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 -- 2.55.0.887.g758fc8c411-goog