linux-kernel - Re: [PATCH v2 10/11] skbuff: reorder capability check last

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Message-ID: <67c708ff63eac_257ad92942d@willemb.c.googlers.com.notmuch>
Date: Tue, 04 Mar 2025 09:06:55 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Christian Göttsche <cgoettsche@...tendoof.de>
Cc: Christian Göttsche <cgzones@...glemail.com>, 
 Serge Hallyn <serge@...lyn.com>, 
 Jan Kara <jack@...e.com>, 
 Julia Lawall <Julia.Lawall@...ia.fr>, 
 Nicolas Palix <nicolas.palix@...g.fr>, 
 linux-kernel@...r.kernel.org, 
 linux-security-module@...r.kernel.org, 
 cocci@...ia.fr, 
 "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>, 
 Willem de Bruijn <willemb@...gle.com>, 
 Mina Almasry <almasrymina@...gle.com>, 
 Pavel Begunkov <asml.silence@...il.com>, 
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
 Christian Hopps <chopps@...n.net>, 
 Alexander Lobakin <aleksander.lobakin@...el.com>, 
 netdev@...r.kernel.org
Subject: Re: [PATCH v2 10/11] skbuff: reorder capability check last

Christian Göttsche wrote:
> 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>
> Reviewed-by: Serge Hallyn <serge@...lyn.com>

Similar to Paolo's response to patch 7: these networking patches
should probably go through net-next.

> ---
>  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 b1c81687e9d8..7ed538e15b56 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1566,7 +1566,7 @@ 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;

Not sure that this case is relevant:

Unlike most other capable checks, this does not protect a privileged
operation and returns with error for unprivileged users.

It offers a shortcut to privileged users to avoid memory accounting,
but continues in the comon case that the user is not privileged.

So the common case here is to generate denial messages when LSMs are
enabled. size 0 is not likely, so swapping the order is unlikely to
significantly change the number of denial messages.

>  
>  	rlim = rlimit(RLIMIT_MEMLOCK);
> -- 
> 2.47.2
>