nfsd: prevent panic for nfsv4.0 closed files in nfs4_show_open

In the Linux kernel, the following vulnerability has been resolved: nfsd: prevent panic for nfsv4.0 closed files in nfs4_show_open Prior to commit 3f29cc82a84c ("nfsd: split sc_status out of sc_type") states_show() relied on sc_type field to be of valid type before calling into a subfunction to show content of a particular stateid. From that commit, we split the validity of the stateid into sc_status and no longer changed sc_type to 0 while unhashing the stateid. This resulted in kernel oopsing for nfsv4.0 opens that stay around and in nfs4_show_open() would derefence sc_file which was NULL. Instead, for closed open stateids forgo displaying information that relies of having a valid sc_file. To reproduce: mount the server with 4.0, read and close a file and then on the server cat /proc/fs/nfsd/clients/2/states [ 513.590804] Call trace: [ 513.590925] _raw_spin_lock+0xcc/0x160 [ 513.591119] nfs4_show_open+0x78/0x2c0 [nfsd] [ 513.591412] states_show+0x44c/0x488 [nfsd] [ 513.591681] seq_read_iter+0x5d8/0x760 [ 513.591896] seq_read+0x188/0x208 [ 513.592075] vfs_read+0x148/0x470 [ 513.592241] ksys_read+0xcc/0x178

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Linux kernel 代码问题漏洞

Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在代码问题漏洞，该漏洞源于在处理已关闭的NFSv4.0文件时未能正确处理状态ID，可能导致内核崩溃。

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