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From: Christian Brauner <christian@brauner.io>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linux Containers <containers@lists.linux-foundation.org>,
	linux-fsdevel@vger.kernel.org,
	Seth Forshee <seth.forshee@canonical.com>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [REVIEW][PATCH 5/6] capabilities: Allow privileged user in s_user_ns to set security.* xattrs
Date: Thu, 24 May 2018 17:57:37 +0200	[thread overview]
Message-ID: <20180524155737.GA19932@mailbox.org> (raw)
In-Reply-To: <20180523232538.4880-5-ebiederm@xmission.com>

On Wed, May 23, 2018 at 06:25:37PM -0500, Eric W. Biederman wrote:
> A privileged user in s_user_ns will generally have the ability to
> manipulate the backing store and insert security.* xattrs into
> the filesystem directly. Therefore the kernel must be prepared to
> handle these xattrs from unprivileged mounts, and it makes little
> sense for commoncap to prevent writing these xattrs to the
> filesystem. The capability and LSM code have already been updated
> to appropriately handle xattrs from unprivileged mounts, so it
> is safe to loosen this restriction on setting xattrs.
> 
> The exception to this logic is that writing xattrs to a mounted
> filesystem may also cause the LSM inode_post_setxattr or
> inode_setsecurity callbacks to be invoked. SELinux will deny the
> xattr update by virtue of applying mountpoint labeling to
> unprivileged userns mounts, and Smack will deny the writes for
> any user without global CAP_MAC_ADMIN, so loosening the
> capability check in commoncap is safe in this respect as well.

Acked-by: Christian Brauner <christian@brauner.io>

> 
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

Note, I just talked to Serge. This should be Acked-by: Serge Hallyn <serge@hallyn.com>

> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
> ---
>  security/commoncap.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 1ce701fcb3f3..f4c33abd9959 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -919,6 +919,8 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
>  int cap_inode_setxattr(struct dentry *dentry, const char *name,
>  		       const void *value, size_t size, int flags)
>  {
> +	struct user_namespace *user_ns = dentry->d_sb->s_user_ns;
> +
>  	/* Ignore non-security xattrs */
>  	if (strncmp(name, XATTR_SECURITY_PREFIX,
>  			sizeof(XATTR_SECURITY_PREFIX) - 1) != 0)
> @@ -931,7 +933,7 @@ int cap_inode_setxattr(struct dentry *dentry, const char *name,
>  	if (strcmp(name, XATTR_NAME_CAPS) == 0)
>  		return 0;
>  
> -	if (!capable(CAP_SYS_ADMIN))
> +	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
>  		return -EPERM;
>  	return 0;
>  }
> @@ -949,6 +951,8 @@ int cap_inode_setxattr(struct dentry *dentry, const char *name,
>   */
>  int cap_inode_removexattr(struct dentry *dentry, const char *name)
>  {
> +	struct user_namespace *user_ns = dentry->d_sb->s_user_ns;
> +
>  	/* Ignore non-security xattrs */
>  	if (strncmp(name, XATTR_SECURITY_PREFIX,
>  			sizeof(XATTR_SECURITY_PREFIX) - 1) != 0)
> @@ -964,7 +968,7 @@ int cap_inode_removexattr(struct dentry *dentry, const char *name)
>  		return 0;
>  	}
>  
> -	if (!capable(CAP_SYS_ADMIN))
> +	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
>  		return -EPERM;
>  	return 0;
>  }
> -- 
> 2.14.1
> 

  reply	other threads:[~2018-05-24 15:57 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-23 23:22 [REVIEW][PATCH 0/6] Wrapping up the vfs support for unprivileged mounts Eric W. Biederman
2018-05-23 23:25 ` [REVIEW][PATCH 1/6] vfs: Don't allow changing the link count of an inode with an invalid uid or gid Eric W. Biederman
2018-05-24 12:58   ` Seth Forshee
2018-05-24 22:30     ` Christian Brauner
2018-05-23 23:25 ` [REVIEW][PATCH 2/6] vfs: Allow userns root to call mknod on owned filesystems Eric W. Biederman
2018-05-24 13:55   ` Seth Forshee
2018-05-24 16:55     ` Eric W. Biederman
2018-05-24 17:22       ` Seth Forshee
2018-05-24 19:12   ` Christian Brauner
2018-05-23 23:25 ` [REVIEW][PATCH 3/6] fs: Allow superblock owner to replace invalid owners of inodes Eric W. Biederman
2018-05-23 23:41   ` [REVIEW][PATCH v2 " Eric W. Biederman
2018-05-24 22:30     ` Christian Brauner
2018-05-23 23:25 ` [REVIEW][PATCH 4/6] fs: Allow superblock owner to access do_remount_sb() Eric W. Biederman
2018-05-24 15:58   ` Christian Brauner
2018-05-24 16:45     ` Eric W. Biederman
2018-05-24 17:28       ` Christian Brauner
2018-05-23 23:25 ` [REVIEW][PATCH 5/6] capabilities: Allow privileged user in s_user_ns to set security.* xattrs Eric W. Biederman
2018-05-24 15:57   ` Christian Brauner [this message]
2018-05-23 23:25 ` [REVIEW][PATCH 6/6] fs: Allow CAP_SYS_ADMIN in s_user_ns to freeze and thaw filesystems Eric W. Biederman
2018-05-24 15:59   ` Christian Brauner
2018-05-24 21:46 ` [REVIEW][PATCH 0/6] Wrapping up the vfs support for unprivileged mounts Theodore Y. Ts'o
2018-05-24 23:23   ` Eric W. Biederman
2018-05-25  3:57     ` Dave Chinner
2018-05-25  4:06       ` Darrick J. Wong
2018-05-29 13:12       ` Eric W. Biederman
2018-05-29 22:17         ` Dave Chinner
2018-05-30  2:34           ` Eric W. Biederman
2018-05-30  4:34             ` Dave Chinner
2018-05-29 15:40 ` Dongsu Park

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