[PATCH 4.9 141/206] mm: khugepaged: close use-after-free race during shmem collapsing

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Johannes Weiner <hannes@xxxxxxxxxxx>

commit 91a45f71078a6569ec3ca5bef74e1ab58121d80e upstream.

Patch series "mm: workingset: radix tree subtleties & single-page file
refaults", v3.

This is another revision of the radix tree / workingset patches based on
feedback from Jan and Kirill.

This is a follow-up to d3798ae8c6f3 ("mm: filemap: don't plant shadow
entries without radix tree node").  That patch fixed an issue that was
caused mainly by the page cache sneaking special shadow page entries
into the radix tree and relying on subtleties in the radix tree code to
make that work.  The fix also had to stop tracking refaults for
single-page files because shadow pages stored as direct pointers in
radix_tree_root->rnode weren't properly handled during tree extension.

These patches make the radix tree code explicitely support and track
such special entries, to eliminate the subtleties and to restore the
thrash detection for single-page files.

This patch (of 9):

When a radix tree iteration drops the tree lock, another thread might
swoop in and free the node holding the current slot.  The iteration
needs to do another tree lookup from the current index to continue.

[kirill.shutemov@xxxxxxxxxxxxxxx: re-lookup for replacement]
Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
Link: http://lkml.kernel.org/r/20161117191138.22769-2-hannes@xxxxxxxxxxx
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/khugepaged.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1403,6 +1403,9 @@ static void collapse_shmem(struct mm_str
 
 		spin_lock_irq(&mapping->tree_lock);
 
+		slot = radix_tree_lookup_slot(&mapping->page_tree, index);
+		VM_BUG_ON_PAGE(page != radix_tree_deref_slot_protected(slot,
+					&mapping->tree_lock), page);
 		VM_BUG_ON_PAGE(page_mapped(page), page);
 
 		/*
@@ -1426,6 +1429,7 @@ static void collapse_shmem(struct mm_str
 		radix_tree_replace_slot(slot,
 				new_page + (index % HPAGE_PMD_NR));
 
+		slot = radix_tree_iter_next(&iter);
 		index++;
 		continue;
 out_lru:
@@ -1537,6 +1541,7 @@ tree_unlocked:
 			putback_lru_page(page);
 			unlock_page(page);
 			spin_lock_irq(&mapping->tree_lock);
+			slot = radix_tree_iter_next(&iter);
 		}
 		VM_BUG_ON(nr_none);
 		spin_unlock_irq(&mapping->tree_lock);