CVE-2025-38475: smc: Fix various oops due to inet_sock type confusion.
In the Linux kernel, the following vulnerability has been resolved:

smc: Fix various oops due to inet_sock type confusion.

syzbot reported weird splats [0][1] in cipso_v4_sock_setattr() while
freeing inet_sk(sk)->inet_opt.

The address was freed multiple times even though it was read-only memory.

cipso_v4_sock_setattr() did nothing wrong, and the root cause was type
confusion.

The cited commit made it possible to create smc_sock as an INET socket.

The issue is that struct smc_sock does not have struct inet_sock as the
first member but hijacks AF_INET and AF_INET6 sk_family, which confuses
various places.

In this case, inet_sock.inet_opt was actually smc_sock.clcsk_data_ready(),
which is an address of a function in the text segment.

  $ pahole -C inet_sock vmlinux
  struct inet_sock {
  ...
          struct ip_options_rcu *    inet_opt;             /*   784     8 */

  $ pahole -C smc_sock vmlinux
  struct smc_sock {
  ...
          void                       (*clcsk_data_ready)(struct sock *); /*   784     8 */

The same issue for another field was reported before. [2][3]

At that time, an ugly hack was suggested [4], but it makes both INET
and SMC code error-prone and hard to change.

Also, yet another variant was fixed by a hacky commit 98d4435efcbf3
("net/smc: prevent NULL pointer dereference in txopt_get").

Instead of papering over the root cause by such hacks, we should not
allow non-INET socket to reuse the INET infra.

Let's add inet_sock as the first member of smc_sock.

[0]:
kvfree_call_rcu(): Double-freed call. rcu_head 000000006921da73
WARNING: CPU: 0 PID: 6718 at mm/slab_common.c:1956 kvfree_call_rcu+0x94/0x3f0 mm/slab_common.c:1955
Modules linked in:
CPU: 0 UID: 0 PID: 6718 Comm: syz.0.17 Tainted: G        W           6.16.0-rc4-syzkaller-g7482bb149b9f #0 PREEMPT
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : kvfree_call_rcu+0x94/0x3f0 mm/slab_common.c:1955
lr : kvfree_call_rcu+0x94/0x3f0 mm/slab_common.c:1955
sp : ffff8000a03a7730
x29: ffff8000a03a7730 x28: 00000000fffffff5 x27: 1fffe000184823d3
x26: dfff800000000000 x25: ffff0000c2411e9e x24: ffff0000dd88da00
x23: ffff8000891ac9a0 x22: 00000000ffffffea x21: ffff8000891ac9a0
x20: ffff8000891ac9a0 x19: ffff80008afc2480 x18: 00000000ffffffff
x17: 0000000000000000 x16: ffff80008ae642c8 x15: ffff700011ede14c
x14: 1ffff00011ede14c x13: 0000000000000004 x12: ffffffffffffffff
x11: ffff700011ede14c x10: 0000000000ff0100 x9 : 5fa3c1ffaf0ff000
x8 : 5fa3c1ffaf0ff000 x7 : 0000000000000001 x6 : 0000000000000001
x5 : ffff8000a03a7078 x4 : ffff80008f766c20 x3 : ffff80008054d360
x2 : 0000000000000000 x1 : 0000000000000201 x0 : 0000000000000000
Call trace:
 kvfree_call_rcu+0x94/0x3f0 mm/slab_common.c:1955 (P)
 cipso_v4_sock_setattr+0x2f0/0x3f4 net/ipv4/cipso_ipv4.c:1914
 netlbl_sock_setattr+0x240/0x334 net/netlabel/netlabel_kapi.c:1000
 smack_netlbl_add+0xa8/0x158 security/smack/smack_lsm.c:2581
 smack_inode_setsecurity+0x378/0x430 security/smack/smack_lsm.c:2912
 security_inode_setsecurity+0x118/0x3c0 security/security.c:2706
 __vfs_setxattr_noperm+0x174/0x5c4 fs/xattr.c:251
 __vfs_setxattr_locked+0x1ec/0x218 fs/xattr.c:295
 vfs_setxattr+0x158/0x2ac fs/xattr.c:321
 do_setxattr fs/xattr.c:636 [inline]
 file_setxattr+0x1b8/0x294 fs/xattr.c:646
 path_setxattrat+0x2ac/0x320 fs/xattr.c:711
 __do_sys_fsetxattr fs/xattr.c:761 [inline]
 __se_sys_fsetxattr fs/xattr.c:758 [inline]
 __arm64_sys_fsetxattr+0xc0/0xdc fs/xattr.c:758
 __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
 invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49
 el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:132
 do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
 el0_svc+0x58/0x180 arch/arm64/kernel/entry-common.c:879
 el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:898
 el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:600

[
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CVSS v3.1: 5.5