[PATCH] mm: mempolicy: fix the wrong return value and potential pages leak of mbind

[Yang Shi]

The commit d883544515aa ("mm: mempolicy: make the behavior consistent
when MPOL_MF_MOVE* and MPOL_MF_STRICT were specified") fixed the return
value of mbind() for a couple of corner cases.  But, it altered the
errno for some other cases, for example, mbind() should return -EFAULT
when part or all of the memory range specified by nodemask and maxnode
points  outside your accessible address space, or there was an unmapped
hole in the specified memory range specified by addr and len.

Fixed this by preserving the errno returned by queue_pages_range().
And, the pagelist may be not empty even though queue_pages_range()
returns error, put the pages back to LRU since mbind_range() is not called
to really apply the policy so those pages should not be migrated, this
is also the old behavior before the problematic commit.

Reported-by: Li Xinhai <lixinhai.lxh@xxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> v4.19 and v5.2+
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
 mm/mempolicy.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 4ae967b..e08c941 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -672,7 +672,9 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
  * 1 - there is unmovable page, but MPOL_MF_MOVE* & MPOL_MF_STRICT were
  *     specified.
  * 0 - queue pages successfully or no misplaced page.
- * -EIO - there is misplaced page and only MPOL_MF_STRICT was specified.
+ * errno - i.e. misplaced pages with MPOL_MF_STRICT specified (-EIO) or
+ *         memory range specified by nodemask and maxnode points outside
+ *         your accessible address space (-EFAULT)
  */
 static int
 queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
@@ -1286,7 +1288,7 @@ static long do_mbind(unsigned long start, unsigned long len,
 			  flags | MPOL_MF_INVERT, &pagelist);
 
 	if (ret < 0) {
-		err = -EIO;
+		err = ret;
 		goto up_out;
 	}
 
@@ -1305,10 +1307,12 @@ static long do_mbind(unsigned long start, unsigned long len,
 
 		if ((ret > 0) || (nr_failed && (flags & MPOL_MF_STRICT)))
 			err = -EIO;
-	} else
-		putback_movable_pages(&pagelist);
-
+	} else {
 up_out:
+		if (!list_empty(&pagelist))
+			putback_movable_pages(&pagelist);
+	}
+
 	up_write(&mm->mmap_sem);
 mpol_out:
 	mpol_put(new);
-- 
1.8.3.1