net: atm: add lec_mutex

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

net: atm: add lec_mutex

syzbot found its way in net/atm/lec.c, and found an error path
in lecd_attach() could leave a dangling pointer in dev_lec[].

Add a mutex to protect dev_lecp[] uses from lecd_attach(),
lec_vcc_attach() and lec_mcast_attach().

Following patch will use this mutex for /proc/net/atm/lec.

BUG: KASAN: slab-use-after-free in lecd_attach net/atm/lec.c:751 [inline]
BUG: KASAN: slab-use-after-free in lane_ioctl+0x2224/0x23e0 net/atm/lec.c:1008
Read of size 8 at addr ffff88807c7b8e68 by task syz.1.17/6142

CPU: 1 UID: 0 PID: 6142 Comm: syz.1.17 Not tainted 6.16.0-rc1-syzkaller-00239-g08215f5486ec #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:408 [inline]
print_report+0xcd/0x680 mm/kasan/report.c:521
kasan_report+0xe0/0x110 mm/kasan/report.c:634
lecd_attach net/atm/lec.c:751 [inline]
lane_ioctl+0x2224/0x23e0 net/atm/lec.c:1008
do_vcc_ioctl+0x12c/0x930 net/atm/ioctl.c:159
sock_do_ioctl+0x118/0x280 net/socket.c:1190
sock_ioctl+0x227/0x6b0 net/socket.c:1311
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:907 [inline]
__se_sys_ioctl fs/ioctl.c:893 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:893
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
</TASK>

Allocated by task 6132:
kasan_save_stack+0x33/0x60 mm/kasan/common.c:47
kasan_save_track+0x14/0x30 mm/kasan/common.c:68
poison_kmalloc_redzone mm/kasan/common.c:377 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:394
kasan_kmalloc include/linux/kasan.h:260 [inline]
__do_kmalloc_node mm/slub.c:4328 [inline]
__kvmalloc_node_noprof+0x27b/0x620 mm/slub.c:5015
alloc_netdev_mqs+0xd2/0x1570 net/core/dev.c:11711
lecd_attach net/atm/lec.c:737 [inline]
lane_ioctl+0x17db/0x23e0 net/atm/lec.c:1008
do_vcc_ioctl+0x12c/0x930 net/atm/ioctl.c:159
sock_do_ioctl+0x118/0x280 net/socket.c:1190
sock_ioctl+0x227/0x6b0 net/socket.c:1311
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:907 [inline]
__se_sys_ioctl fs/ioctl.c:893 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:893
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 6132:
kasan_save_stack+0x33/0x60 mm/kasan/common.c:47
kasan_save_track+0x14/0x30 mm/kasan/common.c:68
kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:576
poison_slab_object mm/kasan/common.c:247 [inline]
__kasan_slab_free+0x51/0x70 mm/kasan/common.c:264
kasan_slab_free include/linux/kasan.h:233 [inline]
slab_free_hook mm/slub.c:2381 [inline]
slab_free mm/slub.c:4643 [inline]
kfree+0x2b4/0x4d0 mm/slub.c:4842
free_netdev+0x6c5/0x910 net/core/dev.c:11892
lecd_attach net/atm/lec.c:744 [inline]
lane_ioctl+0x1ce8/0x23e0 net/atm/lec.c:1008
do_vcc_ioctl+0x12c/0x930 net/atm/ioctl.c:159
sock_do_ioctl+0x118/0x280 net/socket.c:1190
sock_ioctl+0x227/0x6b0 net/socket.c:1311
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:907 [inline]
__se_sys_ioctl fs/ioctl.c:893 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:893