[PATCH bpf 1/2] bpf: fix NPE when computing changes_pkt_data of program w/o subprograms

[Eduard Zingerman <eddyz87@xxxxxxxxx>]

bpf_prog_aux->func field might be NULL if program does not have
subprograms except for main sub-program. The fixed commit does
bpf_prog_aux->func access unconditionally, which might lead to null
pointer dereference.

The bug could be triggered by replacing the following BPF program:

    SEC("tc")
    int main_changes(struct __sk_buff *sk)
    {
        bpf_skb_pull_data(sk, 0);
        return 0;
    }

With the following BPF program:

    SEC("freplace")
    long changes_pkt_data(struct __sk_buff *sk)
    {
        return bpf_skb_pull_data(sk, 0);
    }

bpf_prog_aux instance itself represents the main sub-program,
use this property to fix the bug.

Fixes: 81f6d0530ba0 ("bpf: check changes_pkt_data property for extension programs")
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Closes: https://lore.kernel.org/r/202412111822.qGw6tOyB-lkp@xxxxxxxxx/
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
 kernel/bpf/verifier.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c2e5d0e6e3d0..5e541339b2f6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -22193,6 +22193,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 	}
 	if (tgt_prog) {
 		struct bpf_prog_aux *aux = tgt_prog->aux;
+		bool tgt_changes_pkt_data;
 
 		if (bpf_prog_is_dev_bound(prog->aux) &&
 		    !bpf_prog_dev_bound_match(prog, tgt_prog)) {
@@ -22227,8 +22228,10 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 					"Extension programs should be JITed\n");
 				return -EINVAL;
 			}
-			if (prog->aux->changes_pkt_data &&
-			    !aux->func[subprog]->aux->changes_pkt_data) {
+			tgt_changes_pkt_data = aux->func
+					       ? aux->func[subprog]->aux->changes_pkt_data
+					       : aux->changes_pkt_data;
+			if (prog->aux->changes_pkt_data && !tgt_changes_pkt_data) {
 				bpf_log(log,
 					"Extension program changes packet data, while original does not\n");
 				return -EINVAL;
-- 
2.47.0