Re: [PATCH v4] SUNRPC: ensure the matching upcall is in-flight upon downcall

[Trond Myklebust <trondmy@kernel.org>]

On Tue, 2022-12-13 at 13:14 +0900, minoura makoto wrote:
> Commit 9130b8dbc6ac ("SUNRPC: allow for upcalls for the same uid
> but different gss service") introduced `auth` argument to
> __gss_find_upcall(), but in gss_pipe_downcall() it was left as NULL
> since it (and auth->service) was not (yet) determined.
> 
> When multiple upcalls with the same uid and different service are
> ongoing, it could happen that __gss_find_upcall(), which returns the
> first match found in the pipe->in_downcall list, could not find the
> correct gss_msg corresponding to the downcall we are looking for.
> Moreover, it might return a msg which is not sent to rpc.gssd yet.
> 
> We could see mount.nfs process hung in D state with multiple
> mount.nfs
> are executed in parallel.  The call trace below is of CentOS 7.9
> kernel-3.10.0-1160.24.1.el7.x86_64 but we observed the same hang w/
> elrepo kernel-ml-6.0.7-1.el7.
> 
> PID: 71258  TASK: ffff91ebd4be0000  CPU: 36  COMMAND: "mount.nfs"
>  #0 [ffff9203ca3234f8] __schedule at ffffffffa3b8899f
>  #1 [ffff9203ca323580] schedule at ffffffffa3b88eb9
>  #2 [ffff9203ca323590] gss_cred_init at ffffffffc0355818
> [auth_rpcgss]
>  #3 [ffff9203ca323658] rpcauth_lookup_credcache at ffffffffc0421ebc
> [sunrpc]
>  #4 [ffff9203ca3236d8] gss_lookup_cred at ffffffffc0353633
> [auth_rpcgss]
>  #5 [ffff9203ca3236e8] rpcauth_lookupcred at ffffffffc0421581
> [sunrpc]
>  #6 [ffff9203ca323740] rpcauth_refreshcred at ffffffffc04223d3
> [sunrpc]
>  #7 [ffff9203ca3237a0] call_refresh at ffffffffc04103dc [sunrpc]
>  #8 [ffff9203ca3237b8] __rpc_execute at ffffffffc041e1c9 [sunrpc]
>  #9 [ffff9203ca323820] rpc_execute at ffffffffc0420a48 [sunrpc]
> 
> The scenario is like this. Let's say there are two upcalls for
> services A and B, A -> B in pipe->in_downcall, B -> A in pipe->pipe.
> 
> When rpc.gssd reads pipe to get the upcall msg corresponding to
> service B from pipe->pipe and then writes the response, in
> gss_pipe_downcall the msg corresponding to service A will be picked
> because only uid is used to find the msg and it is before the one for
> B in pipe->in_downcall.  And the process waiting for the msg
> corresponding to service A will be woken up.
> 
> Actual scheduing of that process might be after rpc.gssd processes
> the
> next msg.  In rpc_pipe_generic_upcall it clears msg->errno (for A).
> The process is scheduled to see gss_msg->ctx == NULL and
> gss_msg->msg.errno == 0, therefore it cannot break the loop in
> gss_create_upcall and is never woken up after that.
> 
> This patch adds a simple check to ensure that a msg which is not
> sent to rpc.gssd yet is not chosen as the matching upcall upon
> receiving a downcall.
> 
> Fixes: Commit 9130b8dbc6ac ("SUNRPC: allow for upcalls for the same
> uid
> but different gss service")
> Signed-off-by: minoura makoto <minoura@valinux.co.jp>
> Signed-off-by: Hiroshi Shimamoto <h-shimamoto@nec.com>
> Tested-by: Hiroshi Shimamoto <h-shimamoto@nec.com>
> 

Thanks! I'm queuing this up for the next bugfix pull.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com