x86/kvm: restrict kvm user region memory size

syzbot found WARNING in memslot_rmap_alloc[1] when
struct kvm_userspace_memory_region .memory_size is bigger than 0x40000000000,
which is 4GB, e.g. KMALLOC_MAX_SIZE * 100 * PAGE_SIZE.

Here is the PoC to trigger the warning:

    struct kvm_userspace_memory_region mem = {
        .slot = 0,
        .guest_phys_addr = 0,
        /* + 0x100 extra to trigger kmalloc WARNING */
        .memory_size = 0x40000000000 + 0x100,
        .userspace_addr = 0,
    };

    ioctl(kvm_fd, KVM_SET_USER_MEMORY_REGION, &mem);

I couldn't find any relevant max constant to restrict unsigned long npages.
There might be another solution with chunking big portions of pages, but
there is already KVM_MAX_HUGEPAGE_LEVEL, though warning happens in
memslot_rmap_alloc() when level = 1, base_gfn = 0, e.g.
on the very first KVM_NR_PAGE_SIZES iteration.

This is, seems, valid for early Linux versions as well. Can't tell which is
exactly can be considered for git bisect.
Here is Commit d89cc617b954af ("KVM: Push rmap into kvm_arch_memory_slot") for
example, Linux 3.7.

[1]
Call Trace:
 kvmalloc include/linux/mm.h:806 [inline]
 kvmalloc_array include/linux/mm.h:824 [inline]
 kvcalloc include/linux/mm.h:829 [inline]
 memslot_rmap_alloc+0xf6/0x310 arch/x86/kvm/x86.c:11320
 kvm_alloc_memslot_metadata arch/x86/kvm/x86.c:11388 [inline]
 kvm_arch_prepare_memory_region+0x48d/0x610 arch/x86/kvm/x86.c:11462
 kvm_set_memslot+0xfe/0x1700 arch/x86/kvm/../../../virt/kvm/kvm_main.c:1505
 ...
 kvm_set_memory_region arch/x86/kvm/../../../virt/kvm/kvm_main.c:1689 [inline]
 kvm_vm_ioctl_set_memory_region arch/x86/kvm/../../../virt/kvm/kvm_main.c:1701

Reported-by: syzbot+e0de2333cbf95ea473e8@syzkaller.appspotmail.com
Change-Id: I953b2eb78c5f634eea595da0fd9d86c75f16a8f9
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
index 21427e8..e790bb3 100644
--- a/arch/x86/kvm/mmu/page_track.c
+++ b/arch/x86/kvm/mmu/page_track.c
@@ -35,6 +35,9 @@
 	int  i;
 
 	for (i = 0; i < KVM_PAGE_TRACK_MAX; i++) {
+		if (npages > KMALLOC_MAX_SIZE)
+			return -ENOMEM;
+
 		slot->arch.gfn_track[i] =
 			kvcalloc(npages, sizeof(*slot->arch.gfn_track[i]),
 				 GFP_KERNEL_ACCOUNT);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index aabd3a2..2bad607 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11394,6 +11394,9 @@
 
 		WARN_ON(slot->arch.rmap[i]);
 
+		if (lpages > KMALLOC_MAX_SIZE)
+			return -ENOMEM;
+
 		slot->arch.rmap[i] = kvcalloc(lpages, sz, GFP_KERNEL_ACCOUNT);
 		if (!slot->arch.rmap[i]) {
 			memslot_rmap_free(slot);