Re: [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access from ioapic_write_indirect()

[Eduardo Habkost]

On Thu, Aug 26, 2021 at 02:40:53PM +0200, Vitaly Kuznetsov wrote:
> Eduardo Habkost <ehabkost@xxxxxxxxxx> writes:
> 
> > On Wed, Aug 25, 2021 at 5:43 AM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> >>
> >> Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes:
> >>
> >> > On Wed, 2021-08-25 at 10:21 +0200, Vitaly Kuznetsov wrote:
> >> >> Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes:
> >> >>
> >> >> > On Tue, 2021-08-24 at 16:42 +0200, Vitaly Kuznetsov wrote:
> >> >> ...
> >> >> > Not a classical review but,
> >> >> > I did some digital archaeology with this one, trying to understand what is going on:
> >> >> >
> >> >> >
> >> >> > I think that 16 bit vcpu bitmap is due to the fact that IOAPIC spec states that
> >> >> > it can address up to 16 cpus in physical destination mode.
> >> >> >
> >> >> > In logical destination mode, assuming flat addressing and that logical id = 1 << physical id
> >> >> > which KVM hardcodes, it is also only possible to address 8 CPUs.
> >> >> >
> >> >> > However(!) in flat cluster mode, the logical apic id is split in two.
> >> >> > We have 16 clusters and each have 4 CPUs, so it is possible to address 64 CPUs,
> >> >> > and unlike the logical ID, the KVM does honour cluster ID,
> >> >> > thus one can stick say cluster ID 0 to any vCPU.
> >> >> >
> >> >> >
> >> >> > Let's look at ioapic_write_indirect.
> >> >> > It does:
> >> >> >
> >> >> >     -> bitmap_zero(&vcpu_bitmap, 16);
> >> >> >     -> kvm_bitmap_or_dest_vcpus(ioapic->kvm, &irq, &vcpu_bitmap);
> >> >> >     -> kvm_make_scan_ioapic_request_mask(ioapic->kvm, &vcpu_bitmap); // use of the above bitmap
> >> >> >
> >> >> >
> >> >> > When we call kvm_bitmap_or_dest_vcpus, we can already overflow the bitmap,
> >> >> > since we pass all 8 bit of the destination even when it is physical.
> >> >> >
> >> >> >
> >> >> > Lets examine the kvm_bitmap_or_dest_vcpus:
> >> >> >
> >> >> >   -> It calls the kvm_apic_map_get_dest_lapic which
> >> >> >
> >> >> >        -> for physical destinations, it just sets the bitmap, which can overflow
> >> >> >           if we pass it 8 bit destination (which basically includes reserved bits + 4 bit destination).
> >> >> >
> >> >> >
> >> >> >        -> For logical apic ID, it seems to truncate the result to 16 bit, which isn't correct as I explained
> >> >> >           above, but should not overflow the result.
> >> >> >
> >> >> >
> >> >> >    -> If call to kvm_apic_map_get_dest_lapic fails, it goes over all vcpus and tries to match the destination
> >> >> >        This can overflow as well.
> >> >> >
> >> >> >
> >> >> > I also don't like that ioapic_write_indirect calls the kvm_bitmap_or_dest_vcpus twice,
> >> >> > and second time with 'old_dest_id'
> >> >> >
> >> >> > I am not 100%  sure why old_dest_id/old_dest_mode are needed as I don't see anything in the
> >> >> > function changing them.
> >> >> > I think only the guest can change them, so maybe the code deals with the guest changing them
> >> >> > while the code is running from a different vcpu?
> >> >> >
> >> >> > The commit that introduced this code is 7ee30bc132c683d06a6d9e360e39e483e3990708
> >> >> > Nitesh Narayan Lal, maybe you remember something about it?
> >> >> >
> >> >>
> >> >> Before posting this patch I've contacted Nitesh privately, he's
> >> >> currently on vacation but will take a look when he gets back.
> >> >>
> >> >> > Also I worry a lot about other callers of kvm_apic_map_get_dest_lapic
> >> >> >
> >> >> > It is also called from kvm_irq_delivery_to_apic_fast, and from kvm_intr_is_single_vcpu_fast
> >> >> > and both seem to also use 'unsigned long' for bitmap, and then only use 16 bits of it.
> >> >> >
> >> >> > I haven't dug into them, but these don't seem to be IOAPIC related and I think
> >> >> > can overwrite the stack as well.
> >> >>
> >> >> I'm no expert in this code but when writing the patch I somehow
> >> >> convinced myself that a single unsigned long is always enough. I think
> >> >> that for cluster mode 'bitmap' needs 64-bits (and it is *not* a
> >> >> vcpu_bitmap, we need to convert). I may be completely wrong of course
> >> >> but in any case this is a different issue. In ioapic_write_indirect() we
> >> >> have 'vcpu_bitmap' which should certainly be longer than 64 bits.
> >> >
> >> >
> >> > This code which I mentioned in 'other callers' as far as I see is not IOAPIC related.
> >> > For regular local APIC all bets are off, any vCPU and apic ID are possible
> >> > (xapic I think limits apic id to 255 but x2apic doesn't).
> >> >
> >> > I strongly suspect that this code can overflow as well.
> >>
> >> I've probably missed something but I don't see how
> >> kvm_apic_map_get_dest_lapic() can set bits above 64 in 'bitmap'. If it
> >> can, then we have a problem indeed.
> >
> > It would be nice if the compiler took care of validating bitmap sizes
> > for us. Shouldn't we make the function prototypes explicit about the
> > bitmap sizes they expect?
> >
> > I believe some `typedef DECLARE_BITMAP(...)` or `typedef struct {
> > DECLARE_BITMAP(...) } ...` declarations would be very useful here.
> 
> The fundamental problem here is that bitmap in Linux has 'unsigned long
> *' type, it's supposed to be accompanied with 'int len' parameter but
> it's not always the case.
> 
> In KVM, we usually use 'vcpu_bitmap' (or 'dest_vcpu_bitmap') and these
> are 'KVM_MAX_VCPUS' long. Just 'bitmap' or 'mask' case is a bit more
> complicated. E.g. kvm_apic_map_get_logical_dest() uses 'u16 *mask' and
> this means that only 16 bits in the destination are supposed to be
> set. kvm_apic_map_get_dest_lapic() uses 'unsigned long *bitmap' - go
> figure.
> 
> We could've probably used a declaration like you suggest to e.g. create
> incompatible 'bitmap16','bitmap64',... types and make the compiler do
> the checking but I'm slightly hesitant to introduce such helpers to KVM
> and not the whole kernel. Alternatively, we could've just encoded the
> length in parameters name, e.g. 
> 
> @@ -918,7 +918,7 @@ static bool kvm_apic_is_broadcast_dest(struct kvm *kvm, struct kvm_lapic **src,
>  static inline bool kvm_apic_map_get_dest_lapic(struct kvm *kvm,
>                 struct kvm_lapic **src, struct kvm_lapic_irq *irq,
>                 struct kvm_apic_map *map, struct kvm_lapic ***dst,
> -               unsigned long *bitmap)
> +               unsigned long *bitmap64)

You can communicate the expected bitmap size to the compiler
without typedefs if using DECLARE_BITMAP inside the function
parameter list is acceptable coding style (is it?).

For example, the following would have allowed the compiler to
catch the bug you are fixing:

Signed-off-by: Eduardo Habkost <ehabkost@xxxxxxxxxx>
---
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index d7c25d0c1354..e8c64747121a 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -236,7 +236,7 @@ bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);
 void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu);
 
 void kvm_bitmap_or_dest_vcpus(struct kvm *kvm, struct kvm_lapic_irq *irq,
-			      unsigned long *vcpu_bitmap);
+			      DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS));
 
 bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
 			struct kvm_vcpu **dest_vcpu);
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 76fb00921203..1df113894cba 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1166,7 +1166,7 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
  * each available vcpu to identify the same.
  */
 void kvm_bitmap_or_dest_vcpus(struct kvm *kvm, struct kvm_lapic_irq *irq,
-			      unsigned long *vcpu_bitmap)
+			      DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS))
 {
 	struct kvm_lapic **dest_vcpu = NULL;
 	struct kvm_lapic *src = NULL;

-- 
Eduardo