In the Linux kernel, the following vulnerability has been resolved: smb: client: pin DFS superblock in iterator callback tcon_super_cb() s

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

smb: client: pin DFS superblock in iterator callback

tcon_super_cb() stores a raw superblock pointer, but __cifs_get_super()
takes its active reference only after iterate_supers_type() has dropped
s_umount and its passive reference. Concurrent DFS automount expiry can
therefore free the superblock before cifs_sb_active() uses it.

A deterministic KASAN test reproduces the race as:

BUG: KASAN: slab-use-after-free in cifs_sb_active+0x77/0x80

The same test passes with this change applied.

Take the active reference in the callback while iterate_supers_type()
still holds s_umount shared. cifs_put_tcp_super() remains the matching
release.