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

cifs: fix DFS traversal oops without CONFIG_CIFS_DFS_UPCALL

When compiled with CONFIG_CIFS_DFS_UPCALL disabled, cifs_dfs_d_automount
is NULL. cifs.ko logic for mapping CIFS_FATTR_DFS_REFERRAL attributes to
S_AUTOMOUNT and corresponding dentry flags is retained regardless of
CONFIG_CIFS_DFS_UPCALL, leading to a NULL pointer dereference in
VFS follow_automount() when traversing a DFS referral link:
BUG: kernel NULL pointer dereference, address: 0000000000000000
...
Call Trace:

__traverse_mounts+0xb5/0x220
? cifs_revalidate_mapping+0x65/0xc0 [cifs]
step_into+0x195/0x610
? lookup_fast+0xe2/0xf0
path_lookupat+0x64/0x140
filename_lookup+0xc2/0x140
? __create_object+0x299/0x380
? kmem_cache_alloc+0x119/0x220
? user_path_at_empty+0x31/0x50
user_path_at_empty+0x31/0x50
__x64_sys_chdir+0x2a/0xd0
? exit_to_user_mode_prepare+0xca/0x100
do_syscall_64+0x42/0x90
entry_SYSCALL_64_after_hwframe+0x72/0xdc

This fix adds an inline cifs_dfs_d_automount() {return -EREMOTE} handler
when CONFIG_CIFS_DFS_UPCALL is disabled. An alternative would be to
avoid flagging S_AUTOMOUNT, etc. without CONFIG_CIFS_DFS_UPCALL. This
approach was chosen as it provides more control over the error path.

References

• https://nvd.nist.gov/vuln/detail/CVE-2023-53246
• https://git.kernel.org/stable/c/179a88a8558bbf42991d361595281f3e45d7edfc
• https://git.kernel.org/stable/c/1e144b68208e98fd4602c842a7149ba5f41d87fb
• https://git.kernel.org/stable/c/26a32a212bc540f4773cd6af8cf73e967d72569c
• https://git.kernel.org/stable/c/657d7c215ca974d366ab1808213f716e1e3aa950
• https://git.kernel.org/stable/c/8afb1fabcec1929db46977e84baeee0cc0e79242
• https://git.kernel.org/stable/c/8cd7dbc9c46d51e00a0a8372e07cc1cbb8d24a77
• https://git.kernel.org/stable/c/b64305185b76f1d5145ce594ff48f3f0e70695bd
• https://git.kernel.org/stable/c/b7d854c33ab48e55fc233699bbefe39ec9bb5c05