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

[Paolo Bonzini]

I think we should consider the embargo for CVE-2019-14821 to be broken.
 Since your patch is better, I'll push that one instead as soon as I get
confirmation.

Paolo

On 17/09/19 10:16, Wanpeng Li 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;
>  	if (!coalesced_mmio_has_room(dev)) {
>  		spin_unlock(&dev->kvm->ring_lock);
>  		return -EOPNOTSUPP;
>