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Date:   Tue, 10 Apr 2018 11:32:41 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Jan Kara <jack@suse.cz>
Cc:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Chris Fries <cfries@google.com>
Subject: Re: [PATCH] mm: workingset: fix NULL ptr dereference
Message-ID: <20180410093241.GA21835@dhcp22.suse.cz>
References: <20180409015815.235943-1-minchan@kernel.org>
 <20180410082243.GW21835@dhcp22.suse.cz>
 <20180410085531.m2xvzi7nenbrgbve@quack2.suse.cz>
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On Tue 10-04-18 10:55:31, Jan Kara wrote:
> 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@google.com>
> > > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > > Cc: Jan Kara <jack@suse.cz>
> > > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > 
> > 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@suse.com>
> 
> 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.

I really doubt that LIST_INIT is something to notice for the radix tree
allocation. So I would rather have safe code than rely on the previous
state which is really subtle.

Btw. I am not a huge fan of ctor semantic as we have it. I am not really
sure all users understand when it is called...
-- 
Michal Hocko
SUSE Labs