CVE-2026-99171

In the Linux kernel, the following vulnerability has been resolved:

ntfs3: free the sbi after a grace period

If the "nocase" mount option ntfs3 uses custom hash and dentry compare
functions. Both access ntfs3 superblock private information in rcu
pathwalk. ntfs_put_super() and ntfs3_kill_sb() drop the info without any
rcu delay. A lookup that's still inside the filesystem when the last
reference to the mount is dropped causes a UAF. With a pathwalk held up
in ntfs_d_hash() while the last reference to the mount goes away:

  BUG: KASAN: slab-use-after-free in ntfs_nls_to_utf16+0x752/0x940
  Read of size 8 at addr ff1100010453c850 by task walker/99
  Call Trace:
   <TASK>
   ntfs_nls_to_utf16+0x752/0x940
   ntfs_d_hash+0x22b/0x500
   link_path_walk+0x11ad/0x1d30
   path_lookupat+0x63/0xb00
   filename_lookup+0x1f1/0x5b0
   vfs_statx+0xd7/0x3b0
  [...]
  Allocated by task 95:
   __kmalloc_cache_noprof+0x249/0x560
   ntfs_init_fs_context+0x2db/0x5a0
   alloc_fs_context+0x52e/0xc70
  [...]
  Freed by task 98:
   kfree+0x1db/0x5b0
   deactivate_locked_super+0x9c/0xf0
   cleanup_mnt+0x9f/0x130
   task_work_run+0x131/0x250

The bug is currently masked by the grace period in namespace_unlock()
for umount(2). Keep the necessary information until the filesystem
private info is freed and free it with an rcu delay.

ntfs3_free_sbi() now runs from an rcu callback and drops the upcase
table via ntfs_put_shared(). So switch s_shared_lock over to
spin_lock_bh(). Otherwise the callback can deadlock against a task
holding the lock on the same cpu.