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

[Minchan Kim <minchan@kernel.org>]

On Tue, Apr 10, 2018 at 11:33:39AM +0900, Minchan Kim wrote:
> On Mon, Apr 09, 2018 at 06:12:11PM -0700, Matthew Wilcox wrote:
> > On Tue, Apr 10, 2018 at 08:04:09AM +0900, Minchan Kim wrote:
> > > On Mon, Apr 09, 2018 at 08:20:32AM -0700, Matthew Wilcox wrote:
> > > > I don't think this is something the radix tree should know about.
> > > 
> > > Because shadow entry implementation is hidden by radix tree implemetation.
> > > IOW, radix tree user cannot know how it works.
> > 
> > I have no idea what you mean.
> > 
> > > > SLAB should be checking for it (the patch I posted earlier in this
> > > 
> > > I don't think it's right approach. SLAB constructor can initialize
> > > some metadata for slab page populated as well as page zeroing.
> > > However, __GFP_ZERO means only clearing pages, not metadata.
> > > So it's different semantic. No need to mix out.
> > 
> > No, __GFP_ZERO is specified to clear the allocated memory whether
> > you're allocating from alloc_pages or from slab.  What makes no sense
> > is allocating an object from slab with a constructor *and* __GFP_ZERO.
> > They're in conflict, and slab can't fulfill both of those requirements.
> 
> It's a stable material. If you really think it does make sense,
> please submit patch separately.
> 
> > 
> > > > thread), but the right place to filter this out is in the caller of
> > > > radix_tree_maybe_preload -- it's already filtering out HIGHMEM pages,
> > > > and should filter out GFP_ZERO too.
> > > 
> > > radix_tree_[maybe]_preload is exported API, which are error-prone
> > > for out of modules or upcoming customers.
> > > 
> > > More proper place is __radix_tree_preload.
> > 
> > I could not disagree with you more.  It is the responsibility of the
> > callers of radix_tree_preload to avoid calling it with nonsense flags
> > like __GFP_DMA, __GFP_HIGHMEM or __GFP_ZERO.
> 
> How about this?
> 
> It would fix current problem and warn potential bugs as well.
> radix_tree_preload already has done such warning and
> radix_tree_maybe_preload has skipping for misbehaivor gfp.
> 
> From 27ecf7a009d3570d1155c528c7f08040ede68ed3 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@kernel.org>
> Date: Tue, 10 Apr 2018 11:20:11 +0900
> Subject: [PATCH v2] mm: workingset: fix NULL ptr dereference
> 
> It assumes shadow entries of radix tree rely on the init state
> that node->private_list allocated newly is list_empty state
> for the working. Currently, it's initailized in SLAB constructor
> which means node of radix tree would be initialized only when
> *slub allocates new page*, not *slub alloctes new object*.
> 
> If some FS or subsystem pass gfp_mask to __GFP_ZERO, that means
> newly allocated node can have !list_empty(node->private_list)
> by memset of slab allocator. It ends up calling NULL deference
> at workingset_update_node by failing list_empty check.
> 
> This patch fixes it.
> 
> Fixes: 449dd6984d0e ("mm: keep page cache radix tree nodes in check")
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Jaegeuk Kim <jaegeuk@kernel.org>
> Cc: Chao Yu <yuchao0@huawei.com>
> Cc: Christopher Lameter <cl@linux.com>
> Cc: linux-fsdevel@vger.kernel.org
> Reported-by: Chris Fries <cfries@google.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  lib/radix-tree.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
> index da9e10c827df..9d68f2a7888e 100644
> --- a/lib/radix-tree.c
> +++ b/lib/radix-tree.c
> @@ -511,6 +511,16 @@ int radix_tree_preload(gfp_t gfp_mask)
>  {
>  	/* Warn on non-sensical use... */
>  	WARN_ON_ONCE(!gfpflags_allow_blocking(gfp_mask));
> +	/*
> +	 * New allocate node must have node->private_list as INIT_LIST_HEAD
> +	 * state by workingset shadow memory implementation.
> +	 * If user pass  __GFP_ZERO by mistake, slab allocator will clear
> +	 * node->private_list, which makes a BUG. Rather than going Oops,
> +	 * just fix and warn about it.
> +	 */
> +	if (WARN_ON(gfp_mask & __GFP_ZERO))
> +		gfp_mask &= ~GFP_ZERO
 
Build fail.

If others are okay for this patch, I will resend fixed patch with stable mark.
I will wait feedback from others.

Thanks.