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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 27 Mar 2025, Volodymyr Babchuk wrote:
> Hi Andrew,
> 
> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> writes:
> 
> > 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>
> 
> Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@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;
> >      }
> 
> -- 
> WBR, Volodymyr