CVE-2024-26645: In the Linux kernel, the following vulnerability has been resolved:tracing: Ensure visibility when inserting an element into tracing_mapRunning the following two commands in parallel on a multi-processorAArch64 machine can sporadically produce an unexpected warning aboutduplicate histogram entries: $ while true; do     echo hist:key=id.syscall:val=hitcount > \\\\       /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger     cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist     sleep 0.001   done $ stress-ng --sysbadaddr $(nproc)The warning looks as follows:[ 2911.172474] ------------[ cut here ]------------[ 2911.173111] Duplicates detected: 1[ 2911.173574] WARNING: CPU: 2 PID: 12247 at kernel/trace/tracing_map.c:983 tracing_map_sort_entries+0x3e0/0x408[ 2911.174702] Modules linked in: iscsi_ibft(E) iscsi_boot_sysfs(E) rfkill(E) af_packet(E) nls_iso8859_1(E) nls_cp437(E) vfat(E) fat(E) ena(E) tiny_power_button(E) qemu_fw_cfg(E) button(E) fuse(E) efi_pstore(E) ip_tables(E) x_tables(E) xfs(E) libcrc32c(E) aes_ce_blk(E) aes_ce_cipher(E) crct10dif_ce(E) polyval_ce(E) polyval_generic(E) ghash_ce(E) gf128mul(E) sm4_ce_gcm(E) sm4_ce_ccm(E) sm4_ce(E) sm4_ce_cipher(E) sm4(E) sm3_ce(E) sm3(E) sha3_ce(E) sha512_ce(E) sha512_arm64(E) sha2_ce(E) sha256_arm64(E) nvme(E) sha1_ce(E) nvme_core(E) nvme_auth(E) t10_pi(E) sg(E) scsi_mod(E) scsi_common(E) efivarfs(E)[ 2911.174738] Unloaded tainted modules: cppc_cpufreq(E):1[ 2911.180985] CPU: 2 PID: 12247 Comm: cat Kdump: loaded Tainted: G            E      6.7.0-default #2 1b58bbb22c97e4399dc09f92d309344f69c44a01[ 2911.182398] Hardware name: Amazon EC2 c7g.8xlarge/, BIOS 1.0 11/1/2018[ 2911.183208] pstate: 61400005 (nZCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)[ 2911.184038] pc : tracing_map_sort_entries+0x3e0/0x408[ 2911.184667] lr : tracing_map_sort_entries+0x3e0/0x408[ 2911.185310] sp : ffff8000a1513900[ 2911.185750] x29: ffff8000a1513900 x28: ffff0003f272fe80 x27: 0000000000000001[ 2911.186600] x26: ffff0003f272fe80 x25: 0000000000000030 x24: 0000000000000008[ 2911.187458] x23: ffff0003c5788000 x22: ffff0003c16710c8 x21: ffff80008017f180[ 2911.188310] x20: ffff80008017f000 x19: ffff80008017f180 x18: ffffffffffffffff[ 2911.189160] x17: 0000000000000000 x16: 0000000000000000 x15: ffff8000a15134b8[ 2911.190015] x14: 0000000000000000 x13: 205d373432323154 x12: 5b5d313131333731[ 2911.190844] x11: 00000000fffeffff x10: 00000000fffeffff x9 : ffffd1b78274a13c[ 2911.191716] x8 : 000000000017ffe8 x7 : c0000000fffeffff x6 : 000000000057ffa8[ 2911.192554] x5 : ffff0012f6c24ec0 x4 : 0000000000000000 x3 : ffff2e5b72b5d000[ 2911.193404] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff0003ff254480[ 2911.194259] Call trace:[ 2911.194626]  tracing_map_sort_entries+0x3e0/0x408[ 2911.195220]  hist_show+0x124/0x800[ 2911.195692]  seq_read_iter+0x1d4/0x4e8[ 2911.196193]  seq_read+0xe8/0x138[ 2911.196638]  vfs_read+0xc8/0x300[ 2911.197078]  ksys_read+0x70/0x108[ 2911.197534]  __arm64_sys_read+0x24/0x38[ 2911.198046]  invoke_syscall+0x78/0x108[ 2911.198553]  el0_svc_common.constprop.0+0xd0/0xf8[ 2911.199157]  do_el0_svc+0x28/0x40[ 2911.199613]  el0_svc+0x40/0x178[ 2911.200048]  el0t_64_sync_handler+0x13c/0x158[ 2911.200621]  el0t_64_sync+0x1a8/0x1b0[ 2911.201115] ---[ end trace 0000000000000000 ]---The problem appears to be caused by CPU reordering of writes issued from__tracing_map_insert().The check for the presence of an element with a given key in thisfunction is: val = READ_ONCE(entry->val); if (val && keys_match(key, val->key, map->key_size)) ...The write of a new entry is: elt = get_free_elt(map); memcpy(elt->key, key, map->key_size); entry->val = elt;The "memcpy(elt->key, key, map->key_size);" and "entry->val = elt;"stores may become visible in the reversed order on another CPU. Thissecond CPU might then incorrectly determine that a new key doesn't matchan already present val->key and subse---truncated---