From: Kumar Kartikeya Dwivedi memxor@gmail.com
commit 275c30bcee66a27d1aa97a215d607ad6d49804cb upstream.
The LRU map that is preallocated may have its elements reused while another program holds a pointer to it from bpf_map_lookup_elem. Hence, only check_and_free_fields is appropriate when the element is being deleted, as it ensures proper synchronization against concurrent access of the map value. After that, we cannot call check_and_init_map_value again as it may rewrite bpf_spin_lock, bpf_timer, and kptr fields while they can be concurrently accessed from a BPF program.
This is safe to do as when the map entry is deleted, concurrent access is protected against by check_and_free_fields, i.e. an existing timer would be freed, and any existing kptr will be released by it. The program can create further timers and kptrs after check_and_free_fields, but they will eventually be released once the preallocated items are freed on map destruction, even if the item is never reused again. Hence, the deleted item sitting in the free list can still have resources attached to it, and they would never leak.
With spin_lock, we never touch the field at all on delete or update, as we may end up modifying the state of the lock. Since the verifier ensures that a bpf_spin_lock call is always paired with bpf_spin_unlock call, the program will eventually release the lock so that on reuse the new user of the value can take the lock.
Essentially, for the preallocated case, we must assume that the map value may always be in use by the program, even when it is sitting in the freelist, and handle things accordingly, i.e. use proper synchronization inside check_and_free_fields, and never reinitialize the special fields when it is reused on update.
Fixes: 68134668c17f ("bpf: Add map side support for bpf timers.") Acked-by: Yonghong Song yhs@fb.com Signed-off-by: Kumar Kartikeya Dwivedi memxor@gmail.com Acked-by: Martin KaFai Lau kafai@fb.com Link: https://lore.kernel.org/r/20220809213033.24147-3-memxor@gmail.com Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/bpf/hashtab.c | 6 +----- 1 file changed, 1 insertion(+), 5 deletions(-)
--- a/kernel/bpf/hashtab.c +++ b/kernel/bpf/hashtab.c @@ -311,12 +311,8 @@ static struct htab_elem *prealloc_lru_po struct htab_elem *l;
if (node) { - u32 key_size = htab->map.key_size; - l = container_of(node, struct htab_elem, lru_node); - memcpy(l->key, key, key_size); - check_and_init_map_value(&htab->map, - l->key + round_up(key_size, 8)); + memcpy(l->key, key, htab->map.key_size); return l; }