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nbd: fix null-ptr-dereference while accessing 'nbd->config'

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Memory reordering may occur in nbd_genl_connect(), causing config_refs
to be set to 1 while nbd->config is still empty. Opening nbd at this
time will cause null-ptr-dereference.

   T1                      T2
   nbd_open
    nbd_get_config_unlocked
                 	   nbd_genl_connect
                 	    nbd_alloc_and_init_config
                 	     //memory reordered
                  	     refcount_set(&nbd->config_refs, 1)  // 2
     nbd->config
      ->null point
			     nbd->config = config  // 1

Fix it by adding smp barrier to guarantee the execution sequence.

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20231116162316.1740402-4-linan666@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Li Nan 2023-11-17 00:23:16 +08:00 committed by Jens Axboe
parent 3123ac7792
commit c2da049f41
1 changed files with 17 additions and 1 deletions
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18
drivers/block/nbd.c
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@ -397,8 +397,16 @@ static u32 req_to_nbd_cmd_type(struct request *req)
static struct nbd_config *nbd_get_config_unlocked(struct nbd_device *nbd)
{
if (refcount_inc_not_zero(&nbd->config_refs))
if (refcount_inc_not_zero(&nbd->config_refs)) {
/*
* Add smp_mb__after_atomic to ensure that reading nbd->config_refs
* and reading nbd->config is ordered. The pair is the barrier in
* nbd_alloc_and_init_config(), avoid nbd->config_refs is set
* before nbd->config.
*/
smp_mb__after_atomic();
return nbd->config;
}
return NULL;
}
@ -1559,7 +1567,15 @@ static int nbd_alloc_and_init_config(struct nbd_device *nbd)
init_waitqueue_head(&config->conn_wait);
config->blksize_bits = NBD_DEF_BLKSIZE_BITS;
atomic_set(&config->live_connections, 0);
nbd->config = config;
/*
* Order refcount_set(&nbd->config_refs, 1) and nbd->config assignment,
* its pair is the barrier in nbd_get_config_unlocked().
* So nbd_get_config_unlocked() won't see nbd->config as null after
* refcount_inc_not_zero() succeed.
*/
smp_mb__before_atomic();
refcount_set(&nbd->config_refs, 1);
return 0;