Re: [PATCH v8 3/6] Uprobes: Support SDT markers having reference count (semaphore)

[Song Liu <liu.song.a23@gmail.com>]

On Sun, Aug 12, 2018 at 10:47 PM, Ravi Bangoria
<ravi.bangoria@linux.ibm.com> wrote:
> Hi Song,
>
> On 08/11/2018 01:27 PM, Song Liu wrote:
>>> +
>>> +static void delayed_uprobe_delete(struct delayed_uprobe *du)
>>> +{
>>> +       if (!du)
>>> +               return;
>> Do we really need this check?
>
>
> Not necessary though, but I would still like to keep it for a safety.
>
>
>>
>>> +       list_del(&du->list);
>>> +       kfree(du);
>>> +}
>>> +
>>> +static void delayed_uprobe_remove(struct uprobe *uprobe, struct mm_struct *mm)
>>> +{
>>> +       struct list_head *pos, *q;
>>> +       struct delayed_uprobe *du;
>>> +
>>> +       if (!uprobe && !mm)
>>> +               return;
>> And do we really need this check?
>
>
> Yes. delayed_uprobe_remove(uprobe=NULL, mm=NULL) is an invalid case. If I remove
> this check, code below (or more accurately code suggested by Oleg) will remove
> all entries from delayed_uprobe_list. So I will keep this check but put a comment
> above function.
>
>
> [...]
>>> +
>>> +       ret = get_user_pages_remote(NULL, mm, vaddr, 1,
>>> +                       FOLL_WRITE, &page, &vma, NULL);
>>> +       if (unlikely(ret <= 0)) {
>>> +               /*
>>> +                * We are asking for 1 page. If get_user_pages_remote() fails,
>>> +                * it may return 0, in that case we have to return error.
>>> +                */
>>> +               ret = (ret == 0) ? -EBUSY : ret;
>>> +               pr_warn("Failed to %s ref_ctr. (%d)\n",
>>> +                       d > 0 ? "increment" : "decrement", ret);
>> This warning is not really useful. Seems this function has little information
>> about which uprobe is failing here. Maybe we only need warning in the caller
>> (or caller of caller).
>
>
> Sure, I can move this warning to caller of this function but what are the
> exact fields you would like to print with warning? Something like this is
> fine?
>
>     pr_warn("ref_ctr %s failed for 0x%lx, 0x%lx, 0x%lx, 0x%p",
>                 d > 0 ? "increment" : "decrement", inode->i_ino,
>                 offset, ref_ctr_offset, mm);
>
> More importantly, the reason I didn't print more info is because dmesg is
> accessible to unprivileged users in many distros but uprobes are not. So
> printing this information may be a security violation. No?
>
>
>>
>>> +               return ret;
>>> +       }
>>> +
>>> +       kaddr = kmap_atomic(page);
>>> +       ptr = kaddr + (vaddr & ~PAGE_MASK);
>>> +
>>> +       if (unlikely(*ptr + d < 0)) {
>>> +               pr_warn("ref_ctr going negative. vaddr: 0x%lx, "
>>> +                       "curr val: %d, delta: %d\n", vaddr, *ptr, d);
>>> +               ret = -EINVAL;
>>> +               goto out;
>>> +       }
>>> +
>>> +       *ptr += d;
>>> +       ret = 0;
>>> +out:
>>> +       kunmap_atomic(kaddr);
>>> +       put_page(page);
>>> +       return ret;
>>> +}
>>> +
>>> +static int update_ref_ctr(struct uprobe *uprobe, struct mm_struct *mm,
>>> +                         bool is_register)
>> What's the reason of bool is_register here vs. short d in __update_ref_ctr()?
>> Can we use short for both?
>
>
> Yes, I can use short as well.
>
>
>>
>>> +{
>>> +       struct vm_area_struct *rc_vma;
>>> +       unsigned long rc_vaddr;
>>> +       int ret = 0;
>>> +
>>> +       rc_vma = find_ref_ctr_vma(uprobe, mm);
>>> +
>>> +       if (rc_vma) {
>>> +               rc_vaddr = offset_to_vaddr(rc_vma, uprobe->ref_ctr_offset);
>>> +               ret = __update_ref_ctr(mm, rc_vaddr, is_register ? 1 : -1);
>>> +
>>> +               if (is_register)
>>> +                       return ret;
>>> +       }
>> Mixing __update_ref_ctr() here and delayed_uprobe_add() in the same
>> function is a little confusing (at least for me). How about we always use
>> delayed uprobe for uprobe_mmap() and use non-delayed in other case(s)?
>
>
> No. delayed_uprobe_add() is needed for uprobe_register() case to handle race
> between uprobe_register() and process creation.

I see.

>
>
> [...]
>>>
>>> +static int delayed_uprobe_install(struct vm_area_struct *vma)
>> This function name is confusing. How about we call it delayed_ref_ctr_incr() or
>> something similar? Also, we should add comments to highlight this is vma is not
>> the vma containing the uprobe, but the vma containing the ref_ctr.
>
>
> Sure, I'll do that.
>
>
>>
>>> +{
>>> +       struct list_head *pos, *q;
>>> +       struct delayed_uprobe *du;
>>> +       unsigned long vaddr;
>>> +       int ret = 0, err = 0;
>>> +
>>> +       mutex_lock(&delayed_uprobe_lock);
>>> +       list_for_each_safe(pos, q, &delayed_uprobe_list) {
>>> +               du = list_entry(pos, struct delayed_uprobe, list);
>>> +
>>> +               if (!valid_ref_ctr_vma(du->uprobe, vma))
>>> +                       continue;
>>> +
>>> +               vaddr = offset_to_vaddr(vma, du->uprobe->ref_ctr_offset);
>>> +               ret = __update_ref_ctr(vma->vm_mm, vaddr, 1);
>>> +               /* Record an error and continue. */
>>> +               if (ret && !err)
>>> +                       err = ret;
>> I think this is a good place (when ret != 0) to call pr_warn(). I guess we can
>> print which mm get error for which uprobe (inode+offset).
>
>
> __update_ref_ctr() is already printing warning, so I didn't add anything here.
> In case I remove a warning from __update_ref_ctr(), a warning something like
> below is fine?
>
>     pr_warn("ref_ctr increment failed for 0x%lx, 0x%lx, 0x%lx, 0x%p",
>                 inode->i_ino, offset, ref_ctr_offset, vma->vm_mm);
>

I was thinking about a message like:

ref_ctr increment failed for inode XX offset YY ref_ctr ZZ of mm 0xWWW

I didn't thought about the security part of it, but I guess it is OK.

Thanks,
Song

> Again, can this lead to a security violation?
>
> Thanks for detailed review :)
> -Ravi
>