Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> writes: > 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. Hmm, how about putting the reset (essentially the changes you have above) into bpf_prog_run_xdp() itself, before executing the BPF program? -Toke
