cgroup/dmem: fix NULL pointer dereference when setting max

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

cgroup/dmem: fix NULL pointer dereference when setting max

An issue was triggered:

BUG: kernel NULL pointer dereference, address: 0000000000000000

#PF: supervisor read access in kernel mode

#PF: error_code(0x0000) - not-present page

PGD 0 P4D 0

Oops: Oops: 0000 [#1] SMP NOPTI

CPU: 15 UID: 0 PID: 658 Comm: bash Tainted: 6.19.0-rc6-next-2026012

Tainted: [O]=OOT_MODULE

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),

RIP: 0010:strcmp+0x10/0x30

RSP: 0018:ffffc900017f7dc0 EFLAGS: 00000246

RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff888107cd4358

RDX: 0000000019f73907 RSI: ffffffff82cc381a RDI: 0000000000000000

RBP: ffff8881016bef0d R08: 000000006c0e7145 R09: 0000000056c0e714

R10: 0000000000000001 R11: ffff888107cd4358 R12: 0007ffffffffffff

R13: ffff888101399200 R14: ffff888100fcb360 R15: 0007ffffffffffff

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 0000000000000000 CR3: 0000000105c79000 CR4: 00000000000006f0

Call Trace:

<TASK>

dmemcg_limit_write.constprop.0+0x16d/0x390

? __pfx_set_resource_max+0x10/0x10

kernfs_fop_write_iter+0x14e/0x200

vfs_write+0x367/0x510

ksys_write+0x66/0xe0

do_syscall_64+0x6b/0x390

entry_SYSCALL_64_after_hwframe+0x76/0x7e

RIP: 0033:0x7f42697e1887

It was trriggered setting max without limitation, the command is like:

"echo test/region0 > dmem.max". To fix this issue, add check whether

options is valid after parsing the region_name.