[PATCH] ARM/vgic: Fix out-of-bounds accesses in vgic_mmio_write_sgir()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The switch() statement is over bits 24:25 (unshifted) of the guest provided
value.  This makes case 0x3: dead, and not an implementation of the 4th
possible state.

A guest which writes (3<<24)|(ff<<16) to this register will skip the early
exit, then enter bitmap_for_each() with targets not bound by nr_vcpus.

If the guest has fewer than 8 vCPUs, bitmap_for_each() will read off the end
of d->vcpu[] and use the resulting vcpu pointer to ultimately derive irq, and
perform an out-of-bounds write.

Fix this by changing case 0x3 to default.

Fixes: 08c688ca6422 ("ARM: new VGIC: Add SGIR register handler")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
CC: Michal Orzel <michal.orzel@xxxxxxx>

This vgic driver is explicity not security supported, hence no XSA.
---
 xen/arch/arm/vgic/vgic-mmio-v2.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/vgic/vgic-mmio-v2.c b/xen/arch/arm/vgic/vgic-mmio-v2.c
index 670b335db2c3..7d1391ac9b48 100644
--- a/xen/arch/arm/vgic/vgic-mmio-v2.c
+++ b/xen/arch/arm/vgic/vgic-mmio-v2.c
@@ -104,7 +104,8 @@ static void vgic_mmio_write_sgir(struct vcpu *source_vcpu,
     case GICD_SGI_TARGET_SELF:                    /* this very vCPU only */
         targets = (1U << source_vcpu->vcpu_id);
         break;
-    case 0x3:                                     /* reserved */
+
+    default:
         return;
     }
 
-- 
2.39.5