bpf: fix bpf_prog_array_copy_to_user() issues

1. move copy_to_user out of rcu section to fix the following issue:

./include/linux/rcupdate.h:302 Illegal context switch in RCU read-side critical section!
stack backtrace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x194/0x257 lib/dump_stack.c:53
lockdep_rcu_suspicious+0x123/0x170 kernel/locking/lockdep.c:4592
rcu_preempt_sleep_check include/linux/rcupdate.h:301 [inline]
___might_sleep+0x385/0x470 kernel/sched/core.c:6079
__might_sleep+0x95/0x190 kernel/sched/core.c:6067
__might_fault+0xab/0x1d0 mm/memory.c:4532
_copy_to_user+0x2c/0xc0 lib/usercopy.c:25
copy_to_user include/linux/uaccess.h:155 [inline]
bpf_prog_array_copy_to_user+0x217/0x4d0 kernel/bpf/core.c:1587
bpf_prog_array_copy_info+0x17b/0x1c0 kernel/bpf/core.c:1685
perf_event_query_prog_array+0x196/0x280 kernel/trace/bpf_trace.c:877
_perf_ioctl kernel/events/core.c:4737 [inline]
perf_ioctl+0x3e1/0x1480 kernel/events/core.c:4757

2. move *prog under rcu, since it's not ok to dereference it afterwards

3. in a rare case of prog array being swapped between bpf_prog_array_length()
and bpf_prog_array_copy_to_user() calls make sure to copy zeros to user space,
so the user doesn't walk over uninited prog_ids while kernel reported
uattr->query.prog_cnt > 0

Reported-by: [e-mail]
Fixes: 468e2f64d220 ("bpf: introduce BPF_PROG_QUERY command")
Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

kernel/bpf/core.c

···
 				__u32 __user *prog_ids, u32 cnt)
 {
 	struct bpf_prog **prog;
-	u32 i = 0, id;
+	unsigned long err = 0;
+	u32 i = 0, *ids;
+	bool nospc;
 
+	/* users of this function are doing:
+	 * cnt = bpf_prog_array_length();
+	 * if (cnt > 0)
+	 *     bpf_prog_array_copy_to_user(..., cnt);
+	 * so below kcalloc doesn't need extra cnt > 0 check, but
+	 * bpf_prog_array_length() releases rcu lock and
+	 * prog array could have been swapped with empty or larger array,
+	 * so always copy 'cnt' prog_ids to the user.
+	 * In a rare race the user will see zero prog_ids
+	 */
+	ids = kcalloc(cnt, sizeof(u32), GFP_USER);
+	if (!ids)
+		return -ENOMEM;
 	rcu_read_lock();
 	prog = rcu_dereference(progs)->progs;
 	for (; *prog; prog++) {
 		if (*prog == &dummy_bpf_prog.prog)
 			continue;
-		id = (*prog)->aux->id;
-		if (copy_to_user(prog_ids + i, &id, sizeof(id))) {
-			rcu_read_unlock();
-			return -EFAULT;
-		}
+		ids[i] = (*prog)->aux->id;
 		if (++i == cnt) {
 			prog++;
 			break;
 		}
 	}
+	nospc = !!(*prog);
 	rcu_read_unlock();
-	if (*prog)
+	err = copy_to_user(prog_ids, ids, cnt * sizeof(u32));
+	kfree(ids);
+	if (err)
+		return -EFAULT;
+	if (nospc)
 		return -ENOSPC;
 	return 0;
 }