linux-hardening - [PATCH] netfilter: nf_tables: replace deprecated strncpy with strscpy

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Message-ID: <20240909-strncpy-net-bridge-netfilter-nft_meta_bridge-c-v1-1-946180aa7909@google.com>
Date: Mon, 09 Sep 2024 15:48:39 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Pablo Neira Ayuso <pablo@...filter.org>, Jozsef Kadlecsik <kadlec@...filter.org>, 
	Roopa Prabhu <roopa@...dia.com>, Nikolay Aleksandrov <razor@...ckwall.org>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: netfilter-devel@...r.kernel.org, coreteam@...filter.org, 
	bridge@...ts.linux.dev, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org, Justin Stitt <justinstitt@...gle.com>, 
	Kees Cook <kees@...nel.org>
Subject: [PATCH] netfilter: nf_tables: replace deprecated strncpy with strscpy_pad

strncpy() is deprecated for use on NUL-terminated destination strings [1] and
as such we should prefer more robust and less ambiguous string interfaces.

In this particular instance, the usage of strncpy() is fine and works as
expected. However, towards the goal of [2], we should consider replacing
it with an alternative as many instances of strncpy() are bug-prone. Its
removal from the kernel promotes better long term health for the
codebase.

The current usage of strncpy() likely just wants the NUL-padding
behavior offered by strncpy() and doesn't care about the
NUL-termination. Since the compiler doesn't know the size of @dest, we
can't use strtomem_pad(). Instead, use strscpy_pad() which behaves
functionally the same as strncpy() in this context -- as we expect
br_dev->name to be NUL-terminated itself.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90 [2]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html
Cc: Kees Cook <keescook@...omium.org>
Cc: linux-hardening@...r.kernel.org
Signed-off-by: Justin Stitt <justinstitt@...gle.com>
---
Note: build-tested only.
---
 net/bridge/netfilter/nft_meta_bridge.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bridge/netfilter/nft_meta_bridge.c b/net/bridge/netfilter/nft_meta_bridge.c
index bd4d1b4d745f..2a17e88ab8ee 100644
--- a/net/bridge/netfilter/nft_meta_bridge.c
+++ b/net/bridge/netfilter/nft_meta_bridge.c
@@ -63,7 +63,7 @@ static void nft_meta_bridge_get_eval(const struct nft_expr *expr,
 		return nft_meta_get_eval(expr, regs, pkt);
 	}
 
-	strncpy((char *)dest, br_dev ? br_dev->name : "", IFNAMSIZ);
+	strscpy_pad((char *)dest, br_dev ? br_dev->name : "", IFNAMSIZ);
 	return;
 err:
 	regs->verdict.code = NFT_BREAK;

---
base-commit: 521b1e7f4cf0b05a47995b103596978224b380a8
change-id: 20240909-strncpy-net-bridge-netfilter-nft_meta_bridge-c-09dd8aaad386

Best regards,
--
Justin Stitt <justinstitt@...gle.com>