Messages in this thread | | | Date | Tue, 6 Apr 2010 14:27:37 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3) |
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On Tue, 6 Apr 2010, Borislav Petkov wrote: > > So again, it's actually anon_vma.head.next that is NULL, not any of the > > entries on the list itself. > > > > Now, I can see several cases for this: > > > > - the obvious one: anon_vma just wasn't correctly initialized, and is > > missing a INIT_LIST_HEAD(&anon_vma->head). That's either a slab bug (we > > don't have a whole lot of coverage of constructors), or somebody > > allocated an anon_vma without using the anon_vma_cachep. > > I've added code to verify this and am suspend/resuming now... Wait a > minute, Linus, you're good! :) : > > [ 873.083074] PM: Preallocating image memory... > [ 873.254359] NULL anon_vma->head.next, page 2182681
Yeah, I was pretty sure of that thing.
I still don't see _how_ it happens, though. That 'struct anon_vma' is very simple, and contains literally just the lock and that list_head.
Now, 'head.next' is kind of magical, because it contains that magic low-bit "have I been locked" thing (see "vm_lock_anon_vma()" in mm/mmap.c). But I'm not seeing anything else touching it.
And if you allocate a anon_vma the proper way, the SLUB constructor should have made sure that the head is initialized. And no normal list operation ever sets any list pointer to zero, although a "list_del()" on the first list entry could do it if that first list entry had a NULL next pointer.
> Now, how do we track back to the place which is missing anon_vma->head > init? Can we use the struct page *page arg to page_referenced_anon() > somehow?
You might enable SLUB debugging (both SLUB_DEBUG _and_ SLUB_DEBUG_ON), and then make the "object_err()" function in mm/slub.c be non-static. You could call it when you see the problem, perhaps.
Or you could just add tests to both alloc_anon_vma() and free_anon_vma() to check that 'list_empty(&anon_vma->head)' is true. I dunno.
> > I haven't looked at the kernel config files: do they perhaps share the > > same (odd?) SLUB/SLAB/SLOB config? > > what is an odd SL[AOU]B config?
Probably anything but the default SLUB these days. But Steinar already said he had SLUB, so it's unlikely to be something odd.
> > - anon_vma isn't actually an anonvma at all. 'page->mapping' was crud > > with the low bit set. That sounds unlikely, but who knows. The ksm code > > sets mapping to "stable_node + PAGE_MAPPING_ANON | PAGE_MAPPING_KSM" > > > > Did people have KSM enabled? > > Nope, KSM is off here.
Yeah, wasn't for Steinar either. So it doesn't look like it's any odd corner case that depends on some odd configuration.
Linus
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