In the Linux kernel, the following vulnerability has been resolved:
bpf: Annotate bpf_long_memcpy with data_race
syzbot reported a data race splat between two processes trying to
update the same BPF map value via syscall on different CPUs:
BUG: KCSAN: data-race in bpf_percpu_array_update / bpf_percpu_array_update
write to 0xffffe8fffe7425d8 of 8 bytes by task 8257 on cpu 1:
bpf_long_memcpy include/linux/bpf.h:428 [inline]
bpf_obj_memcpy include/linux/bpf.h:441 [inline]
copy_map_value_long include/linux/bpf.h:464 [inline]
bpf_percpu_array_update+0x3bb/0x500 kernel/bpf/arraymap.c:380
bpf_map_update_value+0x190/0x370 kernel/bpf/syscall.c:175
generic_map_update_batch+0x3ae/0x4f0 kernel/bpf/syscall.c:1749
bpf_map_do_batch+0x2df/0x3d0 kernel/bpf/syscall.c:4648
__sys_bpf+0x28a/0x780
__do_sys_bpf kernel/bpf/syscall.c:5241 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5239 [inline]
__x64_sys_bpf+0x43/0x50 kernel/bpf/syscall.c:5239
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
write to 0xffffe8fffe7425d8 of 8 bytes by task 8268 on cpu 0:
bpf_long_memcpy include/linux/bpf.h:428 [inline]
bpf_obj_memcpy include/linux/bpf.h:441 [inline]
copy_map_value_long include/linux/bpf.h:464 [inline]
bpf_percpu_array_update+0x3bb/0x500 kernel/bpf/arraymap.c:380
bpf_map_update_value+0x190/0x370 kernel/bpf/syscall.c:175
generic_map_update_batch+0x3ae/0x4f0 kernel/bpf/syscall.c:1749
bpf_map_do_batch+0x2df/0x3d0 kernel/bpf/syscall.c:4648
__sys_bpf+0x28a/0x780
__do_sys_bpf kernel/bpf/syscall.c:5241 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5239 [inline]
__x64_sys_bpf+0x43/0x50 kernel/bpf/syscall.c:5239
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
value changed: 0x0000000000000000 -> 0xfffffff000002788
The bpf_long_memcpy is used with 8-byte aligned pointers, power-of-8 size
and forced to use long read/writes to try to atomically copy long counters.
It is best-effort only and no barriers are here since it _will_ race with
concurrent updates from BPF programs. The bpf_long_memcpy() is called from
bpf(2) syscall. Marco suggested that the best way to make this known to
KCSAN would be to use data_race() annotation.
