Re: bpf: use-after-free in array_map_alloc

On 05/23/2016 02:01 PM, Vlastimil Babka wrote:

quoted

if I read the report correctly it's not about bpf, but rather points to
the issue inside percpu logic.
First __alloc_percpu_gfp() is called, then the memory is freed with
free_percpu() which triggers async pcpu_balance_work and then
pcpu_extend_area_map is hitting use-after-free.
I guess bpf percpu array map is stressing this logic the most.

I've been staring at it for a while (not knowing the code at all) and
the first thing that struck me is that pcpu_extend_area_map() is done
outside of pcpu_lock. So what prevents the chunk from being freed during
the extend?

Erm to be precise, pcpu_lock is unlocked just before calling
pcpu_extend_area_map(), which relocks it after an allocation, and
assumes the chunk still exists at that point. Unless I'm missing
something, that's an unlocked window where chunk can be destroyed by the
workfn, as the report suggests?