Re: [REVIEW][PATCH v2 3/6] fs: Allow superblock owner to replace invalid owners of inodes

[Christian Brauner <christian@brauner.io>]

On Wed, May 23, 2018 at 06:41:29PM -0500, Eric W. Biederman wrote:
> 
> Allow users with CAP_SYS_CHOWN over the superblock of a filesystem to
> chown files when inode owner is invalid.  Ordinarily the
> capable_wrt_inode_uidgid check is sufficient to allow access to files
> but when the underlying filesystem has uids or gids that don't map to
> the current user namespace it is not enough, so the chown permission
> checks need to be extended to allow this case.
> 
> Calling chown on filesystem nodes whose uid or gid don't map is
> necessary if those nodes are going to be modified as writing back
> inodes which contain uids or gids that don't map is likely to cause
> filesystem corruption of the uid or gid fields.
> 
> Once chown has been called the existing capable_wrt_inode_uidgid
> checks are sufficient to allow the owner of a superblock to do anything
> the global root user can do with an appropriate set of capabilities.
> 
> An ordinary filesystem mountable by a userns root will limit all uids
> and gids in s_user_ns or the INVALID_UID and INVALID_GID to flag all
> others.  So having this added permission limited to just INVALID_UID
> and INVALID_GID is sufficient to handle every case on an ordinary filesystem.
> 
> Of the virtual filesystems at least proc is known to set s_user_ns to
> something other than &init_user_ns, while at the same time presenting
> some files owned by GLOBAL_ROOT_UID.  Those files the mounter of proc
> in a user namespace should not be able to chown to get access to.
> Limiting the relaxation in permission to just the minimum of allowing
> changing INVALID_UID and INVALID_GID prevents problems with cases like
> that.
> 
> The original version of this patch was written by: Seth Forshee.  I
> have rewritten and rethought this patch enough so it's really not the
> same thing (certainly it needs a different description), but he
> deserves credit for getting out there and getting the conversation
> started, and finding the potential gotcha's and putting up with my
> semi-paranoid feedback.

Ok, took me a little longer to reason about this.

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

> 
> Inspired-by: Seth Forshee <seth.forshee@canonical.com>
> Acked-by: Seth Forshee <seth.forshee@canonical.com>
> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
> ---
> 
> Sigh.  In simplifying this change so it would not require a change to
> proc (or any other similar filesystem) I accidentally introduced some
> badly placed semicolons.  The kbuild test robot was very nice and found
> those for me.  Resend with those unnecessary semicolons removed.
> 
>  fs/attr.c | 36 ++++++++++++++++++++++++++++--------
>  1 file changed, 28 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/attr.c b/fs/attr.c
> index 12ffdb6fb63c..d0b4d34878fb 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -18,6 +18,32 @@
>  #include <linux/evm.h>
>  #include <linux/ima.h>
>  
> +static bool chown_ok(const struct inode *inode, kuid_t uid)
> +{
> +	if (uid_eq(current_fsuid(), inode->i_uid) &&
> +	    uid_eq(uid, inode->i_uid))
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	if (uid_eq(inode->i_uid, INVALID_UID) &&
> +	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +
> +static bool chgrp_ok(const struct inode *inode, kgid_t gid)
> +{
> +	if (uid_eq(current_fsuid(), inode->i_uid) &&
> +	    (in_group_p(gid) || gid_eq(gid, inode->i_gid)))
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	if (gid_eq(inode->i_gid, INVALID_GID) &&
> +	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +
>  /**
>   * setattr_prepare - check if attribute changes to a dentry are allowed
>   * @dentry:	dentry to check
> @@ -52,17 +78,11 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
>  		goto kill_priv;
>  
>  	/* Make sure a caller can chown. */
> -	if ((ia_valid & ATTR_UID) &&
> -	    (!uid_eq(current_fsuid(), inode->i_uid) ||
> -	     !uid_eq(attr->ia_uid, inode->i_uid)) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid))
>  		return -EPERM;
>  
>  	/* Make sure caller can chgrp. */
> -	if ((ia_valid & ATTR_GID) &&
> -	    (!uid_eq(current_fsuid(), inode->i_uid) ||
> -	    (!in_group_p(attr->ia_gid) && !gid_eq(attr->ia_gid, inode->i_gid))) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if ((ia_valid & ATTR_GID) && !chgrp_ok(inode, attr->ia_gid))
>  		return -EPERM;
>  
>  	/* Make sure a caller can chmod. */