Date: Mon, 16 Mar 2009 18:03:42 -0500
From: "Serge E. Hallyn" <serue@us.ibm.com>
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>, Igor Zhbanov <izh1979@gmail.com>,
       Michael Kerrisk <mtk.manpages@gmail.com>, linux-kernel@vger.kernel.org,
       viro@zeniv.linux.org.uk, neilb@suse.de, Trond.Myklebust@netapp.com,
       David Howells <dhowells@redhat.com>, Andrew Morgan <morgan@kernel.org>,
       James Morris <jmorris@namei.org>, linux-security-module@vger.kernel.org,
       SELinux <selinux@tycho.nsa.gov>
Subject: Re: =?utf-8?B?0J7RgtCy0LXRgjogVkZTLCBORlMg?=
	=?utf-8?Q?security_bug=3F_Shoul?=
	=?utf-8?Q?d?= CAP_MKNOD and CAP_LINUX_IMMUTABLE be added to
	CAP_FS_MASK?
Message-ID: <20090316230342.GB15522@us.ibm.com>
References: <f44001920903110553t52ce0ba7l4ba97213e1c51873@mail.gmail.com> <20090311232356.GP13540@fieldses.org> <20090312161047.GA15209@us.ibm.com> <517f3f820903121321sf6d2014q8165b925d5d44db7@mail.gmail.com> <20090313175848.GB27891@fieldses.org> <f44001920903131137j158c5f76y6a71b58ee964df77@mail.gmail.com> <20090313190002.GA16025@us.ibm.com> <1237227705.1035.93.camel@localhost.localdomain> <20090316184926.GA6729@us.ibm.com> <20090316224848.GC17738@fieldses.org>
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Quoting J. Bruce Fields (bfields@fieldses.org):
> Seems this isn't entirely obvious in the general case.  In the specific

I think the MKNOD part is (based on historical behavior as you
suggested) obvious:  both masks should include CAP_MKNOD and
CAP_LINUX_IMMUTABLE.

(references: http://lxr.linux.no/linux-old+v2.0.21/fs/ext2/ioctl.c#L60
for immutable and http://lxr.linux.no/linux-old+v2.0.21/fs/namei.c#L503
for mknod)

-serge

> case of nfsd, however, this is pretty obvious.  So I'm inclined to
> submit the following now (and leave it to be reverted by a later patch
> if CAP_FS_MASK ends up including CAP_MKNOD, as seems likely).
> 
> --b.
> 
> commit 2ec8f8f0c0005ffe3cf93bbf3d9976de76cf4652
> Author: J. Bruce Fields <bfields@citi.umich.edu>
> Date:   Mon Mar 16 18:34:20 2009 -0400
> 
>     nfsd: nfsd should drop CAP_MKNOD for non-root
>     
>     Since creating a device node is normally an operation requiring special
>     privilege, Igor Zhbanov points out that it is surprising (to say the
>     least) that a client can, for example, create a device node on a
>     filesystem exported with root_squash.
>     
>     So, make sure CAP_MKNOD is among the capabilities dropped when an nfsd
>     thread handles a request from a non-root user.
>     
>     Reported-by: Igor Zhbanov <izh1979@gmail.com>
>     Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
> 
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index 02bdb76..7824483 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -393,8 +393,10 @@ struct cpu_vfs_cap_data {
>  # define CAP_FULL_SET     ((kernel_cap_t){{ ~0, ~0 }})
>  # define CAP_INIT_EFF_SET ((kernel_cap_t){{ ~CAP_TO_MASK(CAP_SETPCAP), ~0 }})
>  # define CAP_FS_SET       ((kernel_cap_t){{ CAP_FS_MASK_B0, CAP_FS_MASK_B1 } })
> -# define CAP_NFSD_SET     ((kernel_cap_t){{ CAP_FS_MASK_B0|CAP_TO_MASK(CAP_SYS_RESOURCE), \
> -					CAP_FS_MASK_B1 } })
> +# define CAP_NFSD_SET     ((kernel_cap_t){{ CAP_FS_MASK_B0 \
> +					    | CAP_TO_MASK(CAP_SYS_RESOURCE) \
> +					    | CAP_TO_MASK(CAP_MKNOD), \
> +					    CAP_FS_MASK_B1 } })
> 
>  #endif /* _KERNEL_CAPABILITY_U32S != 2 */
--
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