Re: [PATCH] mm: workingset: fix NULL ptr dereference

[Jan Kara]

On Tue 10-04-18 10:22:43, Michal Hocko wrote:
> On Mon 09-04-18 10:58:15, Minchan Kim wrote:
> > Recently, I got a report like below.
> > 
> > [ 7858.792946] [<ffffff80086f4de0>] __list_del_entry+0x30/0xd0
> > [ 7858.792951] [<ffffff8008362018>] list_lru_del+0xac/0x1ac
> > [ 7858.792957] [<ffffff800830f04c>] page_cache_tree_insert+0xd8/0x110
> > [ 7858.792962] [<ffffff8008310188>] __add_to_page_cache_locked+0xf8/0x4e0
> > [ 7858.792967] [<ffffff800830ff34>] add_to_page_cache_lru+0x50/0x1ac
> > [ 7858.792972] [<ffffff800830fdd0>] pagecache_get_page+0x468/0x57c
> > [ 7858.792979] [<ffffff80085d081c>] __get_node_page+0x84/0x764
> > [ 7858.792986] [<ffffff800859cd94>] f2fs_iget+0x264/0xdc8
> > [ 7858.792991] [<ffffff800859ee00>] f2fs_lookup+0x3b4/0x660
> > [ 7858.792998] [<ffffff80083d2540>] lookup_slow+0x1e4/0x348
> > [ 7858.793003] [<ffffff80083d0eb8>] walk_component+0x21c/0x320
> > [ 7858.793008] [<ffffff80083d0010>] path_lookupat+0x90/0x1bc
> > [ 7858.793013] [<ffffff80083cfe6c>] filename_lookup+0x8c/0x1a0
> > [ 7858.793018] [<ffffff80083c52d0>] vfs_fstatat+0x84/0x10c
> > [ 7858.793023] [<ffffff80083c5b00>] SyS_newfstatat+0x28/0x64
> > 
> > v4.9 kenrel already has the d3798ae8c6f3,("mm: filemap: don't
> > plant shadow entries without radix tree node") so I thought
> > it should be okay. When I was googling, I found others report
> > such problem and I think current kernel still has the problem.
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=1431567
> > https://bugzilla.redhat.com/show_bug.cgi?id=1420335
> > 
> > It assumes shadow entry of radix tree relies on the init state
> > that node->private_list allocated should be list_empty state.
> > Currently, it's initailized in SLAB constructor which means
> > node of radix tree would be initialized only when *slub allocates
> > new page*, not *new object*. So, if some FS or subsystem pass
> > gfp_mask to __GFP_ZERO, slub allocator will do memset blindly.
> > That means allocated node can have !list_empty(node->private_list).
> > It ends up calling NULL deference at workingset_update_node by
> > failing list_empty check.
> > 
> > This patch should fix it.
> > 
> > Fixes: 449dd6984d0e ("mm: keep page cache radix tree nodes in check")
> > Reported-by: Chris Fries <cfries@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Jan Kara <jack@xxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> 
> Regardless of whether it makes sense to use __GFP_ZERO from the upper
> layer or not, it is subtle as hell to rely on the pre-existing state
> for a newly allocated object. So yes this makes perfect sense.
> 
> Do we want CC: stable?
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Well, for hot allocations we do rely on previous state a lot. After all
that's what slab constructor was created for. Whether radix tree node
allocation is such a hot path is a question for debate, I agree.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR