On 2017/5/20 10:02, Hugh Dickins wrote: > On Sat, 20 May 2017, Xishi Qiu wrote: >> On 2017/5/20 6:00, Hugh Dickins wrote: >>> >>> You're ignoring the rcu_read_lock() on entry to page_lock_anon_vma_read(), >>> and the SLAB_DESTROY_BY_RCU (recently renamed SLAB_TYPESAFE_BY_RCU) nature >>> of the anon_vma_cachep kmem cache. It is not safe to muck with anon_vma-> >>> root in anon_vma_free(), others could still be looking at it. >>> >>> Hugh >>> >> >> Hi Hugh, >> >> Thanks for your reply. >> >> SLAB_DESTROY_BY_RCU will let it call call_rcu() in free_slab(), but if the >> anon_vma *reuse* by someone again, access root_anon_vma is not safe, right? > > That is safe, on reuse it is still a struct anon_vma; then the test for > !page_mapped(page) will show that it's no longer a reliable anon_vma for > this page, so page_lock_anon_vma_read() returns NULL. > > But of course, if page->_mapcount has been corrupted or misaccounted, > it may think page_mapped(page) when actually page is not mapped, > and the anon_vma is not good for it. > Hi Hugh, Here is a bug report form redhat: https://bugzilla.redhat.com/show_bug.cgi?id=1305620 And I meet the bug too. However it is hard to reproduce, and 624483f3ea82598("mm: rmap: fix use-after-free in __put_anon_vma") is not help. >From the vmcore, it seems that the page is still mapped(_mapcount=0 and _count=2), and the value of mapping is a valid address(mapping = 0xffff8801b3e2a101), but anon_vma has been corrupted. Any ideas? Thanks, Xishi Qiu >> >> e.g. if I clean the root pointer before free it, then access root_anon_vma >> in page_lock_anon_vma_read() is NULL pointer access, right? > > Yes, cleaning root pointer before free may result in NULL pointer access. > > Hugh > >> >> anon_vma_free() >> ... >> anon_vma->root = NULL; >> kmem_cache_free(anon_vma_cachep, anon_vma); >> ... >> >> Thanks, >> Xishi Qiu > > . > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
