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Thu, 27 Aug 2026 00:12:30 -0700 (PDT) From: Vitaly Kuznetsov To: Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Stefan Teodorescu Subject: Re: [PATCH] KVM: x86: Zero reserved synic flags when delivering message to avoid stack leak In-Reply-To: <20260826184449.814130-1-seanjc@google.com> References: <20260826184449.814130-1-seanjc@google.com> Date: Thu, 27 Aug 2026 09:12:29 +0200 Message-ID: <874iggnijm.fsf@172.16.32.78> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Sean Christopherson writes: > Ensure all reserved flags are zeroed when setting the msg_pending flag as > part of SynIC message delivery, as setting only msg_pending can leak seven > bits of kernel stack data to the guest. E.g. gcc typically uses a > bitwise-OR to set the msg_pending flag, without initializing the on-stack > variable. > > Precisely zero the flags, e.g. as opposed to zeroing the entire structure, > to make it somewhat more obvious that the flags *need* to be zeroed. E.g. > it would be quite easy to misread the initial read of guest memory and > think that zeroing the entire structure is completely superfluous. > > Fixes: 3a0e7731724f ("x86: kvm: hyperv: simplify SynIC message delivery") > Cc: stable@vger.kernel.org > Reported-by: Stefan Teodorescu > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/hyperv.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c > index 604651cb2739..8d2669d8ef34 100644 > --- a/arch/x86/kvm/hyperv.c > +++ b/arch/x86/kvm/hyperv.c > @@ -802,7 +802,9 @@ static int synic_deliver_msg(struct kvm_vcpu_hv_synic *synic, u32 sint, > if (no_retry) > return 0; > > + hv_hdr.message_flags.asu8 = 0; > hv_hdr.message_flags.msg_pending = 1; > + > r = kvm_vcpu_write_guest_page(vcpu, msg_page_gfn, > &hv_hdr.message_flags, > msg_off + > > base-commit: 76671054f9a1ff6abb976583cd8da37650acdc97 Reviewed-by: Vitaly Kuznetsov (although, I kind of like the idea of zeroing the whole structure with a comment that we don't read it as a whole from guest's memory too) -- Vitaly