EL1.E, because > otherwise, you keep tracing, but using an EL2 VA. What could possibly > go wrong? > > Overall, I'm very uneasy about TRBE in the context of pKVM. > > Thanks, > > M. > Don't we already clear TRBLIMITR_EL1.E in __trace_needs_drain() for pKVM when switching to the guest? James