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

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Aug 24, 2021 at 3:13 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Eduardo Habkost <ehabkost@redhat.com> writes:
>
> > On Mon, Aug 23, 2021 at 04:30:28PM +0200, Vitaly Kuznetsov wrote:
> >> KASAN reports the following issue:
> >>
> >>  BUG: KASAN: stack-out-of-bounds in kvm_make_vcpus_request_mask+0x174/0x440 [kvm]
> >>  Read of size 8 at addr ffffc9001364f638 by task qemu-kvm/4798
> >>
> >>  CPU: 0 PID: 4798 Comm: qemu-kvm Tainted: G               X --------- ---
> >>  Hardware name: AMD Corporation DAYTONA_X/DAYTONA_X, BIOS RYM0081C 07/13/2020
> >>  Call Trace:
> >>   dump_stack+0xa5/0xe6
> >>   print_address_description.constprop.0+0x18/0x130
> >>   ? kvm_make_vcpus_request_mask+0x174/0x440 [kvm]
> >>   __kasan_report.cold+0x7f/0x114
> >>   ? kvm_make_vcpus_request_mask+0x174/0x440 [kvm]
> >>   kasan_report+0x38/0x50
> >>   kasan_check_range+0xf5/0x1d0
> >>   kvm_make_vcpus_request_mask+0x174/0x440 [kvm]
> >>   kvm_make_scan_ioapic_request_mask+0x84/0xc0 [kvm]
> >>   ? kvm_arch_exit+0x110/0x110 [kvm]
> >>   ? sched_clock+0x5/0x10
> >>   ioapic_write_indirect+0x59f/0x9e0 [kvm]
> >>   ? static_obj+0xc0/0xc0
> >>   ? __lock_acquired+0x1d2/0x8c0
> >>   ? kvm_ioapic_eoi_inject_work+0x120/0x120 [kvm]
> >>
> >> The problem appears to be that 'vcpu_bitmap' is allocated as a single long
> >> on stack and it should really be KVM_MAX_VCPUS long. We also seem to clear
> >> the lower 16 bits of it with bitmap_zero() for no particular reason (my
> >> guess would be that 'bitmap' and 'vcpu_bitmap' variables in
> >> kvm_bitmap_or_dest_vcpus() caused the confusion: while the later is indeed
> >> 16-bit long, the later should accommodate all possible vCPUs).
> >>
> >> Fixes: 7ee30bc132c6 ("KVM: x86: deliver KVM IOAPIC scan request to target vCPUs")
> >> Fixes: 9a2ae9f6b6bb ("KVM: x86: Zero the IOAPIC scan request dest vCPUs bitmap")
> >> Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> >> ---
> >>  arch/x86/kvm/ioapic.c | 10 +++++-----
> >>  1 file changed, 5 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
> >> index ff005fe738a4..92cd4b02e9ba 100644
> >> --- a/arch/x86/kvm/ioapic.c
> >> +++ b/arch/x86/kvm/ioapic.c
> >> @@ -319,7 +319,7 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
> >>      unsigned index;
> >>      bool mask_before, mask_after;
> >>      union kvm_ioapic_redirect_entry *e;
> >> -    unsigned long vcpu_bitmap;
> >> +    unsigned long vcpu_bitmap[BITS_TO_LONGS(KVM_MAX_VCPUS)];
> >
> > Is there a way to avoid this KVM_MAX_VCPUS-sized variable on the
> > stack?  This might hit us back when we increase KVM_MAX_VCPUS to
> > a few thousand VCPUs (I was planning to submit a patch for that
> > soon).
>
> What's the short- or mid-term target?

Short term target is 2048 (which was already tested). Mid-term target
(not tested yet) is 4096, maybe 8192.

>
> Note, we're allocating KVM_MAX_VCPUS bits (not bytes!) here, this means
> that for e.g. 2048 vCPUs we need 256 bytes of the stack only. In case
> the target much higher than that, we will need to either switch to
> dynamic allocation or e.g. use pre-allocated per-CPU variables and make
> this a preempt-disabled region. I, however, would like to understand if
> the problem with allocating this from stack is real or not first.

Is 256 bytes too much here, or would that be OK?

--
Eduardo