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

[Dmitry Safonov]

-Cc my old @virtuozzo email.
Previously it just ignored emails and now sends those ugly html replies.
Sorry about that - I've updated .mailmap now.

On 8/27/19 6:03 PM, Dmitry Safonov wrote:
> 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..

If no one has concerns over such approach, I'll cook a fix just after
Plumbers week.

Thanks,
          Dmitry