On Fri, May 11, 2018 at 03:28:41AM +0100, Al Viro wrote: > On Fri, May 11, 2018 at 11:20:57AM +1000, Dave Chinner wrote: > > > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > We recently had an oops reported on a 4.14 kernel in > > xfs_reclaim_inodes_count() where sb->s_fs_info pointed to garbage > > and so the m_perag_tree lookup walked into lala land. It produces > > an oops down this path during the failed mount: > > > > radix_tree_gang_lookup_tag+0xc4/0x130 > > xfs_perag_get_tag+0x37/0xf0 > > xfs_reclaim_inodes_count+0x32/0x40 > > xfs_fs_nr_cached_objects+0x11/0x20 > > super_cache_count+0x35/0xc0 > > shrink_slab.part.66+0xb1/0x370 > > shrink_node+0x7e/0x1a0 > > try_to_free_pages+0x199/0x470 > > __alloc_pages_slowpath+0x3a1/0xd20 > > __alloc_pages_nodemask+0x1c3/0x200 > > cache_grow_begin+0x20b/0x2e0 > > fallback_alloc+0x160/0x200 > > kmem_cache_alloc+0x111/0x4e0 > > > > The problem is that the superblock shrinker is running before the > > filesystem structures it depends on have been fully set up. i.e. > > the shrinker is registered in sget(), before ->fill_super() has been > > called, and the shrinker can call into the filesystem before > > fill_super() does it's setup work. Essentially we are exposed to > > both use-after-free and use-before-initialisation bugs here. > > > > To fix this, add a check for the SB_BORN flag in super_cache_count. > > In general, this flag is not set until ->fs_mount() completes > > successfully, so we know that it is set after the filesystem > > setup has completed. This matches the trylock_super() behaviour > > which will not let super_cache_scan() run if SB_BORN is not set, and > > hence will not allow the superblock shrinker from entering the > > filesystem while it is being set up or after it has failed setup > > and is being torn down. > > > > Signed-Off-By: Dave Chinner <dchinner@xxxxxxxxxx> > > Umm... Shouldn't that be Cc: stable, or is there something subtle I'm > missing here? I don't test stable kernels and I know that test coverage of stable kernels for filesystem changes is utterly abysmal. Hence I will not push patches on users that are unlikely to be properly tested. You can add a CC: <stable> when you commit it if you want, but I'm not going to do that. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
