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

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>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://patch.msgid.link/20260826184449.814130-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions