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Subject[PATCH 4.4 019/241] bcache: fix a race between cache register and cacheset unregister
Date
From: Liang Chen <liangchen.linux@gmail.com>

commit a4b732a248d12cbdb46999daf0bf288c011335eb upstream.

There is a race between cache device register and cache set unregister.
For an already registered cache device, register_bcache will call
bch_is_open to iterate through all cachesets and check every cache
there. The race occurs if cache_set_free executes at the same time and
clears the caches right before ca is dereferenced in bch_is_open_cache.
To close the race, let's make sure the clean up work is protected by
the bch_register_lock as well.

This issue can be reproduced as follows,
while true; do echo /dev/XXX> /sys/fs/bcache/register ; done&
while true; do echo 1> /sys/block/XXX/bcache/set/unregister ; done &

and results in the following oops,

[ +0.000053] BUG: unable to handle kernel NULL pointer dereference at 0000000000000998
[ +0.000457] #PF error: [normal kernel read fault]
[ +0.000464] PGD 800000003ca9d067 P4D 800000003ca9d067 PUD 3ca9c067 PMD 0
[ +0.000388] Oops: 0000 [#1] SMP PTI
[ +0.000269] CPU: 1 PID: 3266 Comm: bash Not tainted 5.0.0+ #6
[ +0.000346] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.fc28 04/01/2014
[ +0.000472] RIP: 0010:register_bcache+0x1829/0x1990 [bcache]
[ +0.000344] Code: b0 48 83 e8 50 48 81 fa e0 e1 10 c0 0f 84 a9 00 00 00 48 89 c6 48 89 ca 0f b7 ba 54 04 00 00 4c 8b 82 60 0c 00 00 85 ff 74 2f <49> 3b a8 98 09 00 00 74 4e 44 8d 47 ff 31 ff 49 c1 e0 03 eb 0d
[ +0.000839] RSP: 0018:ffff92ee804cbd88 EFLAGS: 00010202
[ +0.000328] RAX: ffffffffc010e190 RBX: ffff918b5c6b5000 RCX: ffff918b7d8e0000
[ +0.000399] RDX: ffff918b7d8e0000 RSI: ffffffffc010e190 RDI: 0000000000000001
[ +0.000398] RBP: ffff918b7d318340 R08: 0000000000000000 R09: ffffffffb9bd2d7a
[ +0.000385] R10: ffff918b7eb253c0 R11: ffffb95980f51200 R12: ffffffffc010e1a0
[ +0.000411] R13: fffffffffffffff2 R14: 000000000000000b R15: ffff918b7e232620
[ +0.000384] FS: 00007f955bec2740(0000) GS:ffff918b7eb00000(0000) knlGS:0000000000000000
[ +0.000420] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ +0.000801] CR2: 0000000000000998 CR3: 000000003cad6000 CR4: 00000000001406e0
[ +0.000837] Call Trace:
[ +0.000682] ? _cond_resched+0x10/0x20
[ +0.000691] ? __kmalloc+0x131/0x1b0
[ +0.000710] kernfs_fop_write+0xfa/0x170
[ +0.000733] __vfs_write+0x2e/0x190
[ +0.000688] ? inode_security+0x10/0x30
[ +0.000698] ? selinux_file_permission+0xd2/0x120
[ +0.000752] ? security_file_permission+0x2b/0x100
[ +0.000753] vfs_write+0xa8/0x1a0
[ +0.000676] ksys_write+0x4d/0xb0
[ +0.000699] do_syscall_64+0x3a/0xf0
[ +0.000692] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/md/bcache/super.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1355,6 +1355,7 @@ static void cache_set_free(struct closur
bch_btree_cache_free(c);
bch_journal_free(c);

+ mutex_lock(&bch_register_lock);
for_each_cache(ca, c, i)
if (ca) {
ca->set = NULL;
@@ -1377,7 +1378,6 @@ static void cache_set_free(struct closur
mempool_destroy(c->search);
kfree(c->devices);

- mutex_lock(&bch_register_lock);
list_del(&c->list);
mutex_unlock(&bch_register_lock);


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