net/sched: cls_api: fix error handling causing NULL dereference

tcf_exts_miss_cookie_base_alloc() calls xa_alloc_cyclic() which can
return 1 if the allocation succeeded after wrapping. This was treated as
an error, with value 1 returned to caller tcf_exts_init_ex() which sets
exts->actions to NULL and returns 1 to caller fl_change().

fl_change() treats err == 1 as success, calling tcf_exts_validate_ex()
which calls tcf_action_init() with exts->actions as argument, where it
is dereferenced.

Example trace:

BUG: kernel NULL pointer dereference, address: 0000000000000000
CPU: 114 PID: 16151 Comm: handler114 Kdump: loaded Not tainted 5.14.0-503.16.1.el9_5.x86_64 #1
RIP: 0010:tcf_action_init+0x1f8/0x2c0
Call Trace:
 tcf_action_init+0x1f8/0x2c0
 tcf_exts_validate_ex+0x175/0x190
 fl_change+0x537/0x1120 [cls_flower]

Fixes: 80cd22c ("net/sched: cls_api: Support hardware miss to tc action")
Signed-off-by: Pierre Riteau <pierre@stackhpc.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Link: https://patch.msgid.link/20250213223610.320278-1-pierre@stackhpc.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: priteau

net/sched/cls_api.c

@@ -97,7 +97,7 @@ tcf_exts_miss_cookie_base_alloc(struct tcf_exts *exts, struct tcf_proto *tp,
 
 	err = xa_alloc_cyclic(&tcf_exts_miss_cookies_xa, &n->miss_cookie_base,
 			      n, xa_limit_32b, &next, GFP_KERNEL);
-	if (err)
+	if (err < 0)
 		goto err_xa_alloc;
 
 	exts->miss_cookie_node = n;