nfs: handle failure of nfs_get_lock_context in unlock path

Summary

In the Linux kernel, the following vulnerability has been resolved: nfs: handle failure of nfs_get_lock_context in unlock path When memory is insufficient, the allocation of nfs_lock_context in nfs_get_lock_context() fails and returns -ENOMEM. If we mistakenly treat an nfs4_unlockdata structure (whose l_ctx member has been set to -ENOMEM) as valid and proceed to execute rpc_run_task(), this will trigger a NULL pointer dereference in nfs4_locku_prepare. For example: BUG: kernel NULL pointer dereference, address: 000000000000000c PGD 0 P4D 0 Oops: Oops: 0000 [#1] SMP PTI CPU: 15 UID: 0 PID: 12 Comm: kworker/u64:0 Not tainted 6.15.0-rc2-dirty #60 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-2.fc40 Workqueue: rpciod rpc_async_schedule RIP: 0010:nfs4_locku_prepare+0x35/0xc2 Code: 89 f2 48 89 fd 48 c7 c7 68 69 ef b5 53 48 8b 8e 90 00 00 00 48 89 f3 RSP: 0018:ffffbbafc006bdb8 EFLAGS: 00010246 RAX: 000000000000004b RBX: ffff9b964fc1fa00 RCX: 0000000000000000 RDX: 0000000000000000 RSI: fffffffffffffff4 RDI: ffff9ba53fddbf40 RBP: ffff9ba539934000 R08: 0000000000000000 R09: ffffbbafc006bc38 R10: ffffffffb6b689c8 R11: 0000000000000003 R12: ffff9ba539934030 R13: 0000000000000001 R14: 0000000004248060 R15: ffffffffb56d1c30 FS: 0000000000000000(0000) GS:ffff9ba5881f0000(0000) knlGS:00000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 000000000000000c CR3: 000000093f244000 CR4: 00000000000006f0 Call Trace: <TASK> __rpc_execute+0xbc/0x480 rpc_async_schedule+0x2f/0x40 process_one_work+0x232/0x5d0 worker_thread+0x1da/0x3d0 ? __pfx_worker_thread+0x10/0x10 kthread+0x10d/0x240 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x34/0x50 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 </TASK> Modules linked in: CR2: 000000000000000c ---[ end trace 0000000000000000 ]--- Free the allocated nfs4_unlockdata when nfs_get_lock_context() fails and return NULL to terminate subsequent rpc_run_task, preventing NULL pointer dereference.