Daniel Borkmann <daniel@xxxxxxxxxxxxx> writes:
> On 10/25/21 4:28 PM, Lorenzo Bianconi wrote:
>>> Lorenzo noticed that the code testing for program type compatibility of
>>> tail call maps is potentially racy in that two threads could encounter a
>>> map with an unset type simultaneously and both return true even though they
>>> are inserting incompatible programs.
>>>
>>> The race window is quite small, but artificially enlarging it by adding a
>>> usleep_range() inside the check in bpf_prog_array_compatible() makes it
>>> trivial to trigger from userspace with a program that does, essentially:
>>>
>>> map_fd = bpf_create_map(BPF_MAP_TYPE_PROG_ARRAY, 4, 4, 2, 0);
>>> pid = fork();
>>> if (pid) {
>>> key = 0;
>>> value = xdp_fd;
>>> } else {
>>> key = 1;
>>> value = tc_fd;
>>> }
>>> err = bpf_map_update_elem(map_fd, &key, &value, 0);
>>>
>>> While the race window is small, it has potentially serious ramifications in
>>> that triggering it would allow a BPF program to tail call to a program of a
>>> different type. So let's get rid of it by protecting the update with a
>>> spinlock. The commit in the Fixes tag is the last commit that touches the
>>> code in question.
>>>
>>> v2:
>>> - Use a spinlock instead of an atomic variable and cmpxchg() (Alexei)
>>>
>>> Fixes: 3324b584b6f6 ("ebpf: misc core cleanup")
>>> Reported-by: Lorenzo Bianconi <lorenzo.bianconi@xxxxxxxxxx>
>>> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
>>> ---
>>> include/linux/bpf.h | 1 +
>>> kernel/bpf/arraymap.c | 1 +
>>> kernel/bpf/core.c | 14 ++++++++++----
>>> kernel/bpf/syscall.c | 2 ++
>>> 4 files changed, 14 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>>> index 020a7d5bf470..98d906176d89 100644
>>> --- a/include/linux/bpf.h
>>> +++ b/include/linux/bpf.h
>>> @@ -929,6 +929,7 @@ struct bpf_array_aux {
>>> * stored in the map to make sure that all callers and callees have
>>> * the same prog type and JITed flag.
>>> */
>>> + spinlock_t type_check_lock;
>>
>> I was wondering if we can use a mutex instead of a spinlock here since it is
>> run from a syscall AFAIU. The only downside is mutex_lock is run inside
>> aux->used_maps_mutex critical section. Am I missing something?
>
> Hm, potentially it could work, but then it's also 32 vs 4 extra bytes. There's
> also poke_mutex or freeze_mutex, but feels to hacky to 'generalize for reuse',
> so I think the spinlock in bpf_array_aux is fine.
>
>>> enum bpf_prog_type type;
>>> bool jited;
>>> /* Programs with direct jumps into programs part of this array. */
>>> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
>>> index cebd4fb06d19..da9b1e96cadc 100644
>>> --- a/kernel/bpf/arraymap.c
>>> +++ b/kernel/bpf/arraymap.c
>>> @@ -1072,6 +1072,7 @@ static struct bpf_map *prog_array_map_alloc(union bpf_attr *attr)
>>> INIT_WORK(&aux->work, prog_array_map_clear_deferred);
>>> INIT_LIST_HEAD(&aux->poke_progs);
>>> mutex_init(&aux->poke_mutex);
>>> + spin_lock_init(&aux->type_check_lock);
>
> Just as a tiny nit, I would probably name it slightly different, since type_check_lock
> mainly refers to the type property but there's also jit vs non-jit and as pointed out
> there could be other extensions that need checking in future as well. Maybe 'compat_lock'
> would be a more generic one or just:
>
> struct {
> enum bpf_prog_type type;
> bool jited;
> spinlock_t lock;
> } owner;
Uh, I like that! Makes it easier to move as well (which we're doing as
part of the xdp_mb series). Will send a v3 with this :)
-Toke
