mm/swap: fix race on swap_info reuse between swapoff and swapon

swapoff clear swap_info's SWP_USED flag prematurely and free its
resources after that.  A concurrent swapon will reuse this swap_info
while its previous resources are not cleared completely.

These late freed resources are:
 - p->percpu_cluster
 - swap_cgroup_ctrl[type]
 - block_device setting
 - inode->i_flags &= ~S_SWAPFILE

This patch clears the SWP_USED flag after all its resources are freed,
so that swapon can reuse this swap_info by alloc_swap_info() safely.

[akpm@linux-foundation.org: tidy up code comment]
Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index c6c13b050a58..4a7f7e6992b6 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1923,7 +1923,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
         p->swap_map = NULL;
         cluster_info = p->cluster_info;
         p->cluster_info = NULL;
-        p->flags = 0;
         frontswap_map = frontswap_map_get(p);
         spin_unlock(&p->lock);
         spin_unlock(&swap_lock);
@@ -1949,6 +1948,16 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
                 mutex_unlock(&inode->i_mutex);
         }
         filp_close(swap_file, NULL);
+
+        /*
+         * Clear the SWP_USED flag after all resources are freed so that swapon
+         * can reuse this swap_info in alloc_swap_info() safely.  It is ok to
+         * not hold p->lock after we cleared its SWP_WRITEOK.
+         */
+        spin_lock(&swap_lock);
+        p->flags = 0;
+        spin_unlock(&swap_lock);
+
         err = 0;
         atomic_inc(&proc_poll_event);
         wake_up_interruptible(&proc_poll_wait);