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

[Wanpeng Li]

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;
-- 
2.7.4