CVE-2024-45000: In the Linux kernel, the following vulnerability has been resolved:  fs/netfs/fscache_cookie: add missing "n_accesses" check  This fixes a NULL pointer dereference bug due to a data race which looks like this:    BUG: kernel NULL pointer dereference, address: 0000000000000008   #PF: supervisor read access in kernel mode   #PF: error_code(0x0000) - not-present page   PGD 0 P4D 0   Oops: 0000 [#1] SMP PTI   CPU: 33 PID: 16573 Comm: kworker/u97:799 Not tainted 6.8.7-cm4all1-hp+ #43   Hardware name: HP ProLiant DL380 Gen9/ProLiant DL380 Gen9, BIOS P89 10/17/2018   Workqueue: events_unbound netfs_rreq_write_to_cache_work   RIP: 0010:cachefiles_prepare_write+0x30/0xa0   Code: 57 41 56 45 89 ce 41 55 49 89 cd 41 54 49 89 d4 55 53 48 89 fb 48 83 ec 08 48 8b 47 08 48 83 7f 10 00 48 89 34 24 48 8b 68 20 <48> 8b 45 08 4c 8b 38 74 45 49 8b 7f 50 e8 4e a9 b0 ff 48 8b 73 10   RSP: 0018:ffffb4e78113bde0 EFLAGS: 00010286   RAX: ffff976126be6d10 RBX: ffff97615cdb8438 RCX: 0000000000020000   RDX: ffff97605e6c4c68 RSI: ffff97605e6c4c60 RDI: ffff97615cdb8438   RBP: 0000000000000000 R08: 0000000000278333 R09: 0000000000000001   R10: ffff97605e6c4600 R11: 0000000000000001 R12: ffff97605e6c4c68   R13: 0000000000020000 R14: 0000000000000001 R15: ffff976064fe2c00   FS:  0000000000000000(0000) GS:ffff9776dfd40000(0000) knlGS:0000000000000000   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033   CR2: 0000000000000008 CR3: 000000005942c002 CR4: 00000000001706f0   Call Trace:    <TASK>    ? __die+0x1f/0x70    ? page_fault_oops+0x15d/0x440    ? search_module_extables+0xe/0x40    ? fixup_exception+0x22/0x2f0    ? exc_page_fault+0x5f/0x100    ? asm_exc_page_fault+0x22/0x30    ? cachefiles_prepare_write+0x30/0xa0    netfs_rreq_write_to_cache_work+0x135/0x2e0    process_one_work+0x137/0x2c0    worker_thread+0x2e9/0x400    ? __pfx_worker_thread+0x10/0x10    kthread+0xcc/0x100    ? __pfx_kthread+0x10/0x10    ret_from_fork+0x30/0x50    ? __pfx_kthread+0x10/0x10    ret_from_fork_asm+0x1b/0x30    </TASK>   Modules linked in:   CR2: 0000000000000008   ---[ end trace 0000000000000000 ]---  This happened because fscache_cookie_state_machine() was slow and was still running while another process invoked fscache_unuse_cookie(); this led to a fscache_cookie_lru_do_one() call, setting the FSCACHE_COOKIE_DO_LRU_DISCARD flag, which was picked up by fscache_cookie_state_machine(), withdrawing the cookie via cachefiles_withdraw_cookie(), clearing cookie->cache_priv.  At the same time, yet another process invoked cachefiles_prepare_write(), which found a NULL pointer in this code line:    struct cachefiles_object *object = cachefiles_cres_object(cres);  The next line crashes, obviously:    struct cachefiles_cache *cache = object->volume->cache;  During cachefiles_prepare_write(), the "n_accesses" counter is non-zero (via fscache_begin_operation()).  The cookie must not be withdrawn until it drops to zero.  The counter is checked by fscache_cookie_state_machine() before switching to FSCACHE_COOKIE_STATE_RELINQUISHING and FSCACHE_COOKIE_STATE_WITHDRAWING (in "case FSCACHE_COOKIE_STATE_FAILED"), but not for FSCACHE_COOKIE_STATE_LRU_DISCARDING ("case FSCACHE_COOKIE_STATE_ACTIVE").  This patch adds the missing check.  With a non-zero access counter, the function returns and the next fscache_end_cookie_access() call will queue another fscache_cookie_state_machine() call to handle the still-pending FSCACHE_COOKIE_DO_LRU_DISCARD.