On Mon, Nov 26, 2018 at 09:17:50AM -0500, Brian Foster wrote: > On Mon, Nov 26, 2018 at 06:36:19PM +0800, PanBian wrote: > > On Mon, Nov 26, 2018 at 10:31:39AM +0100, Carlos Maiolino wrote: > > > On Sat, Nov 24, 2018 at 05:44:20PM +0800, Pan Bian wrote: > > > > The function xfs_alloc_get_freelist calls xfs_perag_put to drop the > > > > reference. In this case, pag may be released. However, > > > > pag->pagf_btreeblks is read and write after the put operation. This may > > > > result in a use-after-free bug. This patch moves the put operation late. > > > > > > > > > > The patch looks reasonable, can you detail more how did you find it? Via code > > > inspection of you hit this user-after-free in some way? > > > > I wrote a tool to check such bugs statically. It first scans the source code > > to extract paired alloc/free functions. Equipped with such functions, it > > performs an intra-procedural data flow analysis to detect mismatched > > alloc/free bugs and use-after-free bugs. > > > > You should probably drop the "use after free" text from your commit log > because that's not how the perag reference counting works. If you look > at xfs_perag_put(), you'll see it only drops a reference count and > returns. We only ever free the perag structs on unmount (or mount > failure), where we assert the refcount is zero. It looks like some other > serialization mechanism would have to break down for that assert to fail > due to this error in the allocation code because the fs is mostly shut > down at this point in time. I will follow your guidance to correct the commit log and resubmit the patch. Thanks, Pan > > Misleading commit log aside, the change seems fine to me. I think it's > appropriate to follow the traditional/implied _get()/_put() pattern. > > Brian > > > Best regards, > > Pan Bian > >
