Re: get_unmapped_area && in_ia32_syscall (Was: [PATCH] uprobes/x86: fix detection of 32-bit user mode)

From: Dmitry Safonov
Date: Tue Aug 27 2019 - 13:03:28 EST

Hi Oleg,

On 8/27/19 3:00 PM, Oleg Nesterov wrote:
> But to remind, there is another problem with in_ia32_syscall() && uprobes.
> get_unmapped_area() paths use in_ia32_syscall() and this is wrong in case
> when the caller is xol_add_vma(), in this case TS_COMPAT won't be set.>
> Usually the addr = TASK_SIZE - PAGE_SIZE passed to get_unmapped_area() should
> work, mm->get_unmapped_area() won't be even called. But if this addr is already
> occupied get_area() can return addr > TASK_SIZE.

Technically, it's not bigger than TASK_SIZE that's supplied
get_unmapped_area() as an argument..

> if (!area->vaddr) {
> + if(!is_64bit_mm(mm))
> + current_thread_info()->status |= TS_COMPAT;
> /* Try to map as high as possible, this is only a hint. */
> area->vaddr = get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE,
> PAGE_SIZE, 0, 0);
> + if(!is_64bit_mm(mm))
> + current_thread_info()->status &= ~TS_COMPAT;;
> if (area->vaddr & ~PAGE_MASK) {
> ret = area->vaddr;
> goto fail;

It could have been TASK_SIZE_OF(), but that would be not much better in
my POV. I see that arch_uprobe_analyze_insn() uses is_64bit_mm() which
is correct the majority of time, but not for processes those jump
switching CS.. Except criu afair there are at least wine, dosemu.
I had it in my TODO to fix this :)

Do I read the code properly and xol is always one page?
Could that page be reserved on the top of mmap_base/mmap_compat_base at
the binfmt loading time? (I would need than to add .mremap() for
restoring sake). Probably, not reserving it if personality doesn't allow
randomization or providing a way to disable it..