Re: [PATCH 1/3] KVM: Fix coalesced mmio ring buffer out-of-bounds access

[Jim Mattson]

On Tue, Sep 17, 2019 at 1:16 AM Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> Reported by syzkaller:
>
>         #PF: supervisor write access in kernel mode
>         #PF: error_code(0x0002) - not-present page
>         PGD 403c01067 P4D 403c01067 PUD 0
>         Oops: 0002 [#1] SMP PTI
>         CPU: 1 PID: 12564 Comm: a.out Tainted: G           OE     5.3.0-rc4+ #4
>         RIP: 0010:coalesced_mmio_write+0xcc/0x130 [kvm]
>         Call Trace:
>          __kvm_io_bus_write+0x91/0xe0 [kvm]
>          kvm_io_bus_write+0x79/0xf0 [kvm]
>          write_mmio+0xae/0x170 [kvm]
>          emulator_read_write_onepage+0x252/0x430 [kvm]
>          emulator_read_write+0xcd/0x180 [kvm]
>          emulator_write_emulated+0x15/0x20 [kvm]
>          segmented_write+0x59/0x80 [kvm]
>          writeback+0x113/0x250 [kvm]
>          x86_emulate_insn+0x78c/0xd80 [kvm]
>          x86_emulate_instruction+0x386/0x7c0 [kvm]
>          kvm_mmu_page_fault+0xf9/0x9e0 [kvm]
>          handle_ept_violation+0x10a/0x220 [kvm_intel]
>          vmx_handle_exit+0xbe/0x6b0 [kvm_intel]
>          vcpu_enter_guest+0x4dc/0x18d0 [kvm]
>          kvm_arch_vcpu_ioctl_run+0x407/0x660 [kvm]
>          kvm_vcpu_ioctl+0x3ad/0x690 [kvm]
>          do_vfs_ioctl+0xa2/0x690
>          ksys_ioctl+0x6d/0x80
>          __x64_sys_ioctl+0x1a/0x20
>          do_syscall_64+0x74/0x720
>          entry_SYSCALL_64_after_hwframe+0x49/0xbe
>         RIP: 0010:coalesced_mmio_write+0xcc/0x130 [kvm]
>
> Both the coalesced_mmio ring buffer indexs ring->first and ring->last are
> bigger than KVM_COALESCED_MMIO_MAX from the testcase, array out-of-bounds
> access triggers by ring->coalesced_mmio[ring->last].phys_addr = addr;
> assignment. This patch fixes it by mod indexs by KVM_COALESCED_MMIO_MAX.
>
> syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=134b2826a00000
>
> Reported-by: syzbot+983c866c3dd6efa3662a@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
>  virt/kvm/coalesced_mmio.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
> index 5294abb..cff1ec9 100644
> --- a/virt/kvm/coalesced_mmio.c
> +++ b/virt/kvm/coalesced_mmio.c
> @@ -73,6 +73,8 @@ static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
>
>         spin_lock(&dev->kvm->ring_lock);
>
> +       ring->first = ring->first % KVM_COALESCED_MMIO_MAX;
> +       ring->last = ring->last % KVM_COALESCED_MMIO_MAX;

I don't think this is sufficient, since the memory that ring points to
is shared with userspace. Userspace can overwrite your corrected
values with illegal ones before they are used. Not exactly a TOCTTOU
issue, since there isn't technically a 'check' here, but the same
idea.

>         if (!coalesced_mmio_has_room(dev)) {
>                 spin_unlock(&dev->kvm->ring_lock);
>                 return -EOPNOTSUPP;
> --
> 2.7.4
>