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

Ravi Bangoria
2018-08-13T05:48:12Z

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.

[...]
>>
>> +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);

Again, can this lead to a security violation?

Thanks for detailed review :)
-Ravi