[PATCH] smb: client: fix early procfs exposure race

[Fredric Cover <fredric.cover.lkernel@gmail.com>]

Currently, init_cifs() calls cifs_proc_init() near the beginning of
the function, before INIT_LIST_HEAD, leading to a momentarily
uninitialized, but exposed, procfs. If a userspace process reads this
empty procfs, iterating over a zeroed list_head leads to a NULL
pointer dereference and a kernel oops/panic.

For example, if a delay is added between cifs_proc_init() and
INIT_LIST_HEAD, and a userspace process is polling
/proc/fs/cifs/DebugData, the following oops occurs:

BUG: kernel NULL pointer dereference, address: 0000000000000608
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP PTI
CPU: 0 UID: 0 PID: 3477 Comm: cat Not tainted 7.3.0-02212-g4df4d2a577db-dirty #6 PREEMPT(lazy)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 2025.11-3ubuntu7.3 09/23/2026
RIP: 0010:cifs_debug_data_proc_show+0x103/0xd80 [cifs]
Code: 00 31 d2 eb 25 48 89 df 48 c7 c6 4b d4 67 c0 e8 a3 12 f5 cb 48 8b 54 24 28 48 8b 6d 00 48 81 fd 28 3b 60 c0 0f 84 34 0c 00 00 <48> 83 bd 08 06 00 00 00 75 e5 ff c2 48 8b
4d 38 48 89 df 48 c7 c6
RSP: 0018:ffffb603c03c7c38 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff950240d3f2a0 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffffffffc0672ab7 RDI: ffffffffc0603ad8
RBP: 0000000000000000 R08: 737265767265530a R09: 203a737265767265
R10: 6552205346562065 R11: 203a737473657571 R12: 0000000000000000
R13: ffffb603c03c7d68 R14: ffff950240d3f2a0 R15: 0000000000000001
FS:  00007fdae19d9b00(0000) GS:ffff95032d4bb000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000608 CR3: 000000010638e001 CR4: 0000000000370ef0
Call Trace:
<TASK>
 ? seq_read_iter+0x86/0x400
 ? seq_read_iter+0x86/0x400
 seq_read_iter+0x183/0x400
 proc_reg_read_iter+0x62/0x90
 copy_splice_read+0x215/0x310
 splice_file_to_pipe+0xec/0x1c0
 do_splice+0x4b8/0x780
 __se_sys_splice+0x18f/0x250
 do_syscall_64+0x109/0x330
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
</TASK>

Fix this by moving cifs_proc_init() down in init_cifs(), ensuring it runs
only after all internal lists, locks, counters, and workqueues are fully
ready, but before registering the filesystem. Update the error cleanup
ladder and exit_cifs() to maintain proper LIFO order.

Signed-off-by: Fredric Cover <fredric.cover.lkernel@gmail.com>
---
 fs/smb/client/cifsfs.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
index 576155caf888..1880d5134b1e 100644
--- a/fs/smb/client/cifsfs.c
+++ b/fs/smb/client/cifsfs.c
@@ -2019,7 +2019,6 @@ init_cifs(void)
 	if (rc)
 		return rc;
 
-	cifs_proc_init();
 	INIT_LIST_HEAD(&cifs_tcp_ses_list);
 /*
  *  Initialize Global counters
@@ -2069,8 +2068,7 @@ init_cifs(void)
 				     WQ_FREEZABLE | WQ_MEM_RECLAIM | WQ_PERCPU,
 				     0);
 	if (!cifsiod_wq) {
-		rc = -ENOMEM;
-		goto out_clean_proc;
+		return -ENOMEM;
 	}
 
 	/*
@@ -2162,19 +2160,22 @@ init_cifs(void)
 	if (rc)
 		goto out_cifs_swn_init;
 
+	cifs_proc_init();
+
 	rc = register_filesystem(&cifs_fs_type);
 	if (rc)
-		goto out_init_cifs_idmap;
+		goto out_clean_proc;
 
 	rc = register_filesystem(&smb3_fs_type);
 	if (rc) {
 		unregister_filesystem(&cifs_fs_type);
-		goto out_init_cifs_idmap;
+		goto out_clean_proc;
 	}
 
 	return 0;
 
-out_init_cifs_idmap:
+out_clean_proc:
+	cifs_proc_clean();
 	exit_cifs_idmap();
 out_cifs_swn_init:
 #ifdef CONFIG_CIFS_SWN_UPCALL
@@ -2210,8 +2211,6 @@ init_cifs(void)
 	destroy_workqueue(decrypt_wq);
 out_destroy_cifsiod_wq:
 	destroy_workqueue(cifsiod_wq);
-out_clean_proc:
-	cifs_proc_clean();
 	return rc;
 }
 
@@ -2221,6 +2220,7 @@ exit_cifs(void)
 	cifs_dbg(NOISY, "exit_smb3\n");
 	unregister_filesystem(&cifs_fs_type);
 	unregister_filesystem(&smb3_fs_type);
+	cifs_proc_clean();
 	cifs_release_automount_timer();
 	exit_cifs_idmap();
 #ifdef CONFIG_CIFS_SWN_UPCALL
@@ -2243,7 +2243,6 @@ exit_cifs(void)
 	destroy_workqueue(serverclose_wq);
 	destroy_workqueue(cfid_put_wq);
 	destroy_workqueue(cifsiod_wq);
-	cifs_proc_clean();
 }
 
 MODULE_AUTHOR("Steve French");
-- 
2.53.0