Messages in this thread Patch in this message | | | Date | Fri, 3 Aug 2018 13:24:55 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v7 3/6] Uprobes: Support SDT markers having reference count (semaphore) |
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Hi Ravi,
I was going to give up and ack this series, but it seems I noticed a bug...
On 07/31, Ravi Bangoria wrote: > > +static int delayed_uprobe_add(struct uprobe *uprobe, struct mm_struct *mm) > +{ > + struct delayed_uprobe *du; > + > + if (delayed_uprobe_check(uprobe, mm)) > + return 0; > + > + du = kzalloc(sizeof(*du), GFP_KERNEL); > + if (!du) > + return -ENOMEM; > + > + du->uprobe = uprobe; > + du->mm = mm;
I am surprised I didn't notice this before...
So du->mm = mm;
is fine, mm can't go away, uprobe_clear_state() does delayed_uprobe_remove(NULL,mm).
But du->uprobe = uprobe;
doesn't look right, uprobe can go away and it can be freed, its memory can be reused. We can't rely on remove_breakpoint(), the application can unmap the probed page/vma. Yes we do not care about the application in this case, say, the next uprobe_mmap() can wrongly increment the counter, we do not care although this can lead to hard-to-debug problems. But, if nothing else, the kernel can crash if the freed memory is unmapped. So I think put_uprobe() should do delayed_uprobe_remove(uprobe, NULL) before kfree() and delayed_uprobe_remove() should be updated to handle the mm==NULL case.
Also. delayed_uprobe_add() should check the list and avoid duplicates. Otherwise the trivial
for (;;) munmap(mmap(uprobed_file));
will eat the memory until uprobe is unregistered.
> +static bool valid_ref_ctr_vma(struct uprobe *uprobe, > + struct vm_area_struct *vma) > +{ > + unsigned long vaddr = offset_to_vaddr(vma, uprobe->ref_ctr_offset); > + > + return uprobe->ref_ctr_offset && > + vma->vm_file && > + file_inode(vma->vm_file) == uprobe->inode && > + vma->vm_flags & VM_WRITE && > + !(vma->vm_flags & VM_SHARED) &&
vma->vm_flags & (VM_WRITE|VM_SHARED) == VM_WRITE &&
looks a bit better to me, but I won't insist.
> +static int delayed_uprobe_install(struct vm_area_struct *vma) > +{ > + 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. */ > + err = ret & !err ? ret : err;
I try to avoid the cosmetic nits, but I simply can't look at this line ;)
if (ret && !err) err = ret;
> @@ -1072,7 +1281,14 @@ int uprobe_mmap(struct vm_area_struct *vma) > struct uprobe *uprobe, *u; > struct inode *inode; > > - if (no_uprobe_events() || !valid_vma(vma, true)) > + if (no_uprobe_events()) > + return 0; > + > + if (vma->vm_flags & VM_WRITE && > + test_bit(MMF_HAS_UPROBES, &vma->vm_mm->flags)) > + delayed_uprobe_install(vma);
OK, so you also added the VM_WRITE check and I agree. But then I think we should also check VM_SHARED, just like valid_ref_ctr_vma() does?
Oleg.
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