In the Linux kernel, the following vulnerability has been resolved:
rcu/kvfree: Fix data-race in _modtimer / kvfreecallrcu
KCSAN reports a data race when access the krcp->monitorwork.timer.expires variable in the scheduledelayedmonitorwork() function:
<snip> BUG: KCSAN: data-race in _modtimer / kvfreecallrcu
read to 0xffff888237d1cce8 of 8 bytes by task 10149 on cpu 1: scheduledelayedmonitorwork kernel/rcu/tree.c:3520 [inline] kvfreecallrcu+0x3b8/0x510 kernel/rcu/tree.c:3839 trieupdateelem+0x47c/0x620 kernel/bpf/lpmtrie.c:441 bpfmapupdatevalue+0x324/0x350 kernel/bpf/syscall.c:203 genericmapupdatebatch+0x401/0x520 kernel/bpf/syscall.c:1849 bpfmapdobatch+0x28c/0x3f0 kernel/bpf/syscall.c:5143 _sysbpf+0x2e5/0x7a0 _dosysbpf kernel/bpf/syscall.c:5741 [inline] _sesysbpf kernel/bpf/syscall.c:5739 [inline] _x64sysbpf+0x43/0x50 kernel/bpf/syscall.c:5739 x64syscall+0x2625/0x2d60 arch/x86/include/generated/asm/syscalls64.h:322 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0xc9/0x1c0 arch/x86/entry/common.c:83 entrySYSCALL64after_hwframe+0x77/0x7f
write to 0xffff888237d1cce8 of 8 bytes by task 56 on cpu 0: _modtimer+0x578/0x7f0 kernel/time/timer.c:1173 addtimerglobal+0x51/0x70 kernel/time/timer.c:1330 _queuedelayedwork+0x127/0x1a0 kernel/workqueue.c:2523 queuedelayedworkon+0xdf/0x190 kernel/workqueue.c:2552 queuedelayedwork include/linux/workqueue.h:677 [inline] scheduledelayedmonitorwork kernel/rcu/tree.c:3525 [inline] kfreercumonitor+0x5e8/0x660 kernel/rcu/tree.c:3643 processonework kernel/workqueue.c:3229 [inline] processscheduledworks+0x483/0x9a0 kernel/workqueue.c:3310 workerthread+0x51d/0x6f0 kernel/workqueue.c:3391 kthread+0x1d1/0x210 kernel/kthread.c:389 retfromfork+0x4b/0x60 arch/x86/kernel/process.c:147 retfromforkasm+0x1a/0x30 arch/x86/entry/entry64.S:244
Reported by Kernel Concurrency Sanitizer on: CPU: 0 UID: 0 PID: 56 Comm: kworker/u8:4 Not tainted 6.12.0-rc2-syzkaller-00050-g5b7c893ed5ed #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024 Workqueue: eventsunbound kfreercu_monitor <snip>
kfreercumonitor() rearms the work if a "krcp" has to be still offloaded and this is done without holding krcp->lock, whereas the kvfreecallrcu() holds it.
Fix it by acquiring the "krcp->lock" for kfreercumonitor() so both functions do not race anymore.