um: mm: check more comprehensively for stub changes

If userspace tries to change the stub, we need to kill it,
because otherwise it can escape the virtual machine. In a
few cases the stub checks weren't good, e.g. if userspace
just tries to

mmap(0x100000 - 0x1000, 0x3000, ...)

it could succeed to get a new private/anonymous mapping
replacing the stubs. Fix this by checking everywhere, and
checking for _overlap_, not just direct changes.

Cc: [e-mail]
Fixes: 3963333fe676 ("uml: cover stubs with a VMA")
Signed-off-by: Johannes Berg <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/kernel/tlb.c

···
 	struct host_vm_op *last;
 	int fd = -1, ret = 0;
 
+	if (virt + len > STUB_START && virt < STUB_END)
+		return -EINVAL;
+
 	if (hvc->userspace)
 		fd = phys_mapping(phys, &offset);
 	else
···
 	struct host_vm_op *last;
 	int ret = 0;
 
-	if ((addr >= STUB_START) && (addr < STUB_END))
+	if (addr + len > STUB_START && addr < STUB_END)
 		return -EINVAL;
 
 	if (hvc->index != 0) {
···
 {
 	struct host_vm_op *last;
 	int ret = 0;
+
+	if (addr + len > STUB_START && addr < STUB_END)
+		return -EINVAL;
 
 	if (hvc->index != 0) {
 		last = &hvc->ops[hvc->index - 1];
···
 	struct mm_id *mm_id;
 
 	address &= PAGE_MASK;
+
+	if (address >= STUB_START && address < STUB_END)
+		goto kill;
+
 	pgd = pgd_offset(mm, address);
 	if (!pgd_present(*pgd))
 		goto kill;