CVE-2026-23374: blktrace: fix __this_cpu_read/write in preemptible context

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

blktrace: fix thiscpuread/write in preemptible context

tracingrecordcmdline() internally uses thiscpuread() and thiscpuwrite() on the per-CPU variable tracecmdlinesave, and tracesavecmdline() explicitly asserts preemption is disabled via lockdepassertpreemptiondisabled(). These operations are only safe when preemption is off, as they were designed to be called from the scheduler context (probewakeupschedswitch() / probewakeup()).

blkaddtrace() was calling tracingrecordcmdline(current) early in the blktracer path, before ring buffer reservation, from process context where preemption is fully enabled. This triggers the following using blktests/blktrace/002:

blktrace/002 (blktrace ftrace corruption with sysfs trace) [failed] runtime 0.367s ... 0.437s something found in dmesg: [ 81.211018] run blktests blktrace/002 at 2026-02-25 22:24:33 [ 81.239580] nullblk: disk nullb1 created [ 81.357294] BUG: using thiscpuread() in preemptible [00000000] code: dd/2516 [ 81.362842] caller is tracingrecordcmdline+0x10/0x40 [ 81.362872] CPU: 16 UID: 0 PID: 2516 Comm: dd Tainted: G N 7.0.0-rc1lblk+ #84 PREEMPT(full) [ 81.362877] Tainted: [N]=TEST [ 81.362878] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014 [ 81.362881] Call Trace: [ 81.362884] <TASK> [ 81.362886] dumpstacklvl+0x8d/0xb0 ... (See '/mnt/sda/blktests/results/nodev/blktrace/002.dmesg' for the entire message)

[ 81.211018] run blktests blktrace/002 at 2026-02-25 22:24:33 [ 81.239580] nullblk: disk nullb1 created [ 81.357294] BUG: using thiscpuread() in preemptible [00000000] code: dd/2516 [ 81.362842] caller is tracingrecordcmdline+0x10/0x40 [ 81.362872] CPU: 16 UID: 0 PID: 2516 Comm: dd Tainted: G N 7.0.0-rc1lblk+ #84 PREEMPT(full) [ 81.362877] Tainted: [N]=TEST [ 81.362878] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014 [ 81.362881] Call Trace: [ 81.362884] <TASK> [ 81.362886] dumpstacklvl+0x8d/0xb0 [ 81.362895] checkpreemptiondisabled+0xce/0xe0 [ 81.362902] tracingrecordcmdline+0x10/0x40 [ 81.362923] blkaddtrace+0x307/0x5d0 [ 81.362934] ? lockacquire+0xe0/0x300 [ 81.362940] ? ioviterextractpages+0x101/0xa30 [ 81.362959] blkaddtracebio+0x106/0x1e0 [ 81.362968] submitbionoacctnocheck+0x24b/0x3a0 [ 81.362979] ? lockdepinitmaptype+0x58/0x260 [ 81.362988] submitbiowait+0x56/0x90 [ 81.363009] blkdevdirectIOsimple+0x16c/0x250 [ 81.363026] ? pfxsubmitbiowaitendio+0x10/0x10 [ 81.363038] ? rcureadlockanyheld+0x73/0xa0 [ 81.363051] blkdevreaditer+0xc1/0x140 [ 81.363059] vfsread+0x20b/0x330 [ 81.363083] ksysread+0x67/0xe0 [ 81.363090] dosyscall64+0xbf/0xf00 [ 81.363102] entrySYSCALL64afterhwframe+0x76/0x7e [ 81.363106] RIP: 0033:0x7f281906029d [ 81.363111] Code: 31 c0 e9 c6 fe ff ff 50 48 8d 3d 66 63 0a 00 e8 59 ff 01 00 66 0f 1f 84 00 00 00 00 00 80 3d 41 33 0e 00 00 74 17 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 5b c3 66 2e 0f 1f 84 00 00 00 00 00 48 83 ec [ 81.363113] RSP: 002b:00007ffca127dd48 EFLAGS: 00000246 ORIGRAX: 0000000000000000 [ 81.363120] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f281906029d [ 81.363122] RDX: 0000000000001000 RSI: 0000559f8bfae000 RDI: 0000000000000000 [ 81.363123] RBP: 0000000000001000 R08: 0000002863a10a81 R09: 00007f281915f000 [ 81.363124] R10: 00007f2818f77b60 R11: 0000000000000246 R12: 0000559f8bfae000 [ 81.363126] R13: 0000000000000000 R14: 0000000000000000 R15: 000000000000000a [ 81.363142] </TASK>

The same BUG fires from blkaddtraceplug(), blkaddtraceunplug(), and blkaddtracerq() paths as well.

The purpose of tracin ---truncated---