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Subject[PATCH] swap: fix setting PAGE_SIZE blocksize during swapoff/swapon race
Date
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@samsung.com>
---
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


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