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

[Oleg Nesterov <oleg@redhat.com>]

On 08/28, Dmitry Safonov wrote:
>
> > 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..

Hmm. What do you mean?

Just in case, TASK_SIZE checks TIF_ADDR32, not TS_COMPAT.

> >>  	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(),

tsk is always current, why do we need TASK_SIZE_OF() ?

> > 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..

Heh. it's actually even worse. Just suppose a 32-bit application simply
mmaps a 64-bit executable which has a probe. But this is off-topic.

> > Do I read the code properly and xol is always one page?

Yes,

> > Could that page be reserved on the top of mmap_base/mmap_compat_base at
> > the binfmt loading time?

How? I don't understand...

> (I would need than to add .mremap() for
> > restoring sake).

for what? I don't think you can restore a probed process anyway... OK,
right now this is off-topic too.

Oleg.