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Date: Fri, 18 Sep 2015 17:56:11 -0400
From: bfields@...ldses.org (J. Bruce Fields)
To: Andreas Gruenbacher <agruenba@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-nfs@...r.kernel.org, linux-api@...r.kernel.org,
linux-cifs@...r.kernel.org, linux-security-module@...r.kernel.org,
Andreas Gruenbacher <agruen@...nel.org>
Subject: Re: [RFC v7 22/41] richacl: Propagate everyone@ permissions to other
aces
On Sat, Sep 05, 2015 at 12:27:17PM +0200, Andreas Gruenbacher wrote:
> The trailing everyone@ allow ace can grant permissions to all file
> classes including the owner and group class. Before we can apply the
> other mask to this entry to turn it into an "other class" entry, we need
> to ensure that members of the owner or group class will not lose any
> permissions from that ace.
>
> Conceptually, we do this by inserting additional <who>:<allow>::allow
> entries before the trailing everyone@ allow ace with the same
> permissions as the trailing everyone@ allow ace for owner@, group@, and
> all explicitly mentioned users and groups. (In practice, we will rarely
> need to insert any additional aces in this step.)
>
> Signed-off-by: Andreas Gruenbacher <agruen@...nel.org>
> ---
> fs/richacl_compat.c | 195 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 195 insertions(+)
>
> diff --git a/fs/richacl_compat.c b/fs/richacl_compat.c
> index 4f0acf5..9b76fc0 100644
> --- a/fs/richacl_compat.c
> +++ b/fs/richacl_compat.c
> @@ -218,3 +218,198 @@ richacl_move_everyone_aces_down(struct richacl_alloc *alloc)
> }
> return 0;
> }
> +
> +/**
> + * __richacl_propagate_everyone - propagate everyone@ permissions up for @who
> + * @alloc: acl and number of allocated entries
> + * @who: identifier to propagate permissions for
> + * @allow: permissions to propagate up
> + *
> + * Propagate the permissions in @allow up from the end of the acl to the start
> + * for the specified principal @who.
> + *
> + * The simplest possible approach to achieve this would be to insert a
> + * "<who>:<allow>::allow" ace before the final everyone@ allow ace. Since this
> + * would often result in aces which are not needed or which could be merged
> + * with an existing ace, we make the following optimizations:
> + *
> + * - We go through the acl and determine which permissions are already
> + * allowed or denied to @who, and we remove those permissions from
> + * @allow.
> + *
> + * - If the acl contains an allow ace for @who and no aces after this entry
> + * deny permissions in @allow, we add the permissions in @allow to this
> + * ace. (Propagating permissions across a deny ace which can match the
> + * process can elevate permissions.)
> + *
> + * This transformation does not alter the permissions that the acl grants.
> + */
> +static int
> +__richacl_propagate_everyone(struct richacl_alloc *alloc, struct richace *who,
> + unsigned int allow)
> +{
> + struct richace *allow_last = NULL, *ace;
> + struct richacl *acl = alloc->acl;
> +
> + /*
> + * Remove the permissions from allow that are already determined for
> + * this who value, and figure out if there is an allow entry for
> + * this who value that is "reachable" from the trailing everyone@
> + * allow ace.
> + */
> + richacl_for_each_entry(ace, acl) {
> + if (richace_is_inherit_only(ace))
> + continue;
> + if (richace_is_allow(ace)) {
> + if (richace_is_same_identifier(ace, who)) {
> + allow &= ~ace->e_mask;
> + allow_last = ace;
> + }
> + } else if (richace_is_deny(ace)) {
> + if (richace_is_same_identifier(ace, who))
> + allow &= ~ace->e_mask;
> + else if (allow & ace->e_mask)
> + allow_last = NULL;
> + }
> + }
> + ace--;
> +
> + /*
> + * If for group class entries, all the remaining permissions will
> + * remain granted by the trailing everyone@ ace, no additional entry is
> + * needed.
> + */
> + if (!richace_is_owner(who) &&
> + richace_is_everyone(ace) && richace_is_allow(ace) &&
> + !(allow & ~(ace->e_mask & acl->a_other_mask)))
> + allow = 0;
> +
> + if (allow) {
> + if (allow_last)
> + return richace_change_mask(alloc, &allow_last,
> + allow_last->e_mask | allow);
> + else {
> + struct richace who_copy;
> +
> + richace_copy(&who_copy, who);
> + ace = acl->a_entries + acl->a_count - 1;
> + if (richacl_insert_entry(alloc, &ace))
> + return -1;
> + richace_copy(ace, &who_copy);
> + ace->e_type = RICHACE_ACCESS_ALLOWED_ACE_TYPE;
> + ace->e_flags &= ~RICHACE_INHERITANCE_FLAGS;
> + ace->e_mask = allow;
> + }
> + }
> + return 0;
> +}
> +
> +/**
> + * richacl_propagate_everyone - propagate everyone@ permissions up the acl
> + * @alloc: acl and number of allocated entries
> + *
> + * Make sure that group@ and all other users and groups mentioned in the acl
> + * will not lose any permissions when finally applying the other mask to the
> + * everyone@ allow ace at the end of the acl. We modify the permissions of
> + * existing entries or add new entries before the final everyone@ allow ace to
> + * achieve that.
> + *
> + * For example, the following acl implicitly grants everyone rwpx access:
> + *
> + * joe:r::allow
> + * everyone@:rwpx::allow
> + *
> + * When applying mode 0660 to this acl, group@ would lose rwp access, and joe
> + * would lose wp access even though the mode does not exclude those
> + * permissions. After propagating the everyone@ permissions, the result for
> + * applying mode 0660 becomes:
> + *
> + * owner@:rwp::allow
> + * joe:rwp::allow
> + * group@:rwp::allow
> + *
> + * Deny aces complicate the matter. For example, the following acl grants
> + * everyone but joe write access:
> + *
> + * joe:wp::deny
> + * everyone@:rwpx::allow
> + *
> + * When applying mode 0660 to this acl, group@ would lose rwp access, and joe
> + * would lose r access. After propagating the everyone@ permissions, the
> + * result for applying mode 0660 becomes:
> + *
> + * owner@:rwp::allow
> + * joe:w::deny
> + * group@:rwp::allow
> + * joe:r::allow
> + */
> +static int
> +richacl_propagate_everyone(struct richacl_alloc *alloc)
> +{
> + struct richace who = { .e_flags = RICHACE_SPECIAL_WHO };
> + struct richacl *acl = alloc->acl;
> + struct richace *ace;
> + unsigned int owner_allow, group_allow;
> +
> + /*
> + * If the owner mask contains permissions which are not in the group
> + * mask, the group mask contains permissions which are not in the other
> + * mask, or the owner class contains permissions which are not in the
s/owner class/owner mask?
> + * other mask, we may need to propagate permissions up from the
> + * everyone@ allow ace. The third condition is implied by the first
> + * two.
> + */
> + if (!((acl->a_owner_mask & ~acl->a_group_mask) ||
> + (acl->a_group_mask & ~acl->a_other_mask)))
> + return 0;
The code looks right, but I don't understand the preceding comment.
For example,
owner mask: rw
group mask: wx
other mask: rw
satisfies the first two conditions, but not the third.
Also, I don't understand why the first condition would imply that we
might need to propagate permissions.
--b.
> + if (!acl->a_count)
> + return 0;
> + ace = acl->a_entries + acl->a_count - 1;
> + if (richace_is_inherit_only(ace) || !richace_is_everyone(ace))
> + return 0;
> +
> + owner_allow = ace->e_mask & acl->a_owner_mask;
> + group_allow = ace->e_mask & acl->a_group_mask;
> +
> + if (owner_allow & ~(acl->a_group_mask & acl->a_other_mask)) {
> + /* Propagate everyone@ permissions through to owner@. */
> + who.e_id.special = RICHACE_OWNER_SPECIAL_ID;
> + if (__richacl_propagate_everyone(alloc, &who, owner_allow))
> + return -1;
> + acl = alloc->acl;
> + }
> +
> + if (group_allow & ~acl->a_other_mask) {
> + int n;
> +
> + /* Propagate everyone@ permissions through to group@. */
> + who.e_id.special = RICHACE_GROUP_SPECIAL_ID;
> + if (__richacl_propagate_everyone(alloc, &who, group_allow))
> + return -1;
> + acl = alloc->acl;
> +
> + /*
> + * Start from the entry before the trailing everyone@ allow
> + * entry. We will not hit everyone@ entries in the loop.
> + */
> + for (n = acl->a_count - 2; n != -1; n--) {
> + ace = acl->a_entries + n;
> +
> + if (richace_is_inherit_only(ace) ||
> + richace_is_owner(ace) ||
> + richace_is_group(ace))
> + continue;
> + if (richace_is_allow(ace) || richace_is_deny(ace)) {
> + /*
> + * Any inserted entry will end up below the
> + * current entry
> + */
> + if (__richacl_propagate_everyone(alloc, ace,
> + group_allow))
> + return -1;
> + acl = alloc->acl;
> + }
> + }
> + }
> + return 0;
> +}
> --
> 2.4.3
>
> --
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