[PATCH] swap: fix setting PAGE_SIZE blocksize during swapoff/swaponrace

[Krzysztof Kozlowski]

Fix race between swapoff and swapon resulting in setting blocksize of
PAGE_SIZE for block devices during swapoff.

The swapon modifies swap_info->old_block_size before acquiring
swapon_mutex. It reads block_size of bdev, stores it under
swap_info->old_block_size and sets new block_size to PAGE_SIZE.

On the other hand the swapoff sets the device's block_size to
old_block_size after releasing swapon_mutex.

This patch locks the swapon_mutex much earlier during swapon. It also
releases the swapon_mutex later during swapoff.

The effect of race can be triggered by following scenario:
 - One block swap device with block size of 512
 - thread 1: Swapon is called, swap is activated,
   p->old_block_size = block_size(p->bdev); /512/
   block_size(p->bdev) = PAGE_SIZE;
   Thread ends.

 - thread 2: Swapoff is called and it goes just after releasing the
   swapon_mutex. The swap is now fully disabled except of setting the
   block size to old value. The p->bdev->block_size is still equal to
   PAGE_SIZE.

 - thread 3: New swapon is called. This swap is disabled so without
   acquiring the swapon_mutex:
   - p->old_block_size = block_size(p->bdev); /PAGE_SIZE (!!!)/
   - block_size(p->bdev) = PAGE_SIZE;
   Swap is activated and thread ends.

 - thread 2: resumes work and sets blocksize to old value:
   - set_blocksize(bdev, p->old_block_size)
   But now the p->old_block_size is equal to PAGE_SIZE.

The patch swap-fix-set_blocksize-race-during-swapon-swapoff does not fix
this particular issue. It reduces the possibility of races as the swapon
must overwrite p->old_block_size before acquiring swapon_mutex in
swapoff.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>
---
 mm/swapfile.c |   20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 3963fc2..9b64ef4 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1926,7 +1926,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	spin_unlock(&p->lock);
 	spin_unlock(&swap_lock);
 	frontswap_invalidate_area(type);
-	mutex_unlock(&swapon_mutex);
 	free_percpu(p->percpu_cluster);
 	p->percpu_cluster = NULL;
 	vfree(swap_map);
@@ -1946,6 +1945,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 		mutex_unlock(&inode->i_mutex);
 	}
 	filp_close(swap_file, NULL);
+	mutex_unlock(&swapon_mutex);
 	err = 0;
 	atomic_inc(&proc_poll_event);
 	wake_up_interruptible(&proc_poll_wait);
@@ -2402,37 +2402,38 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		}
 	}
 
+	mutex_lock(&swapon_mutex);
 	inode = mapping->host;
 	/* If S_ISREG(inode->i_mode) will do mutex_lock(&inode->i_mutex); */
 	error = claim_swapfile(p, inode);
 	if (unlikely(error))
-		goto bad_swap;
+		goto bad_swap_wmutex;
 
 	/*
 	 * Read the swap header.
 	 */
 	if (!mapping->a_ops->readpage) {
 		error = -EINVAL;
-		goto bad_swap;
+		goto bad_swap_wmutex;
 	}
 	page = read_mapping_page(mapping, 0, swap_file);
 	if (IS_ERR(page)) {
 		error = PTR_ERR(page);
-		goto bad_swap;
+		goto bad_swap_wmutex;
 	}
 	swap_header = kmap(page);
 
 	maxpages = read_swap_header(p, swap_header, inode);
 	if (unlikely(!maxpages)) {
 		error = -EINVAL;
-		goto bad_swap;
+		goto bad_swap_wmutex;
 	}
 
 	/* OK, set up the swap map and apply the bad block list */
 	swap_map = vzalloc(maxpages);
 	if (!swap_map) {
 		error = -ENOMEM;
-		goto bad_swap;
+		goto bad_swap_wmutex;
 	}
 	if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
 		p->flags |= SWP_SOLIDSTATE;
@@ -2462,13 +2463,13 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 
 	error = swap_cgroup_swapon(p->type, maxpages);
 	if (error)
-		goto bad_swap;
+		goto bad_swap_wmutex;
 
 	nr_extents = setup_swap_map_and_extents(p, swap_header, swap_map,
 		cluster_info, maxpages, &span);
 	if (unlikely(nr_extents < 0)) {
 		error = nr_extents;
-		goto bad_swap;
+		goto bad_swap_wmutex;
 	}
 	/* frontswap enabled? set up bit-per-page map for frontswap */
 	if (frontswap_enabled)
@@ -2504,7 +2505,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		}
 	}
 
-	mutex_lock(&swapon_mutex);
 	prio = -1;
 	if (swap_flags & SWAP_FLAG_PREFER)
 		prio =
@@ -2529,6 +2529,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		inode->i_flags |= S_SWAPFILE;
 	error = 0;
 	goto out;
+bad_swap_wmutex:
+	mutex_unlock(&swapon_mutex);
 bad_swap:
 	free_percpu(p->percpu_cluster);
 	p->percpu_cluster = NULL;
-- 
1.7.9.5

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/