[Devel] [PATCH v3 2/6] KVM: x86: fix NULL deref in vcpu_scan_ioapic

[Roman Kagan]

From: Wanpeng Li <wanpeng.li at hotmail.com>

Patchset description:
kvm: hyperv: backport latest stuff from mainstream

These are backports of the latest commits to hyperv subsystem in
mainsteram KVM (as of 4.13).  They address a couple of design flaws, and
are necessary for the latest hyperv code in QEMU to work properly.

v1 -> v2:
 - replace incremental patch with a revert and a fresh backport
 - pull in an intermediate upstream patch to minimize conflicts

Roman Kagan (3):
  Revert "kvm/x86: clear hyperv synic pages only when guest modifies
    MSRs"
  kvm: x86: hyperv: add KVM_CAP_HYPERV_SYNIC2
  kvm: x86: hyperv: make VP_INDEX managed by userspace

Wanpeng Li (1):
  KVM: x86: fix NULL deref in vcpu_scan_ioapic

============================================
This patch description:

Reported by syzkaller:

    BUG: unable to handle kernel NULL pointer dereference at 00000000000001b0
    IP: _raw_spin_lock+0xc/0x30
    PGD 3e28eb067
    PUD 3f0ac6067
    PMD 0
    Oops: 0002 [#1] SMP
    CPU: 0 PID: 2431 Comm: test Tainted: G           OE   4.10.0-rc1+ #3
    Call Trace:
     ? kvm_ioapic_scan_entry+0x3e/0x110 [kvm]
     kvm_arch_vcpu_ioctl_run+0x10a8/0x15f0 [kvm]
     ? pick_next_task_fair+0xe1/0x4e0
     ? kvm_arch_vcpu_load+0xea/0x260 [kvm]
     kvm_vcpu_ioctl+0x33a/0x600 [kvm]
     ? hrtimer_try_to_cancel+0x29/0x130
     ? do_nanosleep+0x97/0xf0
     do_vfs_ioctl+0xa1/0x5d0
     ? __hrtimer_init+0x90/0x90
     ? do_nanosleep+0x5b/0xf0
     SyS_ioctl+0x79/0x90
     do_syscall_64+0x6e/0x180
     entry_SYSCALL64_slow_path+0x25/0x25
    RIP: _raw_spin_lock+0xc/0x30 RSP: ffffa43688973cc0

The syzkaller folks reported a NULL pointer dereference due to
ENABLE_CAP succeeding even without an irqchip.  The Hyper-V
synthetic interrupt controller is activated, resulting in a
wrong request to rescan the ioapic and a NULL pointer dereference.

    #include <sys/ioctl.h>
    #include <sys/mman.h>
    #include <sys/types.h>
    #include <linux/kvm.h>
    #include <pthread.h>
    #include <stddef.h>
    #include <stdint.h>
    #include <stdlib.h>
    #include <string.h>
    #include <unistd.h>

    #ifndef KVM_CAP_HYPERV_SYNIC
    #define KVM_CAP_HYPERV_SYNIC 123
    #endif

    void* thr(void* arg)
    {
	struct kvm_enable_cap cap;
	cap.flags = 0;
	cap.cap = KVM_CAP_HYPERV_SYNIC;
	ioctl((long)arg, KVM_ENABLE_CAP, &cap);
	return 0;
    }

    int main()
    {
	void *host_mem = mmap(0, 0x1000, PROT_READ|PROT_WRITE,
			MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
	int kvmfd = open("/dev/kvm", 0);
	int vmfd = ioctl(kvmfd, KVM_CREATE_VM, 0);
	struct kvm_userspace_memory_region memreg;
	memreg.slot = 0;
	memreg.flags = 0;
	memreg.guest_phys_addr = 0;
	memreg.memory_size = 0x1000;
	memreg.userspace_addr = (unsigned long)host_mem;
	host_mem[0] = 0xf4;
	ioctl(vmfd, KVM_SET_USER_MEMORY_REGION, &memreg);
	int cpufd = ioctl(vmfd, KVM_CREATE_VCPU, 0);
	struct kvm_sregs sregs;
	ioctl(cpufd, KVM_GET_SREGS, &sregs);
	sregs.cr0 = 0;
	sregs.cr4 = 0;
	sregs.efer = 0;
	sregs.cs.selector = 0;
	sregs.cs.base = 0;
	ioctl(cpufd, KVM_SET_SREGS, &sregs);
	struct kvm_regs regs = { .rflags = 2 };
	ioctl(cpufd, KVM_SET_REGS, &regs);
	ioctl(vmfd, KVM_CREATE_IRQCHIP, 0);
	pthread_t th;
	pthread_create(&th, 0, thr, (void*)(long)cpufd);
	usleep(rand() % 10000);
	ioctl(cpufd, KVM_RUN, 0);
	pthread_join(th, 0);
	return 0;
    }

This patch fixes it by failing ENABLE_CAP if without an irqchip.

Reported-by: Dmitry Vyukov <dvyukov at google.com>
Fixes: 5c919412fe61 (kvm/x86: Hyper-V synthetic interrupt controller)
Cc: stable at vger.kernel.org # 4.5+
Cc: Paolo Bonzini <pbonzini at redhat.com>
Cc: Radim Krčmář <rkrcmar at redhat.com>
Cc: Dmitry Vyukov <dvyukov at google.com>
Signed-off-by: Wanpeng Li <wanpeng.li at hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>

(cherry picked from commit 546d87e5c903a7f3ee7b9f998949a94729fbc65b)
Signed-off-by: Roman Kagan <rkagan at virtuozzo.com>
---
 arch/x86/kvm/x86.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 478faf18d1cb..16a8aa071a48 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3296,6 +3296,8 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 
 	switch (cap->cap) {
 	case KVM_CAP_HYPERV_SYNIC:
+		if (!irqchip_in_kernel(vcpu->kvm))
+			return -EINVAL;
 		return kvm_hv_activate_synic(vcpu);
 	default:
 		return -EINVAL;
-- 
2.13.6