[PATCH v3] ocfs2: rebase copied fsdlm LVB pointers in locking_state

The locking_state debugfs iterator snapshots struct ocfs2_lock_res by
value under ocfs2_dlm_tracking_lock and later formats that copy in
ocfs2_dlm_seq_show(). That is fine for the inline fields, but the
userspace fsdlm stack stores the LVB through lksb_fsdlm.sb_lvbptr. Once
the iterator drops the tracking lock, a copied non-NULL sb_lvbptr still
points into the original lockres owner, so teardown can free that
container before the debugfs dump walks the raw LVB bytes.

Rebase the copied sb_lvbptr to the copied l_lksb before dumping the raw
LVB. The seq snapshot already carries the inline LVB storage reserved in
struct ocfs2_dlm_lksb, so the debugfs reader can dump the copied bytes
without borrowing the original lockres lifetime.

The buggy scenario involves two paths, with each column showing the order
within that path:

locking_state reader:                  lockres teardown:
1. ocfs2_dlm_seq_start()/next()        1. file release or another owner
   copies struct ocfs2_lock_res           teardown reaches
2. ocfs2_dlm_seq_show() formats           ocfs2_lock_res_free()
   the copied row                      2. the lockres is removed from the
3. ocfs2_dlm_lvb() follows the            tracking list
   copied sb_lvbptr                   3. the owner frees the original
                                          lockres container

Validation reproduced this kernel report:
KASAN slab-use-after-free in ocfs2_dlm_seq_show+0x1bd/0x430
RIP: 0033:0x7f8ec4b1e29d
The buggy address belongs to the object at ffff88810a1e0800 which belongs
to the cache kmalloc-1k of size 1024
The buggy address is located 368 bytes inside of freed 1024-byte region
[ffff88810a1e0800, ffff88810a1e0c00)
Read of size 1
Call trace:
  dump_stack_lvl+0x66/0xa0
  print_report+0xce/0x630
  ocfs2_dlm_seq_show+0x1bd/0x430 (fs/ocfs2/dlmglue.c:3137)
  srso_alias_return_thunk+0x5/0xfbef5
  __virt_addr_valid+0x19f/0x330
  kasan_report+0xe0/0x110
  seq_read_iter+0x29d/0x790
  seq_read+0x20a/0x280
  find_held_lock+0x2b/0x80
  rcu_read_unlock+0x18/0x70
  full_proxy_read+0x9e/0xd0
  vfs_read+0x12c/0x590
  ksys_read+0xd2/0x170
  do_user_addr_fault+0x65a/0x890
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
Allocated by task stack:
  kasan_save_stack+0x33/0x60
  kasan_save_track+0x14/0x30
  __kasan_kmalloc+0xaa/0xb0
  ocfs2_file_open+0x13e/0x300
  do_dentry_open+0x233/0x7f0
  vfs_open+0x5a/0x1b0
  path_openat+0x66d/0x1540
  do_file_open+0x186/0x2b0
  do_sys_openat2+0xce/0x150
  __x64_sys_openat+0xd0/0x140
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task stack:
  kasan_save_stack+0x33/0x60
  kasan_save_track+0x14/0x30
  kasan_save_free_info+0x3b/0x60
  __kasan_slab_free+0x5f/0x80
  kfree+0x313/0x590
  ocfs2_file_release+0x138/0x260
  __fput+0x1df/0x4b0
  fput_close_sync+0xd2/0x170
  __x64_sys_close+0x55/0x90
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: cf4d8d75d8ab ("ocfs2: add fsdlm to stackglue")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Zhang Cen <[email protected]>
---
v3:
Fix the truncated commit log in v2.
Include the complete KASAN use, allocation, and free stacks from validation.
No code changes.

v2:
Document the introducing fsdlm stackglue commit.

 fs/ocfs2/dlmglue.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
index 7283bb2c5a31..a23dd8f86c89 100644
--- a/fs/ocfs2/dlmglue.c
+++ b/fs/ocfs2/dlmglue.c
@@ -3134,6 +3134,22 @@ static void *ocfs2_dlm_seq_next(struct seq_file *m, void *v, loff_t *pos)
  *	- Add last pr/ex unlock times and first lock wait time in usecs
  */
 #define OCFS2_DLM_DEBUG_STR_VERSION 4
+
+/*
+ * The debug iterator snapshots lockres by value, so a userspace-stack LVB
+ * pointer copied from the original lockres must be rebased to the copied
+ * lksb before the dump walks the raw bytes.
+ */
+static void ocfs2_dlm_seq_rebase_lvb(struct ocfs2_lock_res *lockres)
+{
+	if (!ocfs2_stack_supports_plocks())
+		return;
+
+	if (lockres->l_lksb.lksb_fsdlm.sb_lvbptr)
+		lockres->l_lksb.lksb_fsdlm.sb_lvbptr =
+			(char *)&lockres->l_lksb + sizeof(struct dlm_lksb);
+}
+
 static int ocfs2_dlm_seq_show(struct seq_file *m, void *v)
 {
 	int i;
@@ -3191,6 +3207,7 @@ static int ocfs2_dlm_seq_show(struct seq_file *m, void *v)
 		   lockres->l_blocking);
 
 	/* Dump the raw LVB */
+	ocfs2_dlm_seq_rebase_lvb(lockres);
 	lvb = ocfs2_dlm_lvb(&lockres->l_lksb);
 	for(i = 0; i < DLM_LVB_LEN; i++)
 		seq_printf(m, "0x%x\t", lvb[i]);
-- 
2.43.0