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x86/HVM: avoid reading ioreq state more than once
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Otherwise, especially when the compiler chooses to translate the
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switch() to a jump table, unpredictable behavior (and in the jump table
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case arbitrary code execution) can result.
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This is XSA-166.
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Signed-off-by: Jan Beulich <jbeulich@suse.com>
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Acked-by: Ian Campbell <ian.campbell@citrix.com>
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--- a/xen/arch/x86/hvm/hvm.c
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+++ b/xen/arch/x86/hvm/hvm.c
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@@ -460,7 +460,10 @@ static bool_t hvm_wait_for_io(struct hvm
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{
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while ( sv->pending )
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{
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- switch ( p->state )
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+ unsigned int state = p->state;
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+
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+ rmb();
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+ switch ( state )
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{
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case STATE_IOREQ_NONE:
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/*
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@@ -471,18 +474,15 @@ static bool_t hvm_wait_for_io(struct hvm
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hvm_io_assist(sv, ~0ul);
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break;
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case STATE_IORESP_READY: /* IORESP_READY -> NONE */
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- rmb(); /* see IORESP_READY /then/ read contents of ioreq */
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p->state = STATE_IOREQ_NONE;
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hvm_io_assist(sv, p->data);
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break;
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case STATE_IOREQ_READY: /* IOREQ_{READY,INPROCESS} -> IORESP_READY */
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case STATE_IOREQ_INPROCESS:
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- wait_on_xen_event_channel(sv->ioreq_evtchn,
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- (p->state != STATE_IOREQ_READY) &&
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- (p->state != STATE_IOREQ_INPROCESS));
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+ wait_on_xen_event_channel(sv->ioreq_evtchn, p->state != state);
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break;
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default:
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- gdprintk(XENLOG_ERR, "Weird HVM iorequest state %d.\n", p->state);
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+ gdprintk(XENLOG_ERR, "Weird HVM iorequest state %u\n", state);
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sv->pending = 0;
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domain_crash(sv->vcpu->domain);
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return 0; /* bail */
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