On 2025-03-13 20:28:06 [+0100], Toke Høiland-Jørgensen wrote: > Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> writes: > > > Hi, > > > > Ricardo reported a KASAN related use after free > > https://lore.kernel.org/all/20250226-20250204-kasan-slab-use-after-free-read-in-dev_map_enqueue__submit-v3-0-360efec441ba@xxxxxxxxxx/ > > > > in v6.6 stable and suggest a backport of commits > > 401cb7dae8130 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.") > > fecef4cd42c68 ("tun: Assign missing bpf_net_context.") > > 9da49aa80d686 ("tun: Add missing bpf_net_ctx_clear() in do_xdp_generic()") > > > > as a fix. In the meantime I have the syz reproducer+config and was able > > to investigate. > > It looks as if the syzbot starts a BPF program via xdp_test_run_batch() > > which assigns ri->tgt_value via dev_hash_map_redirect() and the return code > > isn't XDP_REDIRECT it looks like nonsense. So the print in > > bpf_warn_invalid_xdp_action() appears once. Everything goes as planned. > > Then the TUN driver runs another BPF program which returns XDP_REDIRECT > > without setting ri->tgt_value. This appears to be a trick because it > > invoked bpf_trace_printk() which printed four characters. Anyway, this > > is enough to get xdp_do_redirect() going. > > > > The commits in questions do fix it because the bpf_redirect_info becomes > > not only per-task but gets invalidated after the XDP context is left. > > > > Now that I understand it I would suggest something smaller instead as a > > stable fix, (instead the proposed patches). Any objections to the > > following: > > > > diff --git a/net/core/filter.c b/net/core/filter.c > > index be313928d272..1d906b7a541d 100644 > > --- a/net/core/filter.c > > +++ b/net/core/filter.c > > @@ -9000,8 +9000,12 @@ static bool xdp_is_valid_access(int off, int size, > > > > void bpf_warn_invalid_xdp_action(struct net_device *dev, struct bpf_prog *prog, u32 act) > > { > > + struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info); > > const u32 act_max = XDP_REDIRECT; > > > > + ri->map_id = INT_MAX; > > + ri->map_type = BPF_MAP_TYPE_UNSPEC; > > + > > pr_warn_once("%s XDP return value %u on prog %s (id %d) dev %s, expect packet loss!\n", > > act > act_max ? "Illegal" : "Driver unsupported", > > act, prog->aux->name, prog->aux->id, dev ? dev->name : "N/A"); > > From your description above, this will fix the particular error > encountered, but what happens if the initial return code is not in fact > nonsense (so the warn_invalid_action) is not triggered? > > I.e., > > bpf_redirect_map(...); > return XDP_DROP; > > would still leave ri->map_id and ri->map_type set for the later tun > driver invocation, no? Right. So if it returns XDP_PASS or XDP_DROP instead of nonsense then the buffer remains set. And another driver could use it. But this would mean we would have to tackle each bpf_prog_run_xdp() invocation and reset it afterwards… So maybe the backport instead? We have | $ git grep bpf_prog_run_xdp | wc -l | 55 call sites. > -Toke Sebastian
