bpf: Annotate data races on bpf_lru_node type

KCSAN reports a data race on bpf_lru_node::type between the LRU list
rotation and the free path of the common LRU:

  BUG: KCSAN: data-race in __bpf_lru_list_rotate / bpf_lru_push_free

  write to 0xffff88811b758022 of 1 bytes by task 4756 on cpu 1:
   __bpf_lru_node_move
   __bpf_lru_list_rotate_active
   __bpf_lru_list_rotate
   bpf_lru_list_pop_free_to_local
   bpf_common_lru_pop_free
   bpf_lru_pop_free
   htab_lru_map_update_elem

  read to 0xffff88811b758022 of 1 bytes by task 4752 on cpu 0:
   bpf_lru_list_push_free
   bpf_common_lru_push_free
   bpf_lru_push_free
   htab_lru_push_free
   htab_lru_map_update_elem

  value changed: 0x00 -> 0x01

bpf_lru_list_push_free() reads node->type for a WARN_ON_ONCE() sanity
check before taking l->lock. __bpf_lru_node_move(), called right after
under l->lock, does the same check, so drop the unlocked one.

bpf_common_lru_push_free() re-checks node->type under loc_l->lock after
the lockless READ_ONCE() at function entry. By then the node has been
removed from the htab, so only flush and rotate can change its type,
through __bpf_lru_node_move_in() and __bpf_lru_node_move() under
l->lock. Mark that re-check with READ_ONCE() and those two writers with
WRITE_ONCE(), following commit 6df8fb83301d ("bpf_lru_list: Read
double-checked variable once without lock"). The remaining writers
operate on a node with a single owner that nobody else reads and are
left alone.

Tested on bpf-next with CONFIG_KCSAN=y in a 4-vCPU virtme-ng guest,
using a reproducer that runs 8 threads issuing BPF_MAP_UPDATE_BATCH
on a 64-entry BPF_MAP_TYPE_LRU_HASH with overlapping keys.

Reported-by: syzbot+1ea4f564bf6f1f3d1dc4@syzkaller.appspotmail.com
Signed-off-by: Nicholas Velten <nicholasfvelten@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261019-fix-lru-node-type-kcsan-v2-1-c10a410b3565@gmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1ea4f564bf6f1f3d1dc4

0 files changed, 0 insertions, 0 deletions