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Subject[PATCH] bcache: fix NULL pointer reference in cached_dev_detach_finish
Date
Commit 0259d4498ba484("bcache: move calc_cached_dev_sectors to proper
place on backing device detach") tries to fix calc_cached_dev_sectors
when bcache device detaches, but now we have:

cached_dev_detach_finish
...
bcache_device_detach(&dc->disk);
...
closure_put(&d->c->caching);
d->c = NULL; [*explicitly set dc->disk.c to NULL*]
list_move(&dc->list, &uncached_devices);
calc_cached_dev_sectors(dc->disk.c); [*passing a NULL pointer*]
...

Upper codeflows shows how bug happens, this patch fix the problem by
caching dc->disk.c beforehand, and cache_set won't be freed under us
because c->caching closure at least holds a reference count and closure
callback __cache_set_unregister only being called by bch_cache_set_stop
which using closure_queue(&c->caching), that means c->caching closure
callback for destroying cache_set won't be trigger by previous
closure_put(&d->c->caching).
So at this stage(while cached_dev_detach_finish is calling) it's safe to
access cache_set dc->disk.c.

Fixes: 0259d4498ba484("bcache: move calc_cached_dev_sectors to proper place on backing device detach")
Signed-off-by: Lin Feng <linf@wangsu.com>
---
drivers/md/bcache/super.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 4a9a65dff95e..3d9bc7cd27f8 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1139,6 +1139,7 @@ static void cancel_writeback_rate_update_dwork(struct cached_dev *dc)
static void cached_dev_detach_finish(struct work_struct *w)
{
struct cached_dev *dc = container_of(w, struct cached_dev, detach);
+ struct cache_set *c = dc->disk.c;

BUG_ON(!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags));
BUG_ON(refcount_read(&dc->count));
@@ -1156,7 +1157,7 @@ static void cached_dev_detach_finish(struct work_struct *w)

bcache_device_detach(&dc->disk);
list_move(&dc->list, &uncached_devices);
- calc_cached_dev_sectors(dc->disk.c);
+ calc_cached_dev_sectors(c);

clear_bit(BCACHE_DEV_DETACHING, &dc->disk.flags);
clear_bit(BCACHE_DEV_UNLINK_DONE, &dc->disk.flags);
--
2.31.1
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