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    Subject[PATCH 5.4 080/147] nfs: fix NULL deference in nfs4_get_valid_delegation
    Date
    From: J. Bruce Fields <bfields@redhat.com>

    [ Upstream commit 29fe839976266bc7c55b927360a1daae57477723 ]

    We add the new state to the nfsi->open_states list, making it
    potentially visible to other threads, before we've finished initializing
    it.

    That wasn't a problem when all the readers were also taking the i_lock
    (as we do here), but since we switched to RCU, there's now a possibility
    that a reader could see the partially initialized state.

    Symptoms observed were a crash when another thread called
    nfs4_get_valid_delegation() on a NULL inode, resulting in an oops like:

    BUG: unable to handle page fault for address: ffffffffffffffb0 ...
    RIP: 0010:nfs4_get_valid_delegation+0x6/0x30 [nfsv4] ...
    Call Trace:
    nfs4_open_prepare+0x80/0x1c0 [nfsv4]
    __rpc_execute+0x75/0x390 [sunrpc]
    ? finish_task_switch+0x75/0x260
    rpc_async_schedule+0x29/0x40 [sunrpc]
    process_one_work+0x1ad/0x370
    worker_thread+0x30/0x390
    ? create_worker+0x1a0/0x1a0
    kthread+0x10c/0x130
    ? kthread_park+0x80/0x80
    ret_from_fork+0x22/0x30

    Fixes: 9ae075fdd190 "NFSv4: Convert open state lookup to use RCU"
    Reviewed-by: Seiichi Ikarashi <s.ikarashi@fujitsu.com>
    Tested-by: Daisuke Matsuda <matsuda-daisuke@fujitsu.com>
    Tested-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
    Signed-off-by: J. Bruce Fields <bfields@redhat.com>
    Cc: stable@vger.kernel.org # v4.20+
    Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/nfs/nfs4state.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
    index b53bcf40e2a77..ea680f619438b 100644
    --- a/fs/nfs/nfs4state.c
    +++ b/fs/nfs/nfs4state.c
    @@ -733,9 +733,9 @@ nfs4_get_open_state(struct inode *inode, struct nfs4_state_owner *owner)
    state = new;
    state->owner = owner;
    atomic_inc(&owner->so_count);
    - list_add_rcu(&state->inode_states, &nfsi->open_states);
    ihold(inode);
    state->inode = inode;
    + list_add_rcu(&state->inode_states, &nfsi->open_states);
    spin_unlock(&inode->i_lock);
    /* Note: The reclaim code dictates that we add stateless
    * and read-only stateids to the end of the list */
    --
    2.20.1


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