bpf, mprog: Fix maximum program check on mprog attachment

After Paul's recent improvement to syzkaller to improve coverage for
bpf_mprog and tcx, it hit a splat that the program limit was surpassed.
What happened is that the maximum number of progs got added, followed
by another prog add request which adds with BPF_F_BEFORE flag relative
to the last program in the array. The idx >= bpf_mprog_max() check in
bpf_mprog_attach() still passes because the index is below the maximum
but the maximum will be surpassed. We need to add a check upfront for
insertions to catch this situation.

Fixes: 053c8e1 ("bpf: Add generic attach/detach/query API for multi-progs")
Reported-by: [email protected]
Reported-by: [email protected]
Reported-by: [email protected]
Co-developed-by: Nikolay Aleksandrov <[email protected]>
Signed-off-by: Nikolay Aleksandrov <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Tested-by: [email protected]
Tested-by: [email protected]
Link: google/syzkaller#4207
Link: https://lore.kernel.org/bpf/[email protected]

kernel/bpf/mprog.c

@@ -253,6 +253,9 @@ int bpf_mprog_attach(struct bpf_mprog_entry *entry,
 			goto out;
 		}
 		idx = tidx;
+	} else if (bpf_mprog_total(entry) == bpf_mprog_max()) {
+		ret = -ERANGE;
+		goto out;
 	}
 	if (flags & BPF_F_BEFORE) {
 		tidx = bpf_mprog_pos_before(entry, &rtuple);