netdev - [PATCH 10/11] skbuff: reorder capability check last

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Message-ID: <20241125104011.36552-9-cgoettsche@seltendoof.de>
Date: Mon, 25 Nov 2024 11:40:02 +0100
From: Christian Göttsche <cgoettsche@...tendoof.de>
To: linux-security-module@...r.kernel.org
Cc: Christian Göttsche <cgzones@...glemail.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Serge Hallyn <serge@...lyn.com>,
	Julia Lawall <Julia.Lawall@...ia.fr>,
	Nicolas Palix <nicolas.palix@...g.fr>,
	Mina Almasry <almasrymina@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Pavel Begunkov <asml.silence@...il.com>,
	Lorenzo Bianconi <lorenzo@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Liang Chen <liangchen.linux@...il.com>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	cocci@...ia.fr
Subject: [PATCH 10/11] skbuff: reorder capability check last

From: Christian Göttsche <cgzones@...glemail.com>

capable() calls refer to enabled LSMs whether to permit or deny the
request.  This is relevant in connection with SELinux, where a
capability check results in a policy decision and by default a denial
message on insufficient permission is issued.
It can lead to three undesired cases:
  1. A denial message is generated, even in case the operation was an
     unprivileged one and thus the syscall succeeded, creating noise.
  2. To avoid the noise from 1. the policy writer adds a rule to ignore
     those denial messages, hiding future syscalls, where the task
     performs an actual privileged operation, leading to hidden limited
     functionality of that task.
  3. To avoid the noise from 1. the policy writer adds a rule to permit
     the task the requested capability, while it does not need it,
     violating the principle of least privilege.

Signed-off-by: Christian Göttsche <cgzones@...glemail.com>
---
 net/core/skbuff.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 6841e61a6bd0..8bf622744862 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1656,7 +1656,7 @@ int mm_account_pinned_pages(struct mmpin *mmp, size_t size)
 	unsigned long max_pg, num_pg, new_pg, old_pg, rlim;
 	struct user_struct *user;
 
-	if (capable(CAP_IPC_LOCK) || !size)
+	if (!size || capable(CAP_IPC_LOCK))
 		return 0;
 
 	rlim = rlimit(RLIMIT_MEMLOCK);
-- 
2.45.2