Re: [RFC v3 14/45] richacl: Permission check algorithm

[bfields@fieldses.org (J. Bruce Fields)]

On Fri, Apr 24, 2015 at 01:04:11PM +0200, Andreas Gruenbacher wrote:
> A richacl grants a requested access if the NFSv4 acl in the richacl
> grants the requested permissions (according to the NFSv4 permission
> check algorithm) and the file mask that applies to the process
> includes the requested permissions.

Is that right?  Based on that I'd have thought that an acl like

	owner  :r::mask
	group  :-::mask
	other  :-::mask
	bfields:r::allow

would permit read to bfields in the case bfields is the file owner,
because both the mask entries and the NFSv4 ACL would permit access.

But I think it doesn't (because the "bfields" entry is subject to the
group mask).

But if I'm right, I'm not sure how to describe the algorithm concisely.

--b.

> 
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> ---
>  fs/Makefile             |   2 +-
>  fs/richacl_inode.c      | 130 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/richacl.h |   3 ++
>  3 files changed, 134 insertions(+), 1 deletion(-)
>  create mode 100644 fs/richacl_inode.c
> 
> diff --git a/fs/Makefile b/fs/Makefile
> index 8f0a59c..bb96ad7 100644
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -48,7 +48,7 @@ obj-$(CONFIG_SYSCTL)		+= drop_caches.o
>  
>  obj-$(CONFIG_FHANDLE)		+= fhandle.o
>  obj-$(CONFIG_FS_RICHACL)	+= richacl.o
> -richacl-y			:= richacl_base.o
> +richacl-y			:= richacl_base.o richacl_inode.o
>  
>  obj-y				+= quota/
>  
> diff --git a/fs/richacl_inode.c b/fs/richacl_inode.c
> new file mode 100644
> index 0000000..33e591a
> --- /dev/null
> +++ b/fs/richacl_inode.c
> @@ -0,0 +1,130 @@
> +/*
> + * Copyright (C) 2010  Novell, Inc.
> + * Copyright (C) 2015  Red Hat, Inc.
> + * Written by Andreas Gruenbacher <agruen@kernel.org>
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License as published by the
> + * Free Software Foundation; either version 2, or (at your option) any
> + * later version.
> + *
> + * This program is distributed in the hope that it will be useful, but
> + * WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * General Public License for more details.
> + */
> +
> +#include <linux/sched.h>
> +#include <linux/module.h>
> +#include <linux/fs.h>
> +#include <linux/slab.h>
> +#include <linux/richacl.h>
> +
> +/**
> + * richacl_permission  -  richacl permission check algorithm
> + * @inode:	inode to check
> + * @acl:	rich acl of the inode
> + * @want:	requested access (MAY_* flags)
> + *
> + * Checks if the current process is granted @mask flags in @acl.
> + */
> +int
> +richacl_permission(struct inode *inode, const struct richacl *acl,
> +		   int want)
> +{
> +	const struct richace *ace;
> +	unsigned int mask = richacl_want_to_mask(want);
> +	unsigned int requested = mask, denied = 0;
> +	int in_owning_group = in_group_p(inode->i_gid);
> +	int in_owner_or_group_class = in_owning_group;
> +
> +	/*
> +	 * We don't need to know which class the process is in when the acl is
> +	 * not masked.
> +	 */
> +	if (!(acl->a_flags & RICHACL_MASKED))
> +		in_owner_or_group_class = 1;
> +
> +	/*
> +	 * A process is
> +	 *   - in the owner file class if it owns the file,
> +	 *   - in the group file class if it is in the file's owning group or
> +	 *     it matches any of the user or group entries, and
> +	 *   - in the other file class otherwise.
> +	 */
> +
> +	/*
> +	 * Check if the acl grants the requested access and determine which
> +	 * file class the process is in.
> +	 */
> +	richacl_for_each_entry(ace, acl) {
> +		unsigned int ace_mask = ace->e_mask;
> +
> +		if (richace_is_inherit_only(ace))
> +			continue;
> +		if (richace_is_owner(ace)) {
> +			if (!uid_eq(current_fsuid(), inode->i_uid))
> +				continue;
> +			goto is_owner;
> +		} else if (richace_is_group(ace)) {
> +			if (!in_owning_group)
> +				continue;
> +		} else if (richace_is_unix_id(ace)) {
> +			if (ace->e_flags & RICHACE_IDENTIFIER_GROUP) {
> +				if (!in_group_p(ace->e_id.gid))
> +					continue;
> +			} else {
> +				if (!uid_eq(current_fsuid(), ace->e_id.uid))
> +					continue;
> +			}
> +		} else
> +			goto is_everyone;
> +
> +		/*
> +		 * Apply the group file mask to entries other than OWNER@ and
> +		 * EVERYONE@. This is not required for correct access checking
> +		 * but ensures that we grant the same permissions as the acl
> +		 * computed by richacl_apply_masks() would grant.
> +		 */
> +		if ((acl->a_flags & RICHACL_MASKED) && richace_is_allow(ace))
> +			ace_mask &= acl->a_group_mask;
> +
> +is_owner:
> +		/* The process is in the owner or group file class. */
> +		in_owner_or_group_class = 1;
> +
> +is_everyone:
> +		/* Check which mask flags the ACE allows or denies. */
> +		if (richace_is_deny(ace))
> +			denied |= ace_mask & mask;
> +		mask &= ~ace_mask;
> +
> +		/*
> +		 * Keep going until we know which file class
> +		 * the process is in.
> +		 */
> +		if (!mask && in_owner_or_group_class)
> +			break;
> +	}
> +	denied |= mask;
> +
> +	if (acl->a_flags & RICHACL_MASKED) {
> +		unsigned int file_mask;
> +
> +		/*
> +		 * The file class a process is in determines which file mask
> +		 * applies.  Check if that file mask also grants the requested
> +		 * access.
> +		 */
> +		if (uid_eq(current_fsuid(), inode->i_uid))
> +			file_mask = acl->a_owner_mask;
> +		else if (in_owner_or_group_class)
> +			file_mask = acl->a_group_mask;
> +		else
> +			file_mask = acl->a_other_mask;
> +		denied |= requested & ~file_mask;
> +	}
> +
> +	return denied ? -EACCES : 0;
> +}
> +EXPORT_SYMBOL_GPL(richacl_permission);
> diff --git a/include/linux/richacl.h b/include/linux/richacl.h
> index a27ff10..d11e8f3 100644
> --- a/include/linux/richacl.h
> +++ b/include/linux/richacl.h
> @@ -292,4 +292,7 @@ extern unsigned int richacl_want_to_mask(unsigned int);
>  extern void richacl_compute_max_masks(struct richacl *);
>  extern struct richacl *richacl_chmod(struct richacl *, mode_t);
>  
> +/* richacl_inode.c */
> +extern int richacl_permission(struct inode *, const struct richacl *, int);
> +
>  #endif /* __RICHACL_H */
> -- 
> 2.1.0
> 
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