[PATCH] KVM: x86: Zero reserved synic flags when delivering message to avoid stack leak

[Sean Christopherson <seanjc@google.com>]

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 <fane@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 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