[RFC] Use after free in BPF/ XDP during XDP_REDIRECT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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@igalia.com/

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");



Sebastian