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Subject[PATCH 3.5 13/60] mm/swap: fix race on swap_info reuse between swapoff and swapon
Date
3.5.7.31 -stable review patch.  If anyone has any objections, please let me know.

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

From: Weijie Yang <weijie.yang@samsung.com>

commit f893ab41e4dae2fe8991faf5d86d029068d1ef3a upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[ luis: backported to 3.5: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
mm/swapfile.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 71373d0..222928f 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1642,7 +1642,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
p->max = 0;
swap_map = p->swap_map;
p->swap_map = NULL;
- p->flags = 0;
frontswap_invalidate_area(type);
spin_unlock(&swap_lock);
mutex_unlock(&swapon_mutex);
@@ -1662,6 +1661,16 @@ 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);
--
1.9.0


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