x86/vLAPIC: prevent undue recursion of vlapic_error()

With the error vector set to an illegal value, the function invoking
vlapic_set_irq() would bring execution back here, with the non-recursive
lock already held. Avoid the call in this case, merely further updating
ESR (if necessary).

This is XSA-462 / CVE-2024-45817.

Fixes: 5f32d186a8 ("x86/vlapic: don't silently accept bad vectors")
Reported-by: Federico Serafini <federico.serafini@bugseng.com>
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: c42d9ec61f
master date: 2024-09-24 14:23:29 +0200

xen/arch/x86/hvm/vlapic.c

@@ -113,9 +113,24 @@ static void vlapic_error(struct vlapic *vlapic, unsigned int errmask)
     if ( (esr & errmask) != errmask )
     {
         uint32_t lvterr = vlapic_get_reg(vlapic, APIC_LVTERR);
+        bool inj = false;
+
+        if ( !(lvterr & APIC_LVT_MASKED) )
+        {
+            /*
+             * If LVTERR is unmasked and has an illegal vector, vlapic_set_irq()
+             * will end up back here.  Break the cycle by only injecting LVTERR
+             * if it will succeed, and folding in RECVILL otherwise.
+             */
+            if ( (lvterr & APIC_VECTOR_MASK) >= 16 )
+                 inj = true;
+            else
+                 errmask |= APIC_ESR_RECVILL;
+        }
 
         vlapic_set_reg(vlapic, APIC_ESR, esr | errmask);
-        if ( !(lvterr & APIC_LVT_MASKED) )
+
+        if ( inj )
             vlapic_set_irq(vlapic, lvterr & APIC_VECTOR_MASK, 0);
     }
     spin_unlock_irqrestore(&vlapic->esr_lock, flags);