netdev - [PATCH nf-next] netfilter: bpf: Pass string literal as format argument of request

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Message-Id: <20241018-nf-mod-fmt-v1-1-b5a275d6861c@kernel.org>
Date: Fri, 18 Oct 2024 10:42:58 +0100
From: Simon Horman <horms@...nel.org>
To: Pablo Neira Ayuso <pablo@...filter.org>, 
 Jozsef Kadlecsik <kadlec@...filter.org>
Cc: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Nathan Chancellor <nathan@...nel.org>, 
 Nick Desaulniers <ndesaulniers@...gle.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 netfilter-devel@...r.kernel.org, coreteam@...filter.org, 
 netdev@...r.kernel.org, bpf@...r.kernel.org, llvm@...ts.linux.dev
Subject: [PATCH nf-next] netfilter: bpf: Pass string literal as format
 argument of request_module()

Both gcc-14 and clang-18 report that passing a non-string literal as the
format argument of request_module() is potentially insecure.

E.g. clang-18 says:

.../nf_bpf_link.c:46:24: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
   46 |                 err = request_module(mod);
      |                                      ^~~
.../kmod.h:25:55: note: expanded from macro 'request_module'
   25 | #define request_module(mod...) __request_module(true, mod)
      |                                                       ^~~
.../nf_bpf_link.c:46:24: note: treat the string as an argument to avoid this
   46 |                 err = request_module(mod);
      |                                      ^
      |                                      "%s",
.../kmod.h:25:55: note: expanded from macro 'request_module'
   25 | #define request_module(mod...) __request_module(true, mod)
      |                                                       ^

It is always the case where the contents of mod is safe to pass as the
format argument. That is, in my understanding, it never contains any
format escape sequences.

But, it seems better to be safe than sorry. And, as a bonus, compiler
output becomes less verbose by addressing this issue as suggested by
clang-18.

No functional change intended.
Compile tested only.

Signed-off-by: Simon Horman <horms@...nel.org>
---
 net/netfilter/nf_bpf_link.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
index 5257d5e7eb09..6b9c9d71906d 100644
--- a/net/netfilter/nf_bpf_link.c
+++ b/net/netfilter/nf_bpf_link.c
@@ -42,7 +42,7 @@ get_proto_defrag_hook(struct bpf_nf_link *link,
 	hook = rcu_dereference(*ptr_global_hook);
 	if (!hook) {
 		rcu_read_unlock();
-		err = request_module(mod);
+		err = request_module("%s", mod);
 		if (err)
 			return ERR_PTR(err < 0 ? err : -EINVAL);