netdev - [PATCH net v2] net: Fix undefined behavior in netdev name allocation

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Date: Tue, 14 Nov 2023 09:56:18 +0200
From: Gal Pressman <gal@...dia.com>
To: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>
CC: Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, "Jiri
 Pirko" <jiri@...nulli.us>, <netdev@...r.kernel.org>, Simon Horman
	<horms@...nel.org>, Gal Pressman <gal@...dia.com>, Vlad Buslov
	<vladbu@...dia.com>
Subject: [PATCH net v2] net: Fix undefined behavior in netdev name allocation

Cited commit removed the strscpy() call and kept the snprintf() only.

It is common to use 'dev->name' as the format string before a netdev is
registered, this results in 'res' and 'name' pointers being equal.
According to POSIX, if copying takes place between objects that overlap
as a result of a call to sprintf() or snprintf(), the results are
undefined.

Add back the strscpy() and use 'buf' as an intermediate buffer.

Fixes: 7ad17b04dc7b ("net: trust the bitmap in __dev_alloc_name()")
Cc: Jakub Kicinski <kuba@...nel.org>
Reviewed-by: Vlad Buslov <vladbu@...dia.com>
Signed-off-by: Gal Pressman <gal@...dia.com>
Reviewed-by: Jakub Kicinski <kuba@...nel.org>
---
Changelog -
v1->v2: https://lore.kernel.org/all/20231113083544.1685919-1-gal@nvidia.com/
* Mention that dev->name is usually used as the format string in the
  commit message (Jakub).
* Put the right commit in the Fixes tag (Simon).
---
 net/core/dev.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 0d548431f3fa..af53f6d838ce 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1119,7 +1119,9 @@ static int __dev_alloc_name(struct net *net, const char *name, char *res)
 	if (i == max_netdevices)
 		return -ENFILE;
 
-	snprintf(res, IFNAMSIZ, name, i);
+	/* 'res' and 'name' could overlap, use 'buf' as an intermediate buffer */
+	strscpy(buf, name, IFNAMSIZ);
+	snprintf(res, IFNAMSIZ, buf, i);
 	return i;
 }
 
-- 
2.40.1