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    SubjectRe: [PATCH] mm: workingset: fix NULL ptr dereference
    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
    +
    return __radix_tree_preload(gfp_mask, RADIX_TREE_PRELOAD_SIZE);
    }
    EXPORT_SYMBOL(radix_tree_preload);
    @@ -522,7 +532,7 @@ EXPORT_SYMBOL(radix_tree_preload);
    */
    int radix_tree_maybe_preload(gfp_t gfp_mask)
    {
    - if (gfpflags_allow_blocking(gfp_mask))
    + if (gfpflags_allow_blocking(gfp_mask) && !(gfp_mask & __GFP_ZERO))
    return __radix_tree_preload(gfp_mask, RADIX_TREE_PRELOAD_SIZE);
    /* Preloading doesn't help anything with this gfp mask, skip it */
    preempt_disable();
    --
    2.17.0.484.g0c8726318c-goog

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