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

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