[PATCH bpf v2 2/4] bpf: Add check iph->ihl < 5 in lwt

[Leon Hwang <leon.hwang@linux.dev>]

Sashiko pointed out [1]: On architectures like MIPS, the while-loop won't
stop in ip_fast_csum().

To avoid such issues caused by invalid iph->ihl in lwt, add check
"iph->ihl < 5" in bpf_lwt_push_ip_encap() to make sure iph->ihl is valid.

[1] https://lore.kernel.org/bpf/20260525150010.CDEBA1F000E9@smtp.kernel.org/

Fixes: 52f278774e79 ("bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 net/core/lwt_bpf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index 8009e427851f..c306120e11d2 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -613,7 +613,7 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
 	iph = (struct iphdr *)buff;
 	if (iph->version == 4) {
 		ipv4 = true;
-		if (unlikely(len < iph->ihl * 4))
+		if (unlikely(iph->ihl < 5 || len < iph->ihl * 4))
 			return -EINVAL;
 	} else if (iph->version == 6) {
 		ipv4 = false;
-- 
2.54.0