x86/hvm: don't leave emulator in inconsistent state

The fact that handle_mmio(), and thus the instruction emulator, is
being run through twice for emulations that require involvement of the
device model, allows for the second run to see a different guest state
than the first one. Since only the MMIO-specific emulation routines
update the vCPU's io_state, if they get invoked on the second pass,
internal state (and particularly this variable) can be left in a state
making successful emulation of a subsequent MMIO operation impossible.

Consequently, whenever the emulator invocation returns without
requesting a retry of the guest instruction, reset io_state.

[ This is a security issue.  XSA#10. -iwj ]

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>
xen-unstable changeset: 25682:ffcb24876b4f
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

diff --git a/xen/arch/x86/hvm/io.c b/xen/arch/x86/hvm/io.c
index 3c31a6d2e46c1042f496798dd9819db1f17b5de5..d59c237f780027cd92c02c64022cd1854db329ff 100644 (file)
--- a/xen/arch/x86/hvm/io.c
+++ b/xen/arch/x86/hvm/io.c
@@ -176,6 +176,8 @@ int handle_mmio(void)
 
     rc = hvm_emulate_one(&ctxt);
 
+    if ( rc != X86EMUL_RETRY )
+        curr->arch.hvm_vcpu.io_state = HVMIO_none;
     if ( curr->arch.hvm_vcpu.io_state == HVMIO_awaiting_completion )
         curr->arch.hvm_vcpu.io_state = HVMIO_handle_mmio_awaiting_completion;
     else