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System panic on cifs_reconnect() when the SMB server is down.

This document (000019780) is provided subject to the disclaimer at the end of this document.


SUSE Linux Enterprise Server 15 SP1
SUSE Linux Enterprise Server 15 GA
SUSE Linux Enterprise Server 12 SP5
SUSE Linux Enterprise Server 12 SP4


System with cifs mounts crashes when SMB server is not reachable.

Before the panic the following cifs relates errors are logged:
 CIFS: VFS: \\FS0.GANDALF.LOCAL has not responded in 30 seconds. Reconnecting...
 CIFS VFS: \\FS0.GANDALF.LOCAL cifs_reconnect: no target servers for DFS failover
 BUG: unable to handle kernel paging request at fffffffffffffff8
 IP: cifs_reconnect+0x4a5/0xdf0 [cifs]

The panic has been triggered by cifsd task hitting a NULL pointer on cifs_reconnect() during cifs tcp session reconnection, as (struct dfs_cache_tgt_list) tgt_list has not been initialized correctly because no cache entry was found for a given DFS referral path. The panic task stack trace:
PID: 24194  TASK: ffff9644560f55c0  CPU: 6   COMMAND: "cifsd"
 #0 [ffffb3eb959afa58] machine_kexec at ffffffffae05ec22
 #1 [ffffb3eb959afaa8] __crash_kexec at ffffffffae122b2a
 #2 [ffffb3eb959afb68] crash_kexec at ffffffffae123b59
 #3 [ffffb3eb959afb80] oops_end at ffffffffae02e0a1
 #4 [ffffb3eb959afba0] no_context at ffffffffae06ed4b
 #5 [ffffb3eb959afbf0] __do_page_fault at ffffffffae06f22c
 #6 [ffffb3eb959afc58] do_page_fault at ffffffffae06f68b
 #7 [ffffb3eb959afc80] page_fault at ffffffffae8016b5
    [exception RIP: cifs_reconnect+0x4a5]
    RIP: ffffffffc05b5b65  RSP: ffffb3eb959afd38  RFLAGS: 00010282
    RAX: 0000000000000000  RBX: ffff963e07cac000  RCX: 0000000000000001
    RDX: fffffffffffffff8  RSI: ffffb3eb959afd70  RDI: 0000000000000286
    RBP: 00000000ffffff8d   R8: 0000000000000001   R9: 0000000000000000
    R10: 0000000104be6900  R11: 0000000000000000  R12: ffff963e07cac1c0
    R13: fffffffffffffff8  R14: ffff9648d0b64400  R15: ffffb3eb959afd58
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #8 [ffffb3eb959afdb0] cifs_readv_from_socket at ffffffffc05b6627 [cifs]
 #9 [ffffb3eb959afde8] cifs_read_from_socket at ffffffffc05b672d [cifs]
#10 [ffffb3eb959afe60] cifs_demultiplex_thread at ffffffffc05b6a64 [cifs]
#11 [ffffb3eb959aff10] kthread at ffffffffae0aa40d
#12 [ffffb3eb959aff50] ret_from_fork at ffffffffae800235

De-referencing the failing instruction:
crash> dis -rl ffffffffc0722be5|tail
0xffffffffc0722bc0 <cifs_reconnect+0x480>:      je     0xffffffffc0722ff4 <cifs_reconnect+0x8b4>
/usr/src/debug/kernel-default-4.12.14/linux-4.12/linux-obj/../fs/cifs/connect.c: 409
0xffffffffc0722bc6 <cifs_reconnect+0x486>:      testb  $0x1,0xcb037(%rip)        # 0xffffffffc07edc04 <cifsFYI>
0xffffffffc0722bcd <cifs_reconnect+0x48d>:      je     0xffffffffc0722bdc <cifs_reconnect+0x49c>
0xffffffffc0722bcf <cifs_reconnect+0x48f>:      testb  $0x4,0xc23ec(%rip)        # 0xffffffffc07e4fc2 <descriptor.82423+0x22>
0xffffffffc0722bd6 <cifs_reconnect+0x496>:      jne    0xffffffffc07231ab <cifs_reconnect+0xa6b>
/usr/src/debug/kernel-default-4.12.14/linux-4.12/linux-obj/../fs/cifs/dfs_cache.h: 93
0xffffffffc0722bdc <cifs_reconnect+0x49c>:      test   %r13,%r13
0xffffffffc0722bdf <cifs_reconnect+0x49f>:      je     0xffffffffc0722fd8 <cifs_reconnect+0x898>
0xffffffffc0722be5 <cifs_reconnect+0x4a5>:      mov    0x0(%r13),%rbp

The failing instruction on line 93:
 90 static inline const char *
 91 dfs_cache_get_tgt_name(const struct dfs_cache_tgt_iterator *it)
 92 {
 93     return it ? it->it_name : NULL;
 94 }

The null pointer is encountered while trying to dereference it->it_name:
crash> struct dfs_cache_tgt_iterator -ox
struct dfs_cache_tgt_iterator {
   [0x0] char *it_name;
   [0x8] struct list_head it_list;

0xffffffffc0722be5 <cifs_reconnect+0x4a5>:      mov    0x0(%r13),%rbp

As %r13 holds an invalid address (0xfffffffffffffff8).

 390 static void reconn_inval_dfs_target(struct TCP_Server_Info *server,
 391                     struct cifs_sb_info *cifs_sb,
 392                     struct dfs_cache_tgt_list *tgt_list,
 393                     struct dfs_cache_tgt_iterator **tgt_it)
 394 {
 395     const char *name;
 397     if (!cifs_sb || !cifs_sb->origin_fullpath || !tgt_list ||
 398         !server->nr_targets)
 399         return;
 401     if (!*tgt_it) {
 402         *tgt_it = dfs_cache_get_tgt_iterator(tgt_list);
 403     } else {
 404         *tgt_it = dfs_cache_get_next_tgt(tgt_list, *tgt_it);
 405         if (!*tgt_it)
 406             *tgt_it = dfs_cache_get_tgt_iterator(tgt_list);
 407     }
 409     cifs_dbg(FYI, "%s: UNC: %s\n", __func__, cifs_sb->origin_fullpath);
 411     name = dfs_cache_get_tgt_name(*tgt_it);
         ...   ...   ...

The call chain:
 cifs_reconnect(struct TCP_Server_Info *server)
  reconn_inval_dfs_target(server, cifs_sb, &tgt_list, &tgt_it)
    dfs_cache_get_tgt_name(const struct dfs_cache_tgt_iterator *it)



This bug has been fixed in the cifs kernel module starting with the following kernel releases:
  • SLES 15 SP1 - 4.12.14-197.56.1
  • SLES 15 GA  - 4.12.14-150.58.1
  • SLES 12 SP5 - 4.12.14-122.37.1
  • SLES 12 SP4 - 4.12.14-95.60.1


The panic was triggered by hitting a NULL pointer on cifs_reconnect() during cifs tcp session reconnection, as (struct
dfs_cache_tgt_list) tgt_list has not been initialized correctly because no cache entry was found for a given DFS referral path.


Reported to Engineering


This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.

  • Document ID:000019780
  • Creation Date: 11-Nov-2020
  • Modified Date:11-Nov-2020
    • SUSE Linux Enterprise Server
    • SUSE Linux Enterprise Server for SAP Applications

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